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bind9/doc/expired/draft-ietf-ipngwg-rfc2292bis-01.txt

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INTERNET-DRAFT W. Richard Stevens (Consultant)
Expires: December 24, 1999 Matt Thomas (Consultant)
Obsoletes RFC 2292 Erik Nordmark (Sun)
Oct 22, 1999
Advanced Sockets API for IPv6
<draft-ietf-ipngwg-rfc2292bis-01.txt>
Abstract
A separate specification [RFC-2553] contain changes to the sockets
API to support IP version 6. Those changes are for TCP and UDP-based
applications and will support most end-user applications in use
today: Telnet and FTP clients and servers, HTTP clients and servers,
and the like.
But another class of applications exists that will also be run under
IPv6. We call these "advanced" applications and today this includes
programs such as Ping, Traceroute, routing daemons, multicast routing
daemons, router discovery daemons, and the like. The API feature
typically used by these programs that make them "advanced" is a raw
socket to access ICMPv4, IGMPv4, or IPv4, along with some knowledge
of the packet header formats used by these protocols. To provide
portability for applications that use raw sockets under IPv6, some
standardization is needed for the advanced API features.
There are other features of IPv6 that some applications will need to
access: interface identification (specifying the outgoing interface
and determining the incoming interface) and IPv6 extension headers
that are not addressed in [RFC-2553]: The Routing header (source
routing), Hop-by-Hop options, and Destination options. This document
provides API access to these features too.
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
draft-ietf-ipngwg-2292bis-01.txt [Page 1]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet Draft expires April 22, 2000.
draft-ietf-ipngwg-2292bis-01.txt [Page 2]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
Table of Contents
1. Introduction .................................................... 6
2. Common Structures and Definitions ............................... 7
2.1. The ip6_hdr Structure ...................................... 8
2.1.1. IPv6 Next Header Values ............................. 8
2.1.2. IPv6 Extension Headers .............................. 9
2.1.3. IPv6 Options ........................................ 10
2.2. The icmp6_hdr Structure .................................... 12
2.2.1. ICMPv6 Type and Code Values ......................... 13
2.2.2. ICMPv6 Neighbor Discovery Type and Code Values ...... 14
2.2.3. Multicast Listener Discovery Type and Code Values ... 16
2.2.4. ICMPv6 Router Renumbering Type and Code Values ...... 17
2.3. Address Testing Macros ..................................... 19
2.4. Protocols File ............................................. 19
3. IPv6 Raw Sockets ................................................ 20
3.1. Checksums .................................................. 21
3.2. ICMPv6 Type Filtering ...................................... 21
4. Access to IPv6 and Extension Headers ............................ 24
4.1. TCP Implications ........................................... 26
4.2. UDP and Raw Socket Implications ............................ 27
5. Extensions to Socket Ancillary Data ............................. 27
6. Packet Information .............................................. 28
6.1. Specifying/Receiving the Interface ......................... 29
6.2. Specifying/Receiving Source/Destination Address ............ 29
6.3. Specifying/Receiving the Hop Limit ......................... 30
6.4. Specifying the Next Hop Address ............................ 31
6.5. Additional Errors with sendmsg() and setsockopt() .......... 31
7. Routing Header Option ........................................... 32
7.1. inet6_rth_space ............................................ 33
7.2. inet6_rth_init ............................................. 33
7.3. inet6_rth_add .............................................. 34
7.4. inet6_rth_reverse .......................................... 34
7.5. inet6_rth_segments ......................................... 35
7.6. inet6_rth_getaddr .......................................... 35
8. Hop-By-Hop Options .............................................. 35
8.1. Receiving Hop-by-Hop Options ............................... 36
8.2. Sending Hop-by-Hop Options ................................. 37
9. Destination Options ............................................. 37
9.1. Receiving Destination Options .............................. 37
draft-ietf-ipngwg-2292bis-01.txt [Page 3]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
9.2. Sending Destination Options ................................ 38
10. Hop-by-Hop and Destination Options Processing ................... 39
10.1. inet6_opt_init ............................................ 39
10.2. inet6_opt_append .......................................... 39
10.3. inet6_opt_finish .......................................... 40
10.4. inet6_opt_set_val ......................................... 41
10.5. inet6_opt_next ............................................ 41
10.6. inet6_opt_find ............................................ 41
10.7. inet6_opt_get_val ......................................... 42
11. Additional Advanced API Functions ............................... 42
11.1. Sending with the Minimum MTU .............................. 42
11.2. Path MTU Discovery and UDP ................................ 43
11.3. Neighbor Reachability and UDP ............................. 43
12. Ordering of Ancillary Data and IPv6 Extension Headers ........... 44
13. IPv6-Specific Options with IPv4-Mapped IPv6 Addresses ........... 44
14. Extended interfaces for rresvport, rcmd and rexec ............... 45
14.1. rresvport_af .............................................. 45
14.2. rcmd_af ................................................... 45
14.3. rexec_af .................................................. 46
15. Summary of New Definitions ...................................... 46
16. Security Considerations ......................................... 49
17. Change History .................................................. 49
18. TODO and Open Issues ............................................ 52
19. References ...................................................... 52
20. Acknowledgments ................................................. 53
21. Authors' Addresses .............................................. 53
22. Appendix A: Ancillary Data Overview ............................. 54
22.1. The msghdr Structure ...................................... 54
22.2. The cmsghdr Structure ..................................... 55
22.3. Ancillary Data Object Macros .............................. 57
22.3.1. CMSG_FIRSTHDR ...................................... 57
22.3.2. CMSG_NXTHDR ........................................ 58
22.3.3. CMSG_DATA .......................................... 59
22.3.4. CMSG_SPACE ......................................... 60
22.3.5. CMSG_LEN ........................................... 60
draft-ietf-ipngwg-2292bis-01.txt [Page 4]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
23. Appendix B: Examples using the inet6_rth_XXX() functions ........ 61
23.1. Sending a Routing Header .................................. 61
23.2. Receiving Routing Headers ................................. 66
24. Appendix C: Examples using the inet6_opt_XXX() functions ........ 68
24.1. Building options .......................................... 68
24.2. Parsing received options .................................. 70
draft-ietf-ipngwg-2292bis-01.txt [Page 5]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
1. Introduction
A separate specification [RFC-2553] contain changes to the sockets
API to support IP version 6. Those changes are for TCP and UDP-based
applications. This document defines some the "advanced" features of
the sockets API that are required for applications to take advantage
of additional features of IPv6.
Today, the portability of applications using IPv4 raw sockets is
quite high, but this is mainly because most IPv4 implementations
started from a common base (the Berkeley source code) or at least
started with the Berkeley header files. This allows programs such as
Ping and Traceroute, for example, to compile with minimal effort on
many hosts that support the sockets API. With IPv6, however, there
is no common source code base that implementors are starting from,
and the possibility for divergence at this level between different
implementations is high. To avoid a complete lack of portability
amongst applications that use raw IPv6 sockets, some standardization
is necessary.
There are also features from the basic IPv6 specification that are
not addressed in [RFC-2553]: sending and receiving Routing headers,
Hop-by-Hop options, and Destination options, specifying the outgoing
interface, and being told of the receiving interface.
This document can be divided into the following main sections.
1. Definitions of the basic constants and structures required for
applications to use raw IPv6 sockets. This includes structure
definitions for the IPv6 and ICMPv6 headers and all associated
constants (e.g., values for the Next Header field).
2. Some basic semantic definitions for IPv6 raw sockets. For
example, a raw ICMPv4 socket requires the application to
calculate and store the ICMPv4 header checksum. But with IPv6
this would require the application to choose the source IPv6
address because the source address is part of the pseudo header
that ICMPv6 now uses for its checksum computation. It should be
defined that with a raw ICMPv6 socket the kernel always
calculates and stores the ICMPv6 header checksum.
3. Packet information: how applications can obtain the received
interface, destination address, and received hop limit, along
with specifying these values on a per-packet basis. There are a
class of applications that need this capability and the technique
should be portable.
4. Access to the optional Routing header, Hop-by-Hop, and
draft-ietf-ipngwg-2292bis-01.txt [Page 6]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
Destination extension headers.
5. Additional features required for improved IPv6 application
portability.
The packet information along with access to the extension headers
(Routing header, Hop-by-Hop options, and Destination options) are
specified using the "ancillary data" fields that were added to the
4.3BSD Reno sockets API in 1990. The reason is that these ancillary
data fields are part of the Posix.1g standard and should therefore be
adopted by most vendors.
This document does not address application access to either the
authentication header or the encapsulating security payload header.
All examples in this document omit error checking in favor of brevity
and clarity.
We note that many of the functions and socket options defined in this
document may have error returns that are not defined in this
document. Many of these possible error returns will be recognized
only as implementations proceed.
Datatypes in this document follow the Posix.1g format: intN_t means a
signed integer of exactly N bits (e.g., int16_t) and uintN_t means an
unsigned integer of exactly N bits (e.g., uint32_t).
Note that we use the (unofficial) terminology ICMPv4, IGMPv4, and
ARPv4 to avoid any confusion with the newer ICMPv6 protocol.
2. Common Structures and Definitions
Many advanced applications examine fields in the IPv6 header and set
and examine fields in the various ICMPv6 headers. Common structure
definitions for these protocol headers are required, along with
common constant definitions for the structure members.
This API assumes that the fields in the protocol headers are left in
the network byte order, which is big-endian for the Internet
protocols. If not, then either these constants or the fields being
tested must be converted at run-time, using something like htons() or
htonl().
Two new header files are defined: <netinet/ip6.h> and
<netinet/icmp6.h>.
When an include file is specified, that include file is allowed to
draft-ietf-ipngwg-2292bis-01.txt [Page 7]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
include other files that do the actual declaration or definition.
2.1. The ip6_hdr Structure
The following structure is defined as a result of including
<netinet/ip6.h>. Note that this is a new header.
struct ip6_hdr {
union {
struct ip6_hdrctl {
uint32_t ip6_un1_flow; /* 4 bits version, 8 bits TC, 24 bits flow-ID */
uint16_t ip6_un1_plen; /* payload length */
uint8_t ip6_un1_nxt; /* next header */
uint8_t ip6_un1_hlim; /* hop limit */
} ip6_un1;
uint8_t ip6_un2_vfc; /* 4 bits version, top 4 bits tclass */
} ip6_ctlun;
struct in6_addr ip6_src; /* source address */
struct in6_addr ip6_dst; /* destination address */
};
#define ip6_vfc ip6_ctlun.ip6_un2_vfc
#define ip6_flow ip6_ctlun.ip6_un1.ip6_un1_flow
#define ip6_plen ip6_ctlun.ip6_un1.ip6_un1_plen
#define ip6_nxt ip6_ctlun.ip6_un1.ip6_un1_nxt
#define ip6_hlim ip6_ctlun.ip6_un1.ip6_un1_hlim
#define ip6_hops ip6_ctlun.ip6_un1.ip6_un1_hlim
2.1.1. IPv6 Next Header Values
IPv6 defines many new values for the Next Header field. The
following constants are defined as a result of including
<netinet/in.h>.
#define IPPROTO_HOPOPTS 0 /* IPv6 Hop-by-Hop options */
#define IPPROTO_IPV6 41 /* IPv6 header */
#define IPPROTO_ROUTING 43 /* IPv6 Routing header */
#define IPPROTO_FRAGMENT 44 /* IPv6 fragmentation header */
#define IPPROTO_ESP 50 /* encapsulating security payload */
#define IPPROTO_AH 51 /* authentication header */
#define IPPROTO_ICMPV6 58 /* ICMPv6 */
#define IPPROTO_NONE 59 /* IPv6 no next header */
#define IPPROTO_DSTOPTS 60 /* IPv6 Destination options */
Berkeley-derived IPv4 implementations also define IPPROTO_IP to be 0.
draft-ietf-ipngwg-2292bis-01.txt [Page 8]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
This should not be a problem since IPPROTO_IP is used only with IPv4
sockets and IPPROTO_HOPOPTS only with IPv6 sockets.
2.1.2. IPv6 Extension Headers
Six extension headers are defined for IPv6. We define structures for
all except the Authentication header and Encapsulating Security
Payload header, both of which are beyond the scope of this document.
The following structures are defined as a result of including
<netinet/ip6.h>.
/* Hop-by-Hop options header */
struct ip6_hbh {
uint8_t ip6h_nxt; /* next header */
uint8_t ip6h_len; /* length in units of 8 octets */
/* followed by options */
};
/* Destination options header */
struct ip6_dest {
uint8_t ip6d_nxt; /* next header */
uint8_t ip6d_len; /* length in units of 8 octets */
/* followed by options */
};
/* Routing header */
struct ip6_rthdr {
uint8_t ip6r_nxt; /* next header */
uint8_t ip6r_len; /* length in units of 8 octets */
uint8_t ip6r_type; /* routing type */
uint8_t ip6r_segleft; /* segments left */
/* followed by routing type specific data */
};
/* Type 0 Routing header */
struct ip6_rthdr0 {
uint8_t ip6r0_nxt; /* next header */
uint8_t ip6r0_len; /* length in units of 8 octets */
uint8_t ip6r0_type; /* always zero */
uint8_t ip6r0_segleft; /* segments left */
uint32_t ip6r0_reserved; /* reserved field */
/* followed by up to 127 struct in6_addr */
};
/* Fragment header */
struct ip6_frag {
uint8_t ip6f_nxt; /* next header */
draft-ietf-ipngwg-2292bis-01.txt [Page 9]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
uint8_t ip6f_reserved; /* reserved field */
uint16_t ip6f_offlg; /* offset, reserved, and flag */
uint32_t ip6f_ident; /* identification */
};
#if BYTE_ORDER == BIG_ENDIAN
#define IP6F_OFF_MASK 0xfff8 /* mask out offset from _offlg */
#define IP6F_RESERVED_MASK 0x0006 /* reserved bits in ip6f_offlg */
#define IP6F_MORE_FRAG 0x0001 /* more-fragments flag */
#else /* BYTE_ORDER == LITTLE_ENDIAN */
#define IP6F_OFF_MASK 0xf8ff /* mask out offset from _offlg */
#define IP6F_RESERVED_MASK 0x0600 /* reserved bits in ip6f_offlg */
#define IP6F_MORE_FRAG 0x0100 /* more-fragments flag */
#endif
2.1.3. IPv6 Options
Eleven options are defined for IPv6 at the time of writing this
document. We define structures for all except the unspecified EID
option. The following structures are defined as a result of
including <netinet/ip6.h>.
/* IPv6 options */
structip6_opt {
uint8_tip6o_type;
uint8_tip6o_len;
};
/*
* The high-order 3 bits of the option type define the behavior
* when processing an unknown option and whether or not the option
* content changes in flight.
*/
#defineIP6OPT_TYPE(o)((o) & 0xc0)
#defineIP6OPT_TYPE_SKIP0x00
#defineIP6OPT_TYPE_DISCARD0x40
#defineIP6OPT_TYPE_FORCEICMP0x80
#defineIP6OPT_TYPE_ICMP0xc0
#defineIP6OPT_MUTABLE0x20
#defineIP6OPT_PAD10x00/* 00 0 00000 */
#defineIP6OPT_PADN0x01/* 00 0 00001 */
#defineIP6OPT_JUMBO0xc2/* 11 0 00010 = 194 */
#defineIP6OPT_NSAP_ADDR0xc3/* 11 0 00011 */
#defineIP6OPT_TUNNEL_LIMIT0x04/* 00 0 00100 */
#defineIP6OPT_ROUTER_ALERT0x05/* 00 0 00101 */
draft-ietf-ipngwg-2292bis-01.txt [Page 10]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
#defineIP6OPT_BINDING_UPDATE0xc6/* 11 0 00110 */
#defineIP6OPT_BINDING_ACK0x07/* 00 0 00111 */
#defineIP6OPT_BINDING_REQ0x08/* 00 0 01000 */
#defineIP6OPT_HOME_ADDRESS0xc9/* 11 0 01001 */
#defineIP6OPT_EID0x8a/* 10 0 01010 */
/* Jumbo Payload Option */
structip6_opt_jumbo {
uint8_tip6oj_type;
uint8_tip6oj_len;
uint8_t ip6oj_jumbo_len[4];
};
#defineIP6OPT_JUMBO_LEN6
/* NSAP Address Option */
structip6_opt_nsap {
uint8_tip6on_type;
uint8_tip6on_len;
uint8_t ip6on_src_nsap_len;
uint8_t ip6on_dst_nsap_len;
/* followed by source NSAP */
/* followed by destination NSAP */
};
/* Tunnel Limit Option */
structip6_opt_tunnel {
uint8_tip6ot_type;
uint8_tip6ot_len;
uint8_t ip6ot_encap_limit;
};
/* Router Alert Option */
structip6_opt_router {
uint8_tip6or_type;
uint8_tip6or_len;
uint8_t ip6or_value[2];
};
/* Router alert values (in network byte order) */
#ifdef _BIG_ENDIAN
#defineIP6_ALERT_MLD0x0000
#defineIP6_ALERT_RSVP0x0001
#defineIP6_ALERT_AN0x0002
#else
#defineIP6_ALERT_MLD0x0000
#defineIP6_ALERT_RSVP0x0100
#defineIP6_ALERT_AN0x0200
#endif
draft-ietf-ipngwg-2292bis-01.txt [Page 11]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
/* Binding Update Option */
structip6_opt_binding_update {
uint8_tip6ou_type;
uint8_tip6ou_len;
uint8_t ip6ou_flags;
uint8_t ip6ou_prefixlen;
uint8_t ip6ou_seqno[2];
uint8_t ip6ou_lifetime[4];
uint8_t ip6ou_coa[16];/* Optional based on flags */
/* followed by sub-options */
};
/* Binding Update Flags */
#defineIP6_BUF_ACK0x80/* Request a binding ack */
#defineIP6_BUF_HOME0x40/* Home Registration */
#defineIP6_BUF_COA0x20/* Care-of-address present in option */
#defineIP6_BUF_ROUTER0x10/* Sending mobile node is a router */
/* Binding Ack Option */
structip6_opt_binding_ack {
uint8_tip6oa_type;
uint8_tip6oa_len;
uint8_t ip6oa_status;
uint8_t ip6oa_seqno[2];
uint8_t ip6oa_lifetime[4];
uint8_t ip6oa_refresh[4];
/* followed by sub-options */
};
/* Binding Request Option */
structip6_opt_binding_request {
uint8_tip6or_type;
uint8_tip6or_len;
/* followed by sub-options */
};
/* Home Address Option */
structip6_opt_home_address {
uint8_tip6oh_type;
uint8_tip6oh_len;
uint8_t ip6oh_addr[16];/* Home Address */
/* followed by sub-options */
};
2.2. The icmp6_hdr Structure
draft-ietf-ipngwg-2292bis-01.txt [Page 12]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
The ICMPv6 header is needed by numerous IPv6 applications including
Ping, Traceroute, router discovery daemons, and neighbor discovery
daemons. The following structure is defined as a result of including
<netinet/icmp6.h>. Note that this is a new header.
struct icmp6_hdr {
uint8_t icmp6_type; /* type field */
uint8_t icmp6_code; /* code field */
uint16_t icmp6_cksum; /* checksum field */
union {
uint32_t icmp6_un_data32[1]; /* type-specific field */
uint16_t icmp6_un_data16[2]; /* type-specific field */
uint8_t icmp6_un_data8[4]; /* type-specific field */
} icmp6_dataun;
};
#define icmp6_data32 icmp6_dataun.icmp6_un_data32
#define icmp6_data16 icmp6_dataun.icmp6_un_data16
#define icmp6_data8 icmp6_dataun.icmp6_un_data8
#define icmp6_pptr icmp6_data32[0] /* parameter prob */
#define icmp6_mtu icmp6_data32[0] /* packet too big */
#define icmp6_id icmp6_data16[0] /* echo request/reply */
#define icmp6_seq icmp6_data16[1] /* echo request/reply */
#define icmp6_maxdelay icmp6_data16[0] /* mcast group membership */
2.2.1. ICMPv6 Type and Code Values
In addition to a common structure for the ICMPv6 header, common
definitions are required for the ICMPv6 type and code fields. The
following constants are also defined as a result of including
<netinet/icmp6.h>.
#define ICMP6_DST_UNREACH 1
#define ICMP6_PACKET_TOO_BIG 2
#define ICMP6_TIME_EXCEEDED 3
#define ICMP6_PARAM_PROB 4
#define ICMP6_INFOMSG_MASK 0x80 /* all informational messages */
#define ICMP6_ECHO_REQUEST 128
#define ICMP6_ECHO_REPLY 129
#define ICMP6_MEMBERSHIP_QUERY 130
#define ICMP6_MEMBERSHIP_REPORT 131
#define ICMP6_MEMBERSHIP_REDUCTION 132
#define ICMP6_DST_UNREACH_NOROUTE 0 /* no route to destination */
draft-ietf-ipngwg-2292bis-01.txt [Page 13]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
#define ICMP6_DST_UNREACH_ADMIN 1 /* communication with */
/* destination */
/* admin. prohibited */
#define ICMP6_DST_UNREACH_BEYONDSCOPE 2 /* beyond scope of source address */
#define ICMP6_DST_UNREACH_ADDR 3 /* address unreachable */
#define ICMP6_DST_UNREACH_NOPORT 4 /* bad port */
#define ICMP6_TIME_EXCEED_TRANSIT 0 /* Hop Limit == 0 in transit */
#define ICMP6_TIME_EXCEED_REASSEMBLY 1 /* Reassembly time out */
#define ICMP6_PARAMPROB_HEADER 0 /* erroneous header field */
#define ICMP6_PARAMPROB_NEXTHEADER 1 /* unrecognized Next Header */
#define ICMP6_PARAMPROB_OPTION 2 /* unrecognized IPv6 option */
The five ICMP message types defined by IPv6 neighbor discovery
(133-137) are defined in the next section.
2.2.2. ICMPv6 Neighbor Discovery Type and Code Values
The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
#define ND_ROUTER_SOLICIT 133
#define ND_ROUTER_ADVERT 134
#define ND_NEIGHBOR_SOLICIT 135
#define ND_NEIGHBOR_ADVERT 136
#define ND_REDIRECT 137
struct nd_router_solicit { /* router solicitation */
struct icmp6_hdr nd_rs_hdr;
/* could be followed by options */
};
#define nd_rs_type nd_rs_hdr.icmp6_type
#define nd_rs_code nd_rs_hdr.icmp6_code
#define nd_rs_cksum nd_rs_hdr.icmp6_cksum
#define nd_rs_reserved nd_rs_hdr.icmp6_data32[0]
struct nd_router_advert { /* router advertisement */
struct icmp6_hdr nd_ra_hdr;
uint32_t nd_ra_reachable; /* reachable time */
uint32_t nd_ra_retransmit; /* retransmit timer */
/* could be followed by options */
};
#define nd_ra_type nd_ra_hdr.icmp6_type
#define nd_ra_code nd_ra_hdr.icmp6_code
draft-ietf-ipngwg-2292bis-01.txt [Page 14]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
#define nd_ra_cksum nd_ra_hdr.icmp6_cksum
#define nd_ra_curhoplimit nd_ra_hdr.icmp6_data8[0]
#define nd_ra_flags_reserved nd_ra_hdr.icmp6_data8[1]
#define ND_RA_FLAG_MANAGED 0x80
#define ND_RA_FLAG_OTHER 0x40
#define nd_ra_router_lifetime nd_ra_hdr.icmp6_data16[1]
struct nd_neighbor_solicit { /* neighbor solicitation */
struct icmp6_hdr nd_ns_hdr;
struct in6_addr nd_ns_target; /* target address */
/* could be followed by options */
};
#define nd_ns_type nd_ns_hdr.icmp6_type
#define nd_ns_code nd_ns_hdr.icmp6_code
#define nd_ns_cksum nd_ns_hdr.icmp6_cksum
#define nd_ns_reserved nd_ns_hdr.icmp6_data32[0]
struct nd_neighbor_advert { /* neighbor advertisement */
struct icmp6_hdr nd_na_hdr;
struct in6_addr nd_na_target; /* target address */
/* could be followed by options */
};
#define nd_na_type nd_na_hdr.icmp6_type
#define nd_na_code nd_na_hdr.icmp6_code
#define nd_na_cksum nd_na_hdr.icmp6_cksum
#define nd_na_flags_reserved nd_na_hdr.icmp6_data32[0]
#if BYTE_ORDER == BIG_ENDIAN
#define ND_NA_FLAG_ROUTER 0x80000000
#define ND_NA_FLAG_SOLICITED 0x40000000
#define ND_NA_FLAG_OVERRIDE 0x20000000
#else /* BYTE_ORDER == LITTLE_ENDIAN */
#define ND_NA_FLAG_ROUTER 0x00000080
#define ND_NA_FLAG_SOLICITED 0x00000040
#define ND_NA_FLAG_OVERRIDE 0x00000020
#endif
struct nd_redirect { /* redirect */
struct icmp6_hdr nd_rd_hdr;
struct in6_addr nd_rd_target; /* target address */
struct in6_addr nd_rd_dst; /* destination address */
/* could be followed by options */
};
#define nd_rd_type nd_rd_hdr.icmp6_type
#define nd_rd_code nd_rd_hdr.icmp6_code
#define nd_rd_cksum nd_rd_hdr.icmp6_cksum
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#define nd_rd_reserved nd_rd_hdr.icmp6_data32[0]
struct nd_opt_hdr { /* Neighbor discovery option header */
uint8_t nd_opt_type;
uint8_t nd_opt_len; /* in units of 8 octets */
/* followed by option specific data */
};
#define ND_OPT_SOURCE_LINKADDR 1
#define ND_OPT_TARGET_LINKADDR 2
#define ND_OPT_PREFIX_INFORMATION 3
#define ND_OPT_REDIRECTED_HEADER 4
#define ND_OPT_MTU 5
struct nd_opt_prefix_info { /* prefix information */
uint8_t nd_opt_pi_type;
uint8_t nd_opt_pi_len;
uint8_t nd_opt_pi_prefix_len;
uint8_t nd_opt_pi_flags_reserved;
uint32_t nd_opt_pi_valid_time;
uint32_t nd_opt_pi_preferred_time;
uint32_t nd_opt_pi_reserved2;
struct in6_addr nd_opt_pi_prefix;
};
#define ND_OPT_PI_FLAG_ONLINK 0x80
#define ND_OPT_PI_FLAG_AUTO 0x40
struct nd_opt_rd_hdr { /* redirected header */
uint8_t nd_opt_rh_type;
uint8_t nd_opt_rh_len;
uint16_t nd_opt_rh_reserved1;
uint32_t nd_opt_rh_reserved2;
/* followed by IP header and data */
};
struct nd_opt_mtu { /* MTU option */
uint8_t nd_opt_mtu_type;
uint8_t nd_opt_mtu_len;
uint16_t nd_opt_mtu_reserved;
uint32_t nd_opt_mtu_mtu;
};
We note that the nd_na_flags_reserved flags have the same byte
ordering problems as we discussed with ip6f_offlg.
2.2.3. Multicast Listener Discovery Type and Code Values
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The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
struct mld_hdr {
struct icmp6_hdr mld_hdr;
struct in6_addr mld_addr; /* multicast address */
};
#define mld_type mld_hdr.icmp6_type
#define mld_code mld_hdr.icmp6_code
#define mld_cksum mld_hdr.icmp6_cksum
#define mld_maxdelay mld_hdr.icmp6_data16[0]
#define mld_reserved mld_hdr.icmp6_data16[1]
2.2.4. ICMPv6 Router Renumbering Type and Code Values
The following structures and definitions are defined as a result of
including <netinet/icmp6.h>.
struct icmp6_router_renum { /* router renumbering header */
struct icmp6_hdr rr_hdr;
u_int8_t rr_segnum;
u_int8_t rr_flags;
u_int16_t rr_maxdelay;
u_int32_t rr_reserved;
};
#define rr_type rr_hdr.icmp6_type
#define rr_code rr_hdr.icmp6_code
#define rr_cksum rr_hdr.icmp6_cksum
#define rr_seqnum rr_hdr.icmp6_data32[0]
/* Router renumbering flags */
#define ICMP6_RR_FLAGS_TEST 0x80
#define ICMP6_RR_FLAGS_REQRESULT 0x40
#define ICMP6_RR_FLAGS_FORCEAPPLY 0x20
#define ICMP6_RR_FLAGS_SPECSITE 0x10
#define ICMP6_RR_FLAGS_PREVDONE 0x08
struct rr_pco_match { /* match prefix part */
u_int8_t rpm_code;
u_int8_t rpm_len;
u_int8_t rpm_ordinal;
u_int8_t rpm_matchlen;
u_int8_t rpm_minlen;
u_int8_t rpm_maxlen;
u_int16_t rpm_reserved;
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struct in6_addr rpm_prefix;
};
/* PCI code values */
#define RPM_PCO_ADD 1
#define RPM_PCO_CHANGE 2
#define RPM_PCO_SETGLOBAL 3
struct rr_pco_use { /* use prefix part */
u_int8_t rpu_uselen;
u_int8_t rpu_keeplen;
u_int8_t rpu_ramask;
u_int8_t rpu_raflags;
u_int32_t rpu_vltime;
u_int32_t rpu_pltime;
u_int32_t rpu_flags;
struct in6_addr rpu_prefix;
};
#define ICMP6_RR_PCOUSE_RAFLAGS_ONLINK 0x20
#define ICMP6_RR_PCOUSE_RAFLAGS_AUTO 0x10
#if BYTE_ORDER == BIG_ENDIAN
#define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80000000
#define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40000000
#elif BYTE_ORDER == LITTLE_ENDIAN
#define ICMP6_RR_PCOUSE_FLAGS_DECRVLTIME 0x80
#define ICMP6_RR_PCOUSE_FLAGS_DECRPLTIME 0x40
#endif
struct rr_result { /* router renumbering result message */
u_int16_t rrr_flags;
u_int8_t rrr_ordinal;
u_int8_t rrr_matchedlen;
u_int32_t rrr_ifid;
struct in6_addr rrr_prefix;
};
#if BYTE_ORDER == BIG_ENDIAN
#define ICMP6_RR_RESULT_FLAGS_OOB 0x0002
#define ICMP6_RR_RESULT_FLAGS_FORBIDDEN 0x0001
#elif BYTE_ORDER == LITTLE_ENDIAN
#define ICMP6_RR_RESULT_FLAGS_OOB 0x0200
#define ICMP6_RR_RESULT_FLAGS_FORBIDDEN 0x0100
#endif
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2.3. Address Testing Macros
The basic API ([RFC-2553]) defines some macros for testing an IPv6
address for certain properties. This API extends those definitions
with additional address testing macros, defined as a result of
including <netinet/in.h>.
int IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
const struct in6_addr *);
This macro returns non-zero if the addresses are equal; otherwise it
returns zero.
2.4. Protocols File
Many hosts provide the file /etc/protocols that contains the names of
the various IP protocols and their protocol number (e.g., the value
of the protocol field in the IPv4 header for that protocol, such as 1
for ICMP). Some programs then call the function getprotobyname() to
obtain the protocol value that is then specified as the third
argument to the socket() function. For example, the Ping program
contains code of the form
struct protoent *proto;
proto = getprotobyname("icmp");
s = socket(PF_INET, SOCK_RAW, proto->p_proto);
Common names are required for the new IPv6 protocols in this file, to
provide portability of applications that call the getprotoXXX()
functions.
We define the following protocol names with the values shown. These
are taken from ftp://ftp.isi.edu/in-notes/iana/assignments/protocol-
numbers.
hopopt 0 # hop-by-hop options for ipv6
ipv6 41 # ipv6
ipv6-route 43 # routing header for ipv6
ipv6-frag 44 # fragment header for ipv6
esp 50 # encapsulating security payload for ipv6
ah 51 # authentication header for ipv6
ipv6-icmp 58 # icmp for ipv6
ipv6-nonxt 59 # no next header for ipv6
ipv6-opts 60 # destination options for ipv6
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3. IPv6 Raw Sockets
Raw sockets bypass the transport layer (TCP or UDP). With IPv4, raw
sockets are used to access ICMPv4, IGMPv4, and to read and write IPv4
datagrams containing a protocol field that the kernel does not
process. An example of the latter is a routing daemon for OSPF,
since it uses IPv4 protocol field 89. With IPv6 raw sockets will be
used for ICMPv6 and to read and write IPv6 datagrams containing a
Next Header field that the kernel does not process. Examples of the
latter are a routing daemon for OSPF for IPv6 and RSVP (protocol
field 46).
All data sent via raw sockets MUST be in network byte order and all
data received via raw sockets will be in network byte order. This
differs from the IPv4 raw sockets, which did not specify a byte
ordering and used the host's byte order for certain IP header fields.
Another difference from IPv4 raw sockets is that complete packets
(that is, IPv6 packets with extension headers) cannot be sent or
received using the IPv6 raw sockets API. Instead, ancillary data
objects are used to transfer the extension headers and hoplimit
information, as described in Section 6. Should an application need
access to the complete IPv6 packet, some other technique, such as the
datalink interfaces BPF or DLPI, must be used.
All fields in the IPv6 header that an application might want to
change (i.e., everything other than the version number) can be
modified using ancillary data and/or socket options by the
application for output. All fields in a received IPv6 header (other
than the version number and Next Header fields) and all extension
headers are also made available to the application as ancillary data
on input. Hence there is no need for a socket option similar to the
IPv4 IP_HDRINCL socket option and on receipt the application will
only receive the payload i.e. the data after the IPv6 header and all
the extension headers.
When writing to a raw socket the kernel will automatically fragment
the packet if its size exceeds the path MTU, inserting the required
fragmentation headers. On input the kernel reassembles received
fragments, so the reader of a raw socket never sees any fragment
headers.
When we say "an ICMPv6 raw socket" we mean a socket created by
calling the socket function with the three arguments PF_INET6,
SOCK_RAW, and IPPROTO_ICMPV6.
Most IPv4 implementations give special treatment to a raw socket
created with a third argument to socket() of IPPROTO_RAW, whose value
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INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
is normally 255, to have it mean that the application will send down
complete packets including the IPv4 header. (Note: This feature was
added to IPv4 in 1988 by Van Jacobson to support traceroute, allowing
a complete IP header to be passed by the application, before the
IP_HDRINCL socket option was added.) We note that this value has no
special meaning to an IPv6 raw socket (and the IANA currently
reserves the value of 255 when used as a next-header field).
3.1. Checksums
The kernel will calculate and insert the ICMPv6 checksum for ICMPv6
raw sockets, since this checksum is mandatory.
For other raw IPv6 sockets (that is, for raw IPv6 sockets created
with a third argument other than IPPROTO_ICMPV6), the application
must set the new IPV6_CHECKSUM socket option to have the kernel (1)
compute and store a checksum for output, and (2) verify the received
checksum on input, discarding the packet if the checksum is in error.
This option prevents applications from having to perform source
address selection on the packets they send. The checksum will
incorporate the IPv6 pseudo-header, defined in Section 8.1 of
[RFC-2460]. This new socket option also specifies an integer offset
into the user data of where the checksum is located.
int offset = 2;
setsockopt(fd, IPPROTO_IPV6, IPV6_CHECKSUM, &offset, sizeof(offset));
By default, this socket option is disabled. Setting the offset to -1
also disables the option. By disabled we mean (1) the kernel will
not calculate and store a checksum for outgoing packets, and (2) the
kernel will not verify a checksum for received packets.
An attempt to set IPV6_CHECKSUM for an ICMPv6 socket will fail.
(Note: Since the checksum is always calculated by the kernel for an
ICMPv6 socket, applications are not able to generate ICMPv6 packets
with incorrect checksums (presumably for testing purposes) using this
API.)
3.2. ICMPv6 Type Filtering
ICMPv4 raw sockets receive most ICMPv4 messages received by the
kernel. (We say "most" and not "all" because Berkeley-derived
kernels never pass echo requests, timestamp requests, or address mask
requests to a raw socket. Instead these three messages are processed
entirely by the kernel.) But ICMPv6 is a superset of ICMPv4, also
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including the functionality of IGMPv4 and ARPv4. This means that an
ICMPv6 raw socket can potentially receive many more messages than
would be received with an ICMPv4 raw socket: ICMP messages similar to
ICMPv4, along with neighbor solicitations, neighbor advertisements,
and the three multicast listener discovery messages.
Most applications using an ICMPv6 raw socket care about only a small
subset of the ICMPv6 message types. To transfer extraneous ICMPv6
messages from the kernel to user can incur a significant overhead.
Therefore this API includes a method of filtering ICMPv6 messages by
the ICMPv6 type field.
Each ICMPv6 raw socket has an associated filter whose datatype is
defined as
struct icmp6_filter;
This structure, along with the macros and constants defined later in
this section, are defined as a result of including the
<netinet/icmp6.h>.
The current filter is fetched and stored using getsockopt() and
setsockopt() with a level of IPPROTO_ICMPV6 and an option name of
ICMP6_FILTER.
Six macros operate on an icmp6_filter structure:
void ICMP6_FILTER_SETPASSALL (struct icmp6_filter *);
void ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);
void ICMP6_FILTER_SETPASS ( int, struct icmp6_filter *);
void ICMP6_FILTER_SETBLOCK( int, struct icmp6_filter *);
int ICMP6_FILTER_WILLPASS (int,
const struct icmp6_filter *);
int ICMP6_FILTER_WILLBLOCK(int,
const struct icmp6_filter *);
The first argument to the last four macros (an integer) is an ICMPv6
message type, between 0 and 255. The pointer argument to all six
macros is a pointer to a filter that is modified by the first four
macros examined by the last two macros.
The first two macros, SETPASSALL and SETBLOCKALL, let us specify that
all ICMPv6 messages are passed to the application or that all ICMPv6
messages are blocked from being passed to the application.
The next two macros, SETPASS and SETBLOCK, let us specify that
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messages of a given ICMPv6 type should be passed to the application
or not passed to the application (blocked).
The final two macros, WILLPASS and WILLBLOCK, return true or false
depending whether the specified message type is passed to the
application or blocked from being passed to the application by the
filter pointed to by the second argument.
When an ICMPv6 raw socket is created, it will by default pass all
ICMPv6 message types to the application.
As an example, a program that wants to receive only router
advertisements could execute the following:
struct icmp6_filter myfilt;
fd = socket(PF_INET6, SOCK_RAW, IPPROTO_ICMPV6);
ICMP6_FILTER_SETBLOCKALL(&myfilt);
ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &myfilt);
setsockopt(fd, IPPROTO_ICMPV6, ICMP6_FILTER, &myfilt, sizeof(myfilt));
The filter structure is declared and then initialized to block all
messages types. The filter structure is then changed to allow router
advertisement messages to be passed to the application and the filter
is installed using setsockopt().
The icmp6_filter structure is similar to the fd_set datatype used
with the select() function in the sockets API. The icmp6_filter
structure is an opaque datatype and the application should not care
how it is implemented. All the application does with this datatype
is allocate a variable of this type, pass a pointer to a variable of
this type to getsockopt() and setsockopt(), and operate on a variable
of this type using the six macros that we just defined.
Nevertheless, it is worth showing a simple implementation of this
datatype and the six macros.
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INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
struct icmp6_filter {
uint32_t icmp6_filt[8]; /* 8*32 = 256 bits */
};
#define ICMP6_FILTER_WILLPASS(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) & (1 << ((type) & 31))) != 0)
#define ICMP6_FILTER_WILLBLOCK(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) & (1 << ((type) & 31))) == 0)
#define ICMP6_FILTER_SETPASS(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) |= (1 << ((type) & 31))))
#define ICMP6_FILTER_SETBLOCK(type, filterp) \
((((filterp)->icmp6_filt[(type) >> 5]) &= ~(1 << ((type) & 31))))
#define ICMP6_FILTER_SETPASSALL(filterp) \
memset((filterp), 0xFF, sizeof(struct icmp6_filter))
#define ICMP6_FILTER_SETBLOCKALL(filterp) \
memset((filterp), 0, sizeof(struct icmp6_filter))
(Note: These sample definitions have two limitations that an
implementation may want to change. The first four macros evaluate
their first argument two times. The second two macros require the
inclusion of the <string.h> header for the memset() function.)
4. Access to IPv6 and Extension Headers
Applications need to be able to control IPv6 header and extension
header content when sending as well as being able to receive the
content of these headers. This is done by defining socket option
types which can be used both with setsockopt and with ancillary data.
Ancillary data is discussed in Appendix A. The following optional
information can be exchanged between the application and the kernel:
1. The send/receive interface and source/destination address,
2. The hop limit,
3. Next hop address,
4. Routing header.
5. Hop-by-Hop options, and
6. Destination options (both before and after a Routing header).
First, to receive any of this optional information (other than the
next hop address, which can only be set), the application must call
setsockopt() to turn on the corresponding flag:
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int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS, &on, sizeof(on));
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDRDSTOPTS,
&on, sizeof(on));
When any of these options are enabled, the corresponding data is
returned as control information by recvmsg(), as one or more
ancillary data objects.
Two different mechanisms exist for sending this optional information:
1. Using setsockopt to specify the option content for a socket.
These are known an "sticky" options since they affect all
transmitted packets on the socket until either the a new
setsockopt is done or the options are overridden using ancillary
data.
2. Using ancillary data to specify the option content for a single
datagram. This only applies to datagram and raw sockets; not to
TCP sockets.
The three socket option parameters and the three cmsghdr fields that
describe the options/ancillary data objects are summarized as:
opt level/ optname/ optval/
cmsg_level cmsg_type cmsg_data[]
------------ ------------ ------------------------
IPPROTO_IPV6 IPV6_PKTINFO in6_pktinfo structure
IPPROTO_IPV6 IPV6_HOPLIMIT int
IPPROTO_IPV6 IPV6_NEXTHOP socket address structure
IPPROTO_IPV6 IPV6_RTHDR ip6_rthdr structure
IPPROTO_IPV6 IPV6_HOPOPTS ip6_hbh structure
IPPROTO_IPV6 IPV6_DSTOPTS ip6_dest structure
IPPROTO_IPV6 IPV6_RTHDRDSTOPTS ip6_dest structure
All these options are described in detail in Section 6, 7, 8 and 9.
All the constants beginning with IPV6_ are defined as a result of
including the <netinet/in.h>.
Issuing getsockopt() for the above options will return the sticky
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option value i.e. the value set with setsockopt().
Note: We intentionally use the same constant for the cmsg_level
member as is used as the second argument to getsockopt() and
setsockopt() (what is called the "level"), and the same constant for
the cmsg_type member as is used as the third argument to getsockopt()
and setsockopt() (what is called the "option name").
The application does not explicitly need to access the data
structures for the Routing header option, Hop-by-Hop option, and
Destination options, since the API to these features is through a set
of inet6_rth_XXX() and inet6_opt_XXX() functions that we define in
Section 8 and Section 10. Those functions simplify the interface to
these features instead of requiring the application to know the
intimate details of the extension header formats.
4.1. TCP Implications
It is not possible to use ancillary data to transmit the above
options for TCP since there is not a one-to-one mapping between send
operations and the TCP segments being transmitted. Instead an
application can use setsockopt to specify them as sticky options.
When the application uses setsockopt to specify the above options it
is expected that TCP will start using the new information when
sending segments. However, TCP may or may not use the new
information when retransmitting segments that were originally sent
when the old sticky options were in effect.
Applications using TCP can use ancillary data (after enabling the
desired IPV6_RECVxxx options) to receive the IPv6 and extension
header information. However, since there is not a one-to-one mapping
between received TCP segments and recv operations seen by the
application, when different TCP segments have different IPv6 and
extension headers the application might not be able to observe all
received headers. For efficiency reasons it is recommended that a
TCP implementation not send ancillary data items with every received
segment but instead try to detect the points in the data stream when
the requested IPv6 and extension header content changes and only send
a single ancillary data item at the time of the change. Also, TCP
should send ancillary data items at the start of the connection and
when the application enables a new IPV6_RECVxxx option.
For example, assume an application has enabled IPV6_RECVHOPLIMIT
before a connection is established. Then the first recvmsg() would
have an IPV6_HOPLIMIT item indicating the hop limit in the first data
segment. Should the hoplimit in the received data segment change a
subsequent recvmsg() will also have an IPV6_HOPLIMIT item. However,
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the application must be prepared to handle ancillary data items even
though the hop limit did not change. Note that should the hop limit
in received ACK-only segments be different than the hop limit in data
segments the application might only be able to observe the hop limit
in the received data segments.
The above example was for hop limit but the application should be
prepared to handle the corresponding behavior for the other option
information.
The above recvmsg() behavior allows the application to detect changes
in the received IPv6 and extension headers without resorting to
periodic getsockopt() calls.
4.2. UDP and Raw Socket Implications
The receive behavior for UDP and raw sockets is quite
straightforward. After the application has enabled an IPV6_RECVxxx
socket option it will receive ancillary data items for every
recvmsg() call containing the requested information. However, if the
information is not present in the packet the ancillary data item will
not be included. For example, if the application enables
IPV6_RECVRTHDR and a received datagram does not contain a Routing
header there will not be an IPV6_RTHDR ancillary data item. Note
that due to buffering in the socket implementation there might be
some packets queued when an IPV6_RECVxxx option is enabled and they
might not have the ancillary data information.
For sending the application has the choice between using sticky
options and ancillary data. The application can also use both having
the sticky options specify the "default" and using ancillary data to
override the default options. Note that if any ancillary data is
specified in a call to sendmsg(), all of the sticky options are
overridden for that datagram. For example, if the application has
set IPV6_RTHDR using a sticky option and later passes IPV6_HOPLIMIT
as ancillary data this will override the IPV6_RTHDR sticky option and
no Routing header will be sent with that datagram.
5. Extensions to Socket Ancillary Data
This specification uses ancillary data as defined in Posix.1g which
the following compatible extensions:
- CMSG_NEXTHDR has been extended to handle a NULL 2nd argument to
mean "get the first header". See Section 22.3.2.
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INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
- A new CMSG_SPACE macro is defined. It is used to determine how
much space need to be allocated for an ancillary data item. See
Section 22.3.4.
- A new CMSG_LEN macro is defined. It returns the value to store
in the cmsg_len member of the cmsghdr structure, taking into
account any padding needed to satisfy alignment requirements.
See Section 22.3.5.
6. Packet Information
There are four pieces of information that an application can specify
for an outgoing packet using ancillary data:
1. the source IPv6 address,
2. the outgoing interface index,
3. the outgoing hop limit, and
4. the next hop address.
Three similar pieces of information can be returned for a received
packet as ancillary data:
1. the destination IPv6 address,
2. the arriving interface index, and
3. the arriving hop limit.
The first two pieces of information are contained in an in6_pktinfo
structure that is set with setsockopt() or sent as ancillary data
with sendmsg() and received as ancillary data with recvmsg(). This
structure is defined as a result of including the <netinet/in.h>.
struct in6_pktinfo {
struct in6_addr ipi6_addr; /* src/dst IPv6 address */
unsigned int ipi6_ifindex; /* send/recv interface index */
};
In the socket option and cmsghdr level will be IPPROTO_IPV6, the type
will be IPV6_PKTINFO, and the first byte of the option value and
cmsg_data[] will be the first byte of the in6_pktinfo structure. An
application can clear any sticky IPV6_PKTINFO option by either doing
a setsockopt for option with optlen being zero, or by doing a
"regular" setsockopt with ipi6_addr being in6addr_any and
ipi6_ifindex being zero.
This information is returned as ancillary data by recvmsg() only if
the application has enabled the IPV6_RECVPKTINFO socket option:
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int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPKTINFO, &on, sizeof(on));
(Note: The hop limit is not contained in the in6_pktinfo structure
for the following reason. Some UDP servers want to respond to client
requests by sending their reply out the same interface on which the
request was received and with the source IPv6 address of the reply
equal to the destination IPv6 address of the request. To do this the
application can enable just the IPV6_RECVPKTINFO socket option and
then use the received control information from recvmsg() as the
outgoing control information for sendmsg(). The application need not
examine or modify the in6_pktinfo structure at all. But if the hop
limit were contained in this structure, the application would have to
parse the received control information and change the hop limit
member, since the received hop limit is not the desired value for an
outgoing packet.)
6.1. Specifying/Receiving the Interface
Interfaces on an IPv6 node are identified by a small positive
integer, as described in Section 4 of [RFC-2553]. That document also
describes a function to map an interface name to its interface index,
a function to map an interface index to its interface name, and a
function to return all the interface names and indexes. Notice from
this document that no interface is ever assigned an index of 0.
When specifying the outgoing interface, if the ipi6_ifindex value is
0, the kernel will choose the outgoing interface. If the application
specifies an outgoing interface for a multicast packet, the interface
specified by the ancillary data overrides any interface specified by
the IPV6_MULTICAST_IF socket option (described in [RFC-2553]), for
that call to sendmsg() only.
When the IPV6_PKTINFO socket option is enabled, the received
interface index is always returned as the ipi6_ifindex member of the
in6_pktinfo structure.
6.2. Specifying/Receiving Source/Destination Address
The source IPv6 address can be specified by calling bind() before
each output operation, but supplying the source address together with
the data requires less overhead (i.e., fewer system calls) and
requires less state to be stored and protected in a multithreaded
application.
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When specifying the source IPv6 address as ancillary data, if the
ipi6_addr member of the in6_pktinfo structure is the unspecified
address (IN6ADDR_ANY_INIT or in6addr_any), then (a) if an address is
currently bound to the socket, it is used as the source address, or
(b) if no address is currently bound to the socket, the kernel will
choose the source address. If the ipi6_addr member is not the
unspecified address, but the socket has already bound a source
address, then the ipi6_addr value overrides the already-bound source
address for this output operation only.
The kernel must verify that the requested source address is indeed a
unicast address assigned to the node.
When the in6_pktinfo structure is returned as ancillary data by
recvmsg(), the ipi6_addr member contains the destination IPv6 address
from the received packet.
6.3. Specifying/Receiving the Hop Limit
The outgoing hop limit is normally specified with either the
IPV6_UNICAST_HOPS socket option or the IPV6_MULTICAST_HOPS socket
option, both of which are described in [RFC-2553]. Specifying the
hop limit as ancillary data lets the application override either the
kernel's default or a previously specified value, for either a
unicast destination or a multicast destination, for a single output
operation. Returning the received hop limit is useful for programs
such as Traceroute and for IPv6 applications that need to verify that
the received hop limit is 255 (e.g., that the packet has not been
forwarded).
The received hop limit is returned as ancillary data by recvmsg()
only if the application has enabled the IPV6_RECVHOPLIMIT socket
option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &on, sizeof(on));
In the cmsghdr structure containing this ancillary data, the
cmsg_level member will be IPPROTO_IPV6, the cmsg_type member will be
IPV6_HOPLIMIT, and the first byte of cmsg_data[] will be the first
byte of the integer hop limit.
Nothing special need be done to specify the outgoing hop limit: just
specify the control information as ancillary data for sendmsg() or
using setsockopt(). As specified in [RFC-2553], the interpretation
of the integer hop limit value is
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x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
6.4. Specifying the Next Hop Address
The IPV6_NEXTHOP ancillary data object specifies the next hop for the
datagram as a socket address structure. In the cmsghdr structure
containing this ancillary data, the cmsg_level member will be
IPPROTO_IPV6, the cmsg_type member will be IPV6_NEXTHOP, and the
first byte of cmsg_data[] will be the first byte of the socket
address structure.
This is a privileged option. (Note: It is implementation defined and
beyond the scope of this document to define what "privileged" means.
Unix systems use this term to mean the process must have an effective
user ID of 0.)
If the socket address structure contains an IPv6 address (e.g., the
sin6_family member is AF_INET6), then the node identified by that
address must be a neighbor of the sending host. If that address
equals the destination IPv6 address of the datagram, then this is
equivalent to the existing SO_DONTROUTE socket option.
6.5. Additional Errors with sendmsg() and setsockopt()
With the IPV6_PKTINFO socket option there are no additional errors
possible with the call to recvmsg(). But when specifying the
outgoing interface or the source address, additional errors are
possible from sendmsg() or setsockopt(). Note that some
implementations might only be able to return this type of errors for
setsockopt(). The following are examples, but some of these may not
be provided by some implementations, and some implementations may
define additional errors:
ENXIO The interface specified by ipi6_ifindex does not exist.
ENETDOWN The interface specified by ipi6_ifindex is not enabled
for IPv6 use.
EADDRNOTAVAIL ipi6_ifindex specifies an interface but the address
ipi6_addr is not available for use on that interface.
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EHOSTUNREACH No route to the destination exists over the interface
specified by ifi6_ifindex.
7. Routing Header Option
Source routing in IPv6 is accomplished by specifying a Routing header
as an extension header. There can be different types of Routing
headers, but IPv6 currently defines only the Type 0 Routing header
[RFC-2460]. This type supports up to 127 intermediate nodes (limited
by the length field in the extension header). With this maximum
number of intermediate nodes, a source, and a destination, there are
128 hops.
Source routing with IPv4 sockets API (the IP_OPTIONS socket option)
requires the application to build the source route in the format that
appears as the IPv4 header option, requiring intimate knowledge of
the IPv4 options format. This IPv6 API, however, defines eight
functions that the application calls to build and examine a Routing
header, and the ability to use sticky options or ancillary data to
communicate this information between the application and the kernel
using the IPV6_RTHDR option.
Three functions build a Routing header:
inet6_rth_space() - return #bytes required for Routing header
inet6_rth_init() - initialize buffer data for Routing header
inet6_rth_add() - add one IPv6 address to the Routing header
Three functions deal with a returned Routing header:
inet6_rth_reverse() - reverse a Routing header
inet6_rth_segments() - return #segments in a Routing header
inet6_rth_getaddr() - fetch one address from a Routing header
The function prototypes for these functions are defined as a result
of including the <netinet/in.h>.
To receive a Routing header the application must enable the
IPV6_RECVRTHDR socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDR, &on, sizeof(on));
To send a Routing header the application specifies it either as
ancillary data in a call to sendmsg() or using setsockopt().
The application can remove any sticky Routing header by calling
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setsockopt() for IPV6_RTHDR with a zero option length.
When using ancillary data a Routing header is passed between the
application and the kernel as follows: The cmsg_level member has a
value of IPPROTO_IPV6 and the cmsg_type member has a value of
IPV6_RTHDR. The contents of the cmsg_data[] member is implementation
dependent and should not be accessed directly by the application, but
should be accessed using the six functions that we are about to
describe.
The following constant is defined as a result of including the
<netinet/in.h>:
#define IPV6_RTHDR_TYPE_0 0 /* IPv6 Routing header type 0 */
When a Routing header is specified, the destination address specified
for connect(), sendto(), or sendmsg() is the final destination
address of the datagram. The Routing header then contains the
addresses of all the intermediate nodes.
7.1. inet6_rth_space
size_t inet6_rth_space(int type, int segments);
This function returns the number of bytes required to hold a Routing
header of the specified type containing the specified number of
segments (addresses). For an IPv6 Type 0 Routing header, the number
of segments must be between 0 and 127, inclusive. The return value
is just the space for the Routing header. When the application uses
ancillary data it must pass the returned length to CMSG_LEN() to
determine how much memory is needed for the ancillary data object
(including the cmsghdr structure).
If the return value is 0, then either the type of the Routing header
is not supported by this implementation or the number of segments is
invalid for this type of Routing header.
(Note: This function returns the size but does not allocate the space
required for the ancillary data. This allows an application to
allocate a larger buffer, if other ancillary data objects are
desired, since all the ancillary data objects must be specified to
sendmsg() as a single msg_control buffer.)
7.2. inet6_rth_init
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void *inet6_rth_init(void *bp, int bp_len, int type, int segments);
This function initializes the buffer pointed to by bp to contain a
Routing header of the specified type and sets ip6r0_len based on the
segments parameter. bp_len is only used to verify that the buffer is
large enough. The ip6r0_segleft field is set to zero;
inet6_rth_add() will increment it.
When the application uses ancillary data the application must
initialize any cmsghdr fields.
The caller must allocate the buffer and its size can be determined by
calling inet6_rth_space().
Upon success the return value is the pointer to the buffer (bp), and
this is then used as the first argument to the next two functions.
Upon an error the return value is NULL.
7.3. inet6_rth_add
int inet6_rth_add(void *bp, const struct in6_addr *addr);
This function adds the IPv6 address pointed to by addr to the end of
the Routing header being constructed.
If successful, the segleft member of the Routing Header is updated to
account for the new address in the Routing header and the return
value of the function is 0. Upon an error the return value of the
function is -1.
7.4. inet6_rth_reverse
int inet6_rth_reverse(const void *in, void *out)
This function takes a Routing header extension header (pointed to by
the first argument) and writes a new Routing header that sends
datagrams along the reverse of that route. Both arguments are
allowed to point to the same buffer (that is, the reversal can occur
in place).
The return value of the function is 0 on success, or -1 upon an
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error.
7.5. inet6_rth_segments
int inet6_rth_segments(const void *bp);
This function returns the number of segments (addresses) contained in
the Routing header described by bp. On success the return value is
zero or greater. The return value of the function is -1 upon an
error.
7.6. inet6_rth_getaddr
struct in6_addr *inet6_rth_getaddr(const void *bp, int index);
This function returns a pointer to the IPv6 address specified by
index (which must have a value between 0 and one less than the value
returned by inet6_rth_segments()) in the Routing header described by
bp. An application should first call inet6_rth_segments() to obtain
the number of segments in the Routing header.
Upon an error the return value of the function is NULL.
8. Hop-By-Hop Options
A variable number of Hop-by-Hop options can appear in a single Hop-
by-Hop options header. Each option in the header is TLV-encoded with
a type, length, and value.
Today only four Hop-by-Hop options are defined for IPv6 [RFC-2460]:
Jumbo Payload, Pad1, PadN, and router-alert. The two pad options are
for alignment purposes and are automatically inserted by the
inet6_opt_XXX() routines and ignored by the inet6_opt_XXX() routines
on the receive side.
This section of the API is therefore defined for future Hop-by-Hop
options (as well as destination options) that an application may need
to specify and receive. This IPv6 API defines seven functions that
the application calls to build and examine a Hop-by_Hop options
header, and the ability to use sticky options or ancillary data to
communicate this information between the application and the kernel.
This uses the IPV6_HOPOPTS for hob-by-hop options.
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Four functions build a options header:
inet6_opt_init() - initialize buffer data for options header
inet6_opt_append() - add one TLV option to the options header
inet6_opt_finish() - finish adding TLV options to the options header
inet6_opt_set_val() - add one component of the option content to the option
Three functions deal with a returned options header:
inet6_opt_next() - extract the next option from the options header
inet6_opt_find() - extract an option of a specified type from the header
inet6_opt_get_val() - retrieve one component of the option content
Individual Hop-by-Hop options (and Destination options, which are
described in Section 9 and are very similar to the Hop-by-Hop
options) may have specific alignment requirements. For example, the
4-byte Jumbo Payload length should appear on a 4-byte boundary, and
IPv6 addresses are normally aligned on an 8-byte boundary. These
requirements and the terminology used with these options are
discussed in Section 4.2 and Appendix B of [RFC-2460]. The alignment
of first byte of each option is specified by two values, called x and
y, written as "xn + y". This states that the option must appear at
an integer multiple of x bytes from the beginning of the options
header (x can have the values 1, 2, 4, or 8), plus y bytes (y can
have a value between 0 and 7, inclusive). The Pad1 and PadN options
are inserted as needed to maintain the required alignment. The
functions below need to know the alignment of the end of the option
(which is always in the form "xn," where x can have the values 1, 2,
4, or 8) and the total size of the data portion of the option. These
are passed as the "align" and "len" arguments to inet6_opt_append().
Multiple Hop-by-Hop options must be specified by the application by
placing them in a single extension header.
Finally, we note that use of some Hop-by-Hop options or some
Destination options, might require special privilege. That is,
normal applications (without special privilege) might be forbidden
from setting certain options in outgoing packets, and might never see
certain options in received packets.
8.1. Receiving Hop-by-Hop Options
To receive Hop-by-Hop options the application must enable the
IPV6_RECVHOPOPTS socket option:
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int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPOPTS, &on, sizeof(on));
When using ancillary data a Hop-by-hop options is passed between the
application and the kernel as follows: The cmsg_level member will be
IPPROTO_IPV6 and the cmsg_type member will be IPV6_HOPOPTS. These
options are then processed by calling the inet6_opt_next(),
inet6_opt_find(), and inet6_opt_get_val() functions, described in
Section 10.6.
8.2. Sending Hop-by-Hop Options
To send a Hop-by-Hop options header, the application specifies the
header either as ancillary data in a call to sendmsg() or using
setsockopt().
The application can remove any sticky Hop-by-Hop extension header by
calling setsockopt() for IPV6_HOPOPTS with a zero option length.
All the Hop-by-Hop options must specified by a single ancillary data
object. The cmsg_level member is set to IPPROTO_IPV6 and the
cmsg_type member is set to IPV6_HOPOPTS. The option is normally
constructed using the inet6_opt_init(), inet6_opt_append(),
inet6_opt_finish(), and inet6_set_val() functions, described in
Section 10.
Additional errors may be possible from sendmsg() and setsockopt() if
the specified option is in error.
9. Destination Options
A variable number of Destination options can appear in one or more
Destination option headers. As defined in [RFC-2460], a Destination
options header appearing before a Routing header is processed by the
first destination plus any subsequent destinations specified in the
Routing header, while a Destination options header appearing after a
Routing header is processed only by the final destination. As with
the Hop-by-Hop options, each option in a Destination options header
is TLV-encoded with a type, length, and value.
Today no Destination options are defined for IPv6 [RFC-2460],
although proposals exist to use Destination options with Mobile IPv6.
9.1. Receiving Destination Options
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To receive Destination options appearing after a Routing header (or
in a packet without a Routing header) the application must enable the
IPV6_RECVDSTOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVDSTOPTS, &on, sizeof(on));
To receive Destination options appearing before a Routing header the
application must enable the IPV6_RECVRTHDRDSTOPTS socket option:
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVRTHDRDSTOPTS,
&on, sizeof(on));
All the Destination options appearing before a Routing header are
returned as one ancillary data object described by a cmsghdr
structure (with cmsg_type set to IPV6_RTHDRDSTOPTS) and all the
Destination options appearing after a Routing header (or in a packet
without a Routing header) are returned as another ancillary data
object described by a cmsghdr structure (with cmsg_type set to
IPV6_DSTOPTS). For all these ancillary data objects, the cmsg_level
member will be IPPROTO_IPV6.
These options are then processed by calling the inet6_opt_next(),
inet6_opt_find(), and inet6_opt_get_value() functions.
9.2. Sending Destination Options
To send a Destination options header, the application specifies it
either as ancillary data in a call to sendmsg() or using
setsockopt().
The application can remove any sticky Destination extension header by
calling setsockopt() for IPV6_RTHDRDSTOPTS/IPV6_DSTOPTS with a zero
option length.
As described in Section 6 one set of Destination options can appear
before a Routing header, and one set can appear after a Routing
header (or in a packet with no Routing header). Each set can consist
of one or more options but each set is a single extension header.
When using ancillary data a Destination options header is passed
between the application and the kernel as follows: The set preceding
a Routing header are specified with the cmsg_level member is set to
IPPROTO_IPV6 and the cmsg_type member is set to IPV6_RTHDRDSTOPTS.
Any setsockopt or ancillary data for IPV6_RTHDRDSTOPTS is silently
ignore when sending packets unless a Routing header is also
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specified.
The set of Destination options after a Routing header, which are also
used when no Routing header is present, are specified with the
cmsg_level member is set to IPPROTO_IPV6 and the cmsg_type member is
set to IPV6_DSTOPTS.
The Destination options are normally constructed using the
inet6_opt_init(), inet6_opt_append(), inet6_opt_finish(), and
inet6_set_val() functions, described in Section 10.
Additional errors may be possible from sendmsg() and setsockopt() if
the specified option is in error.
10. Hop-by-Hop and Destination Options Processing
Building and parsing the Hop-by-Hop and Destination options is
complicated for the reasons given earlier. We therefore define a set
of functions to help the application. These functions assume the
formatting rules specified in Appendix B in [RFC-2460] i.e. that the
largest field is placed last in the option.
The function prototypes for these functions are defined as a result
of including the <netinet/in.h>.
The first 3 functions (init, append, and finish) are used both to
calculate the needed buffer size for the options, and to actually
encode the options once the application has allocated a buffer for
the header. In order to only calculate the size the application must
pass a NULL extbuf and a zero extlen to those functions.
10.1. inet6_opt_init
int inet6_opt_init(void *extbuf, size_t extlen);
This function returns the number of bytes needed for the empty
extension header i.e. without any options. If extbuf is not NULL it
also initializes the extension header to have the correct length
field. If the extlen value is not a positive (i.e., non-zero)
multiple of 8 the function fails and returns -1.
10.2. inet6_opt_append
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int inet6_opt_append(void *extbuf, size_t extlen, int prevlen,
uint8_t type, size_t len, uint_t align,
void **databufp);
Prevlen should be the length returned by inet6_opt_init() or a
previous inet6_opt_append(). This function returns the updated total
length taking into account adding an option with length 'len' and
alignment 'align'. If extbuf is not NULL then, in addition to
returning the length, the function inserts any needed pad option,
initializes the option (setting the type and length fields) and
returns a pointer to the location for the option content in databufp.
If the option does not fit in the extension header buffer the
function returns -1.
type is the 8-bit option type. len is the length of the option data
(i.e. excluding the option type and option length fields).
Once inet6_opt_append() has been called the application can use the
databuf directly, or use inet6_opt_set_val() to specify the content
of the option.
The option type must have a value from 2 to 255, inclusive. (0 and 1
are reserved for the Pad1 and PadN options, respectively.)
The option data length must have a value between 0 and 255,
inclusive, and is the length of the option data that follows.
The align parameter must have a value of 1, 2, 4, or 8. The align
value can not exceed the value of len.
10.3. inet6_opt_finish
int inet6_opt_finish(void *extbuf, size_t extlen, int prevlen);
Prevlen should be the length returned by inet6_opt_init() or
inet6_opt_append(). This function returns the updated total length
taking into account the final padding of the extension header to make
it a multiple of 8 bytes. If extbuf is not NULL the function also
initializes the option by inserting a Pad1 or PadN option of the
proper length.
If the necessary pad does not fit in the extension header buffer the
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function returns -1.
10.4. inet6_opt_set_val
int inet6_opt_set_val(void *databuf, size_t offset, void *val,
int vallen);
Databuf should be a pointer returned by inet6_opt_append(). This
function inserts data items of various sizes (1, 2, 4, or 8 bytes) in
the data portion of the option. val should point to the data to be
inserted. Offset specifies where in the data portion of the option
the value should be inserted; the first byte after the option type
and length is accessed by specifying an offset of zero.
The function returns the offset for the next field (i.e., offset +
vallen) which can be used when composing option content with multiple
fields.
10.5. inet6_opt_next
int inet6_opt_next(void *extbuf, size_t extlen, int prevlen,
uint8_t *typep, size_t *lenp,
void **databufp);
This function parses received extension headers returning the next
option. Extbuf and extlen specifies the extension header. Prevlen
should either be zero (for the first option) or the length returned
by a previous call to inet6_opt_next() or inet6_opt_find(). It
specifies the position where to continue scanning the extension
buffer. The next option is returned by updating typep, lenp, and
databufp. This function returns the updated "previous" length
computed by advancing past the option that was returned. This
returned "previous" length can then be passed to subsequent calls to
inet6_opt_next(). This function does not return any PAD1 or PADN
options. When there are no more options the return value is -1.
10.6. inet6_opt_find
int inet6_opt_find(void *extbuf, size_t extlen, int prevlen,
uint8_t type, size_t *lenp,
void **databufp);
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This function is similar to the previously described inet6_opt_next()
function, except this function lets the caller specify the option
type to be searched for, instead of always returning the next option
in the extension header.
If an option of the specified type is located, the function returns
the updated "previous" total length computed by advancing past the
option that was returned and past any options that didn't match the
type. This returned "previous" length can then be passed to
subsequent calls to inet6_opt_find() for finding the next occurance
of the same option type.
If an option of the specified type is not located, the return value
is -1. If an error occurs, the return value is -1.
10.7. inet6_opt_get_val
int inet6_opt_get_val(void *databuf, size_t offset, void *val,
int vallen);
Databuf should be a pointer returned by inet6_opt_next() or
inet6_opt_find(). This function extracts data items of various sizes
(1, 2, 4, or 8 bytes) in the data portion of the option. val should
point to the destination for the extracted data. Offset specifies
from where in the data portion of the option the value should be
extracted; the first byte after the option type and length is
accessed by specifying an offset of zero.
The function returns the offset for the next field (i.e., offset +
vallen) which can be used when extracting option content with
multiple fields. XXX Perhaps we should add a note to point out that
robust receivers should verify alignment before calling
inet6_opt_get_val(). XXX Or check alignment and fail by returning
-1?
11. Additional Advanced API Functions
11.1. Sending with the Minimum MTU
Some applications might not want to incur the overhead of path MTU
discovery, especially if the applications only send a single datagram
to a destination. A potential example is a DNS server.
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This specification defines a mechanism to avoid fragmentation by
sending at the minimum IPv6 MTU (1280 bytes). This can be enabled
using the IPV6_USE_MIN_MTU socket option.
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_USE_MIN_MTU, &on, sizeof(on));
By default, this socket option is disabled. Setting the value to 0
also disables the option. This option can also be sent as ancillary
data.
11.2. Path MTU Discovery and UDP
UDP and raw socket applications need to be able to determine the
"maximum send transport-message size" (Section 5.1 of [RFC-1981]) to
a given destination so that those applications can participate in
path MTU discovery. This lets those applications send smaller
datagrams to the destination, avoiding fragmentation.
This is accomplished using a new ancillary data item (IPV6_PATHMTU)
which is delivered to recvmsg() without any actual data. The
application enable the receipt of IPV6_PATHMTU ancillary data items
by enabing IPV6_RECVPATHMTU.
int on = 1;
setsockopt(fd, IPPROTO_IPV6, IPV6_RECVPATHMTU, &on, sizeof(on));
By default, this socket option is disabled. Setting the value to 0
also disables the option.
When the application is sending packets too big for the path MTU
recvmsg will return zero (indicating no data) but there will be a
cmsghdr with cmsg_type set to IPV6_PATHMTU, and cmsg_len will
indicate that cmsg_data is 4 bytes long. CMSG_DATA will point to an
integer carrying the path MTU to use.
11.3. Neighbor Reachability and UDP
UDP and raw socket application might know that communication is
making forward progress i.e. that the path from the node to the next
hop is working. In such a case the applications, similarly to TCP as
specified in [RFC-2461], has the option indicate to the internet
layer that the neighbor is reachable. See section 7.3.1 of
[RFC-2461]. This could save unneeded neighbor solicitation and
neighbor advertisement messages.
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This is done by including an ancilary data item with cmsg_type being
IPV6_REACHCONF and with no attached CMSG_DATA.
12. Ordering of Ancillary Data and IPv6 Extension Headers
Three IPv6 extension headers can be specified by the application and
returned to the application using ancillary data with sendmsg() and
recvmsg(): the Routing header, Hop-by-Hop options, and Destination
options. When multiple ancillary data objects are transferred via
recvmsg() and these objects represent any of these three extension
headers, their placement in the control buffer is directly tied to
their location in the corresponding IPv6 datagram. This API imposes
some ordering constraints for using these ancillary data objects with
sendmsg().
All Hop-by-Hop options must be specified in a single ancillary data
object. Should multiple hop-by-hop ancillary data objects be
specified the implementation might choose an arbitrary one or drop
the packet.
All Destination options that precede a Routing header must be
specified in a single ancillary data object. If there is no Routing
header ancillary data object the IPV6_RTHDRDSTOPTS object will be
silently ignored.
All Destination options that follow a Routing header (or are used
without a Routing header) must be specified in a single ancillary
data object.
If Destination options are specified in the control buffer after a
Routing header, or if Destination options are specified without a
Routing header, the kernel will place those Destination options after
an authentication header and/or an encapsulating security payload
header, if present.
13. IPv6-Specific Options with IPv4-Mapped IPv6 Addresses
The various socket options and ancillary data specifications defined
in this document apply only to true IPv6 sockets. It is possible to
create an IPv6 socket that actually sends and receives IPv4 packets,
using IPv4-mapped IPv6 addresses, but the mapping of the options
defined in this document to an IPv4 datagram is beyond the scope of
this document.
In general, attempting to specify an IPv6-only option, such as the
Hop-by-Hop options, Destination options, or Routing header on an IPv6
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socket that is using IPv4-mapped IPv6 addresses, will probably result
in an error. Some implementations, however, may provide access to
the packet information (source/destination address, send/receive
interface, and hop limit) on an IPv6 socket that is using IPv4-mapped
IPv6 addresses.
14. Extended interfaces for rresvport, rcmd and rexec
TBD
14.1. rresvport_af
The rresvport() function is used by the rcmd() function, and this
function is in turn called by many of the "r" commands such as
rlogin. While new applications are not being written to use the
rcmd() function, legacy applications such as rlogin will continue to
use it and these will be ported to IPv6.
rresvport() creates an IPv4/TCP socket and binds a "reserved port" to
the socket. Instead of defining an IPv6 version of this function we
define a new function that takes an address family as its argument.
#include <unistd.h>
int rresvport_af(int *port, int family);
This function behaves the same as the existing rresvport() function,
but instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is either AF_INET or
AF_INET6, and a new error return is EAFNOSUPPORT if the address
family is not supported.
(Note: There is little consensus on which header defines the
rresvport() and rcmd() function prototypes. 4.4BSD defines it in
<unistd.h>, others in <netdb.h>, and others don't define the function
prototypes at all.)
14.2. rcmd_af
The existing rcmd() function can not transparently use AF_INET6
sockets since the an application would not be prepared to handle
AF_INET6 addresses returned by e.g. getpeername on the file
descriptor created by rcmd. Thus a new function is needed.
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int rcmd_af(char **ahost, unsigned short rport, const char *locuser,
const char *remuser, const char *cmd, int *fd2p, int af)
This function behaves the same as the existing rcmd() function, but
instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is either AF_INET or
AF_INET6, and a new error return is EAFNOSUPPORT if the address
family is not supported.
14.3. rexec_af
The existing rexec() function can not transparently use AF_INET6
sockets since the an application would not be prepared to handle
AF_INET6 addresses returned by e.g. getpeername on the file
descriptor created by rexec. Thus a new function is needed.
int rexec_af(char **ahost, unsigned short rport, const char *name,
const char *pass, const char *cmd, int *fd2p, int af)
This function behaves the same as the existing rexec() function, but
instead of creating an AF_INET TCP socket, it can also create an
AF_INET6 TCP socket. The family argument is either AF_INET or
AF_INET6, and a new error return is EAFNOSUPPORT if the address
family is not supported.
15. Summary of New Definitions
The following list summarizes the constants and structure,
definitions discussed in this memo, sorted by header.
<netinet/icmp6.h> ICMP6_DST_UNREACH
<netinet/icmp6.h> ICMP6_DST_UNREACH_ADDR
<netinet/icmp6.h> ICMP6_DST_UNREACH_ADMIN
<netinet/icmp6.h> ICMP6_DST_UNREACH_NOPORT
<netinet/icmp6.h> ICMP6_DST_UNREACH_NOROUTE
<netinet/icmp6.h> ICMP6_DST_UNREACH_BEYONDSCOPE
<netinet/icmp6.h> ICMP6_ECHO_REPLY
<netinet/icmp6.h> ICMP6_ECHO_REQUEST
<netinet/icmp6.h> ICMP6_INFOMSG_MASK
<netinet/icmp6.h> ICMP6_MEMBERSHIP_QUERY
<netinet/icmp6.h> ICMP6_MEMBERSHIP_REDUCTION
<netinet/icmp6.h> ICMP6_MEMBERSHIP_REPORT
<netinet/icmp6.h> ICMP6_PACKET_TOO_BIG
<netinet/icmp6.h> ICMP6_PARAMPROB_HEADER
<netinet/icmp6.h> ICMP6_PARAMPROB_NEXTHEADER
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<netinet/icmp6.h> ICMP6_PARAMPROB_OPTION
<netinet/icmp6.h> ICMP6_PARAM_PROB
<netinet/icmp6.h> ICMP6_TIME_EXCEEDED
<netinet/icmp6.h> ICMP6_TIME_EXCEED_REASSEMBLY
<netinet/icmp6.h> ICMP6_TIME_EXCEED_TRANSIT
<netinet/icmp6.h> ND_NA_FLAG_OVERRIDE
<netinet/icmp6.h> ND_NA_FLAG_ROUTER
<netinet/icmp6.h> ND_NA_FLAG_SOLICITED
<netinet/icmp6.h> ND_NEIGHBOR_ADVERT
<netinet/icmp6.h> ND_NEIGHBOR_SOLICIT
<netinet/icmp6.h> ND_OPT_MTU
<netinet/icmp6.h> ND_OPT_PI_FLAG_AUTO
<netinet/icmp6.h> ND_OPT_PI_FLAG_ONLINK
<netinet/icmp6.h> ND_OPT_PREFIX_INFORMATION
<netinet/icmp6.h> ND_OPT_REDIRECTED_HEADER
<netinet/icmp6.h> ND_OPT_SOURCE_LINKADDR
<netinet/icmp6.h> ND_OPT_TARGET_LINKADDR
<netinet/icmp6.h> ND_RA_FLAG_MANAGED
<netinet/icmp6.h> ND_RA_FLAG_OTHER
<netinet/icmp6.h> ND_REDIRECT
<netinet/icmp6.h> ND_ROUTER_ADVERT
<netinet/icmp6.h> ND_ROUTER_SOLICIT
<netinet/icmp6.h> struct icmp6_filter{};
<netinet/icmp6.h> struct icmp6_hdr{};
<netinet/icmp6.h> struct nd_neighbor_advert{};
<netinet/icmp6.h> struct nd_neighbor_solicit{};
<netinet/icmp6.h> struct nd_opt_hdr{};
<netinet/icmp6.h> struct nd_opt_mtu{};
<netinet/icmp6.h> struct nd_opt_prefix_info{};
<netinet/icmp6.h> struct nd_opt_rd_hdr{};
<netinet/icmp6.h> struct nd_redirect{};
<netinet/icmp6.h> struct nd_router_advert{};
<netinet/icmp6.h> struct nd_router_solicit{};
<netinet/in.h> IPPROTO_AH
<netinet/in.h> IPPROTO_DSTOPTS
<netinet/in.h> IPPROTO_ESP
<netinet/in.h> IPPROTO_FRAGMENT
<netinet/in.h> IPPROTO_HOPOPTS
<netinet/in.h> IPPROTO_ICMPV6
<netinet/in.h> IPPROTO_IPV6
<netinet/in.h> IPPROTO_NONE
<netinet/in.h> IPPROTO_ROUTING
<netinet/in.h> IPV6_RECVDSTOPTS
<netinet/in.h> IPV6_RECVHOPLIMIT
<netinet/in.h> IPV6_RECVHOPOPTS
<netinet/in.h> IPV6_RECVPKTINFO
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<netinet/in.h> IPV6_RECVRTHDR
<netinet/in.h> IPV6_RECVRTHDRDSTOPTS
<netinet/in.h> IPV6_DSTOPTS
<netinet/in.h> IPV6_HOPLIMIT
<netinet/in.h> IPV6_HOPOPTS
<netinet/in.h> IPV6_NEXTHOP
<netinet/in.h> IPV6_PKTINFO
<netinet/in.h> IPV6_RTHDR
<netinet/in.h> IPV6_RTHDRDSTOPTS
<netinet/in.h> IPV6_RTHDR_TYPE_0
<netinet/in.h> IPV6_USE_MIN_MTU
<netinet/in.h> IPV6_RECVPATHMTU
<netinet/in.h> IPV6_PATHMTU
<netinet/in.h> IPV6_REACHCONF
<netinet/in.h> struct in6_pktinfo{};
<netinet/ip6.h> IP6F_OFF_MASK
<netinet/ip6.h> IP6F_RESERVED_MASK
<netinet/ip6.h> IP6F_MORE_FRAG
<netinet/ip6.h> struct ip6_dest{};
<netinet/ip6.h> struct ip6_frag{};
<netinet/ip6.h> struct ip6_hbh{};
<netinet/ip6.h> struct ip6_hdr{};
<netinet/ip6.h> struct ip6_rthdr{};
<netinet/ip6.h> struct ip6_rthdr0{};
<sys/socket.h> struct cmsghdr{};
<sys/socket.h> struct msghdr{};
The following list summarizes the function and macro prototypes
discussed in this memo, sorted by header.
<netinet/icmp6.h> void ICMP6_FILTER_SETBLOCK(int, struct icmp6_filter *);
<netinet/icmp6.h> void ICMP6_FILTER_SETBLOCKALL(struct icmp6_filter *);
<netinet/icmp6.h> void ICMP6_FILTER_SETPASS(int, struct icmp6_filter *);
<netinet/icmp6.h> void ICMP6_FILTER_SETPASSALL(struct icmp6_filter *);
<netinet/icmp6.h> int ICMP6_FILTER_WILLBLOCK(int,
const struct icmp6_filter *);
<netinet/icmp6.h> int ICMP6_FILTER_WILLPASS(int,
const struct icmp6_filter *);
<netinet/in.h> int IN6_ARE_ADDR_EQUAL(const struct in6_addr *,
const struct in6_addr *);
<netinet/in.h> int inet6_opt_append(void *, size_t, int,
uint8_t, size_t, uint_8, void **);
<netinet/in.h> int inet6_opt_get_val(void *, size_t, void *, int);
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<netinet/in.h> int inet6_opt_find(void *, size_t, int, uint8_t ,
size_t *, void **);
<netinet/in.h> int inet6_opt_finish(void *, size_t, int);
<netinet/in.h> int inet6_opt_init(void *, size_t);
<netinet/in.h> int inet6_opt_next(void *, size_t, int, uint8_t *,
size_t *, void **);
<netinet/in.h> int inet6_opt_set_val(void *, size_t, void *, int);
<netinet/in.h> int inet6_rth_add(void *,
const struct in6_addr *);
<netinet/in.h> struct in6_addr inet6_rth_getaddr(const void *,
int);
<netinet/in.h> void *inet6_rth_init(void *, int, int, int);
<netinet/in.h> int inet6_rth_reverse(const void *, void *);
<netinet/in.h> int inet6_rth_segments(const void *);
<netinet/in.h> size_t inet6_rth_space(int, int);
<sys/socket.h> unsigned char *CMSG_DATA(const struct cmsghdr *);
<sys/socket.h> struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *);
<sys/socket.h> unsigned int CMSG_LEN(unsigned int);
<sys/socket.h> struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
const struct cmsghdr *);
<sys/socket.h> unsigned int CMSG_SPACE(unsigned int);
<unistd.h> int rresvport_af(int *, int);
<unistd.h> int rcmd_af(char **, unsigned short, const char *,
const char *, const char *, int *, int);
<unistd.h> int rexec_af(char **, unsigned short , const char *,
const char *, const char *, int *, int);
16. Security Considerations
The setting of certain Hop-by-Hop options and Destination options may
be restricted to privileged processes. Similarly some Hop-by-Hop
options and Destination options may not be returned to nonprivileged
applications.
The ability to specify an arbitrary source address using IPV6_PKTINFO
must be prevented; at least for non-privileged processes.
17. Change History
Changes from RFC 2292:
- Removed the IPV6_PKTOPTIONS socket option by allowing sticky
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options to be set with individual setsockopt calls. This
simplifies the protocol stack implementation by not having to
handle options within options and also clarifies the failure
semantics when some option is incorrectly formatted.
- Added the IPV6_RTHDRDSTOPTS for a Destination header before the
Routing header. This is necessary to allow setting these
Destination headers without IPV6_PKTOPTIONS.
- Removed the ability to be able to specify Hop-by-Hop and
Destination options using multiple ancillary data items. The
application, using the inet6_option_*() routines, is responsible
for formatting the whole extension header. This removes the need
for the protocol stack to somehow guess the alignment
restrictions on options when concatenating them together.
- Added separate IPV6_RECVxxx options to enable the receipt of the
corresponding ancillary data items. This makes the API cleaner
since it allows the application to retrieve with getsockopt the
sticky options it has set with setsockopt.
- Clarified how sticky options are turned off.
- Clarified how and when TCP returns ancillary data.
- Removed the support for the loose/strict Routing header since
that has been removed from the IPv6 specification.
- Modified the inet6_rthdr_XXX() functions to not assume a cmsghdr
structure in order to work with both sticky options and ancillary
data. Renamed the functions to inet6_rth_XXX() to allow
implementations to provide both the old and new functions.
- Modified the inet6_option_XXX() functions to not assume a cmsghdr
structure in order to work with both sticky options and ancillary
data. Renamed the functions to inet6_opt_XXX() to allow
implementations to provide both the old and new functions.
- The new inet6_opt_XXX() functions were made different that the
old as to not require structure declarations but instead use
functions to add the individual fields to the option.
- Changed inet6_rthdr_getaddr() to operate on index O through N-1
(used to be 1 through N).
- Changed the comments in the struct ip6_hdr from "priority" to
"traffic class".
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- Clarified the alignment issues involving ancillary data to allow
for separate alignment of cmsghdr structures and the data. Made
CMSG_SPACE() return an upper bound on the needed space.
- Added rcmd_af() and rexec_af().
Changes since -00:
- Changed ICMP unreachable code 2 name to be "beyond scope of
source address".
- Added motivation for rcmd_af() and rexec_af().
- Added option definitions (IP6OPT_PAD1 etc) to ip6.h.
- Added MLD and router renumbering definitions to icmp6.h
- Removed ip6r0_addr field - replaced by a comment.
- Made the content of IPV6_RTHDR, IPV6_HOPOPTS etc be specified as
the extension header format (struct ip6_rthdr etc) instead of the
previous "implementation dependent".
- Removed attempt at RFC 2292 compatibility.
- Excluded pad options from inet6_opt_next().
- Added IPV6_USE_MIN_MTU socket option for applications to avoid
fragmentation by sending at the minimum IPv6 MTU.
- Added MTU notification so that UDP and raw socket applications
can participate in path MTU discovery.
- Added Reachability confirmation for UDP and raw socket
applications.
- Clarified that if the application asks for e.g., IPV6_RTHDR and a
received datagram does not contain a Routing header an
implementation will exclude the IPV6_RTHDR ancillary data item.
- Removed the constraints for jumbo option.
- Moved the new CMSG macros and changes from the appendix.
- Add text about inet6_opt_ depending on 2260 appendix B formatting
rules i.e. largest field last in the option.
- Specified that getsockopt() of a sticky option returns what was
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set with setsockopt().
18. TODO and Open Issues
Items left to do:
- Add credits for UDP MTU stuff
- Move information about mapping from inet6_opt to setsockopt and
cmsg.
- Clarify IPV6_RTHDRDSTOPTS's interaction with IPV6_RTHDR.
- Make the new inet6_opt_set_val() and inet6_opt_get_val() check
the length of the data item.
Open issues:
- Anything special for mobility options? Restrict setting at API?
Filter out on receipt? If received what does the home address
option contain?
- Specify "change" for TCP especially when there are multiple HBH
option headers etc.
- Specify binding to scope-id => implies filtering of addresses
with that scope if the address you are bound to is link-local
etc. What about conflicts with bound scope-id and sendto/connect
scope-id?
- Specify order for ifindex selection. Put in separate section.
Different cases for sending to link-local (scope_id including
nexthop scope_id) and global. Is multicast different?
- bind() and sin6_scope_id. Should have been in Basic API. Error
checks when bind/sendto sin6_scope_id does not match?
- Specify notion of default multicast interface? In Basic API?
- What about site names and site ids? Need for interfaces to map?
Requires that site-prefixes pass name - does name need to use DNS
format to handle character sets?
19. References
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[RFC-2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
(IPv6), Specification", RFC 2460, Dec. 1998.
[RFC-2553] Gilligan, R. E., Thomson, S., Bound, J., Stevens, W.,
"Basic Socket Interface Extensions for IPv6", RFC 2553,
March 1999.
[RFC-1981] McCann, J., Deering, S., Mogul, J, "Path MTU Discovery
for IP version 6", RFC 1981, Aug. 1996.
[RFC-2461] Narten, T., Nordmark, E., Simpson, W., "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, Dec. 1998.
20. Acknowledgments
Matt Thomas and Jim Bound have been working on the technical details
in this draft for over a year. Keith Sklower is the original
implementor of ancillary data in the BSD networking code. Craig Metz
provided lots of feedback, suggestions, and comments based on his
implementing many of these features as the document was being
written.
The following provided comments on earlier drafts: Pascal Anelli,
Hamid Asayesh, Ran Atkinson, Karl Auerbach, Hamid Asayesh, Jim Bound,
Don Coolidge, Matt Crawford, Sam T. Denton, Richard Draves, Francis
Dupont, Bob Gilligan, Jun-ichiro Hagino, Gerri Harter, Tim Hartrick,
Bob Halley, Masaki Hirabaru, Yoshinobu Inoue, Mukesh Kacker, A. N.
Kuznetsov, Sam Manthorpe, Pedro Marques, Jack McCann, der Mouse, John
Moy, Thomas Narten, Steve Parker, Charles Perkins, Ken Powell, Tom
Pusateri, Pedro Roque, Sameer Shah, Peter Sjodin, Stephen P.
Spackman, Jinmei Tatuya, Karen Tracey, Sowmini Varadhan, Quaizar
Vohra, Carl Williams, Steve Wise, Eric Wong, Farrell Woods, Kazu
Yamamoto, and Vlad Yasevich.
21. Authors' Addresses
W. Richard Stevens
1202 E. Paseo del Zorro
Tucson, AZ 85718
Email: rstevens@kohala.com
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Matt Thomas
3am Software Foundry
8053 Park Villa Circle
Cupertino, CA 95014
Email: matt@3am-software.com
Erik Nordmark
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303, USA
Email: erik.nordmark@eng.sun.com
22. Appendix A: Ancillary Data Overview
4.2BSD allowed file descriptors to be transferred between separate
processes across a UNIX domain socket using the sendmsg() and
recvmsg() functions. Two members of the msghdr structure,
msg_accrights and msg_accrightslen, were used to send and receive the
descriptors. When the OSI protocols were added to 4.3BSD Reno in
1990 the names of these two fields in the msghdr structure were
changed to msg_control and msg_controllen, because they were used by
the OSI protocols for "control information", although the comments in
the source code call this "ancillary data".
Other than the OSI protocols, the use of ancillary data has been
rare. In 4.4BSD, for example, the only use of ancillary data with
IPv4 is to return the destination address of a received UDP datagram
if the IP_RECVDSTADDR socket option is set. With Unix domain sockets
ancillary data is still used to send and receive descriptors.
Nevertheless the ancillary data fields of the msghdr structure
provide a clean way to pass information in addition to the data that
is being read or written. The inclusion of the msg_control and
msg_controllen members of the msghdr structure along with the cmsghdr
structure that is pointed to by the msg_control member is required by
the Posix.1g sockets API standard.
22.1. The msghdr Structure
The msghdr structure is used by the recvmsg() and sendmsg()
functions. Its Posix.1g definition is:
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struct msghdr {
void *msg_name; /* ptr to socket address structure */
socklen_t msg_namelen; /* size of socket address structure */
struct iovec *msg_iov; /* scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data */
socklen_t msg_controllen; /* ancillary data buffer length */
int msg_flags; /* flags on received message */
};
The structure is declared as a result of including <sys/socket.h>.
(Note: Before Posix.1g the two "void *" pointers were typically "char
*", and the two socklen_t members and the size_t member were
typically integers. Earlier drafts of Posix.1g had the two socklen_t
members as size_t, but Draft 6.6 of Posix.1g, apparently the final
draft, changed these to socklen_t to simplify binary portability for
64-bit implementations and to align Posix.1g with X/Open's Networking
Services, Issue 5. The change in msg_control to a "void *" pointer
affects any code that increments this pointer.)
(Note: Before Posix.1g the cmsg_len member was an integer, and not a
socklen_t. See the Note in the previous section for why socklen_t is
used here.)
Most Berkeley-derived implementations limit the amount of ancillary
data in a call to sendmsg() to no more than 108 bytes (an mbuf).
This API requires a minimum of 10240 bytes of ancillary data, but it
is recommended that the amount be limited only by the buffer space
reserved by the socket (which can be modified by the SO_SNDBUF socket
option). (Note: This magic number 10240 was picked as a value that
should always be large enough. 108 bytes is clearly too small as the
maximum size of a Routing header is 2048 bytes.)
22.2. The cmsghdr Structure
The cmsghdr structure describes ancillary data objects transferred by
recvmsg() and sendmsg(). Its Posix.1g definition is:
struct cmsghdr {
socklen_t cmsg_len; /* #bytes, including this header */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by unsigned char cmsg_data[]; */
};
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This structure is declared as a result of including <sys/socket.h>.
As shown in this definition, normally there is no member with the
name cmsg_data[]. Instead, the data portion is accessed using the
CMSG_xxx() macros, as described in Section 22.3. Nevertheless, it is
common to refer to the cmsg_data[] member.
When ancillary data is sent or received, any number of ancillary data
objects can be specified by the msg_control and msg_controllen
members of the msghdr structure, because each object is preceded by a
cmsghdr structure defining the object's length (the cmsg_len member).
Historically Berkeley-derived implementations have passed only one
object at a time, but this API allows multiple objects to be passed
in a single call to sendmsg() or recvmsg(). The following example
shows two ancillary data objects in a control buffer.
|<--------------------------- msg_controllen -------------------------->|
| OR |
|<--------------------------- msg_controllen ----------------------->|
| |
|<----- ancillary data object ----->|<----- ancillary data object ----->|
|<------ min CMSG_SPACE() --------->|<------ min CMSG_SPACE() --------->|
| | |
|<---------- cmsg_len ---------->| |<--------- cmsg_len ----------->| |
|<--------- CMSG_LEN() --------->| |<-------- CMSG_LEN() ---------->| |
| | | | |
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
|cmsg_|cmsg_|cmsg_|XX| |XX|cmsg_|cmsg_|cmsg_|XX| |XX|
|len |level|type |XX|cmsg_data[]|XX|len |level|type |XX|cmsg_data[]|XX|
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
^
|
msg_control
points here
The fields shown as "XX" are possible padding, between the cmsghdr
structure and the data, and between the data and the next cmsghdr
structure, if required by the implementation. While sending an
application may or may not include padding at the end of last
ancillary data in msg_controllen and implementations must accept both
as valid. On receiving a portable application must provide space for
padding at the end of the last ancillary data as implementations may
copy out the padding at the end of the control message buffer and
include it in the received msg_controllen. When recvmsg() is called
if msg_controllen is too small for all the ancillary data items
including any trailing padding after the last item an implementation
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may set MSG_CTRUNC.
22.3. Ancillary Data Object Macros
To aid in the manipulation of ancillary data objects, three macros
from 4.4BSD are defined by Posix.1g: CMSG_DATA(), CMSG_NXTHDR(), and
CMSG_FIRSTHDR(). Before describing these macros, we show the
following example of how they might be used with a call to recvmsg().
struct msghdr msg;
struct cmsghdr *cmsgptr;
/* fill in msg */
/* call recvmsg() */
for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
u_char *ptr;
ptr = CMSG_DATA(cmsgptr);
/* process data pointed to by ptr */
}
}
We now describe the three Posix.1g macros, followed by two more that
are new with this API: CMSG_SPACE() and CMSG_LEN(). All these macros
are defined as a result of including <sys/socket.h>.
22.3.1. CMSG_FIRSTHDR
struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *mhdr);
CMSG_FIRSTHDR() returns a pointer to the first cmsghdr structure in
the msghdr structure pointed to by mhdr. The macro returns NULL if
there is no ancillary data pointed to by the msghdr structure (that
is, if either msg_control is NULL or if msg_controllen is less than
the size of a cmsghdr structure).
One possible implementation could be
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#define CMSG_FIRSTHDR(mhdr) \
( (mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
(struct cmsghdr *)(mhdr)->msg_control : \
(struct cmsghdr *)NULL )
(Note: Most existing implementations do not test the value of
msg_controllen, and just return the value of msg_control. The value
of msg_controllen must be tested, because if the application asks
recvmsg() to return ancillary data, by setting msg_control to point
to the application's buffer and setting msg_controllen to the length
of this buffer, the kernel indicates that no ancillary data is
available by setting msg_controllen to 0 on return. It is also
easier to put this test into this macro, than making the application
perform the test.)
22.3.2. CMSG_NXTHDR
struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
const struct cmsghdr *cmsg);
CMSG_NXTHDR() returns a pointer to the cmsghdr structure describing
the next ancillary data object. mhdr is a pointer to a msghdr
structure and cmsg is a pointer to a cmsghdr structure. If there is
not another ancillary data object, the return value is NULL.
The following behavior of this macro is new to this API: if the value
of the cmsg pointer is NULL, a pointer to the cmsghdr structure
describing the first ancillary data object is returned. That is,
CMSG_NXTHDR(mhdr, NULL) is equivalent to CMSG_FIRSTHDR(mhdr). If
there are no ancillary data objects, the return value is NULL. This
provides an alternative way of coding the processing loop shown
earlier:
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struct msghdr msg;
struct cmsghdr *cmsgptr = NULL;
/* fill in msg */
/* call recvmsg() */
while ((cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) != NULL) {
if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
u_char *ptr;
ptr = CMSG_DATA(cmsgptr);
/* process data pointed to by ptr */
}
}
One possible implementation could be:
#define CMSG_NXTHDR(mhdr, cmsg) \
(((cmsg) == NULL) ? CMSG_FIRSTHDR(mhdr) : \
(((u_char *)(cmsg) + ALIGN_H((cmsg)->cmsg_len) \
+ ALIGN_D(sizeof(struct cmsghdr)) > \
(u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
(struct cmsghdr *)NULL : \
(struct cmsghdr *)((u_char *)(cmsg) + ALIGN_H((cmsg)->cmsg_len))))
The macros ALIGN_H() and ALIGN_D(), which are implementation
dependent, round their arguments up to the next even multiple of
whatever alignment is required for the start of the cmsghdr structure
and the data, respectively. (This is probably a multiple of 4 or 8
bytes.) They are often the same macro in implementations platforms
where alignment requirement for header and data is chosen to be
identical.
22.3.3. CMSG_DATA
unsigned char *CMSG_DATA(const struct cmsghdr *cmsg);
CMSG_DATA() returns a pointer to the data (what is called the
cmsg_data[] member, even though such a member is not defined in the
structure) following a cmsghdr structure.
One possible implementation could be:
draft-ietf-ipngwg-2292bis-01.txt [Page 59]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
#define CMSG_DATA(cmsg) ( (u_char *)(cmsg) + \
ALIGN_D(sizeof(struct cmsghdr)) )
22.3.4. CMSG_SPACE
unsigned int CMSG_SPACE(unsigned int length);
This macro is new with this API. Given the length of an ancillary
data object, CMSG_SPACE() returns an upper bound on the space
required by the object and its cmsghdr structure, including any
padding needed to satisfy alignment requirements. This macro can be
used, for example, when allocating space dynamically for the
ancillary data. This macro should not be used to initialize the
cmsg_len member of a cmsghdr structure; instead use the CMSG_LEN()
macro.
One possible implementation could be:
#define CMSG_SPACE(length) ( ALIGN_D(sizeof(struct cmsghdr)) + \
ALIGN_H(length) )
22.3.5. CMSG_LEN
unsigned int CMSG_LEN(unsigned int length);
This macro is new with this API. Given the length of an ancillary
data object, CMSG_LEN() returns the value to store in the cmsg_len
member of the cmsghdr structure, taking into account any padding
needed to satisfy alignment requirements.
One possible implementation could be:
#define CMSG_LEN(length) ( ALIGN_D(sizeof(struct cmsghdr)) + length )
Note the difference between CMSG_SPACE() and CMSG_LEN(), shown also
in the figure in Section 4.2: the former accounts for any required
padding at the end of the ancillary data object and the latter is the
actual length to store in the cmsg_len member of the ancillary data
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object.
23. Appendix B: Examples using the inet6_rth_XXX() functions
Here we show an example for both sending Routing headers and
processing and reversing a received Routing header.
23.1. Sending a Routing Header
As an example of these Routing header functions defined in this
document, we go through the function calls for the example on p. 17
of [RFC-2460]. The source is S, the destination is D, and the three
intermediate nodes are I1, I2, and I3.
S -----> I1 -----> I2 -----> I3 -----> D
src: * S S S S S
dst: D I1 I2 I3 D D
A[1]: I1 I2 I1 I1 I1 I1
A[2]: I2 I3 I3 I2 I2 I2
A[3]: I3 D D D I3 I3
#seg: 3 3 2 1 0 3
src and dst are the source and destination IPv6 addresses in the IPv6
header. A[1], A[2], and A[3] are the three addresses in the Routing
header. #seg is the Segments Left field in the Routing header.
The six values in the column beneath node S are the values in the
Routing header specified by the sending application using sendmsg()
of setsockopt(). The function calls by the sender would look like:
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void *extptr;
int extlen;
struct msghdr msg;
struct cmsghdr *cmsgptr;
int cmsglen;
struct sockaddr_in6 I1, I2, I3, D;
extlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, 3);
cmsglen = CMSG_SPACE(extlen);
cmsgptr = malloc(cmsglen);
cmsgptr->cmsg_len = CMSG_LEN(extlen);
cmsgptr->cmsg_level = IPPROTO_IPV6;
cmsgptr->cmsg_type = IPV6_RTHDR;
optptr = CMSG_DATA(cmsgptr);
optptr = inet6_rth_init(optptr, optlen, IPV6_RTHDR_TYPE_0, 3);
inet6_rth_add(optptr, &I1.sin6_addr);
inet6_rth_add(optptr, &I2.sin6_addr);
inet6_rth_add(optptr, &I3.sin6_addr);
msg.msg_control = cmsgptr;
msg.msg_controllen = cmsglen;
/* finish filling in msg{}, msg_name = D */
/* call sendmsg() */
We also assume that the source address for the socket is not
specified (i.e., the asterisk in the figure).
The four columns of six values that are then shown between the five
nodes are the values of the fields in the packet while the packet is
in transit between the two nodes. Notice that before the packet is
sent by the source node S, the source address is chosen (replacing
the asterisk), I1 becomes the destination address of the datagram,
the two addresses A[2] and A[3] are "shifted up", and D is moved to
A[3].
The columns of values that are shown beneath the destination node are
the values returned by recvmsg(), assuming the application has
enabled both the IPV6_RECVPKTINFO and IPV6_RECVRTHDR socket options.
The source address is S (contained in the sockaddr_in6 structure
pointed to by the msg_name member), the destination address is D
(returned as an ancillary data object in an in6_pktinfo structure),
and the ancillary data object specifying the Routing header will
contain three addresses (I1, I2, and I3). The number of segments in
the Routing header is known from the Hdr Ext Len field in the Routing
draft-ietf-ipngwg-2292bis-01.txt [Page 62]
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header (a value of 6, indicating 3 addresses).
The return value from inet6_rth_segments() will be 3 and
inet6_rth_getaddr(0) will return I1, inet6_rth_getaddr(1) will return
I2, and inet6_rth_getaddr(2) will return I3,
If the receiving application then calls inet6_rth_reverse(), the
order of the three addresses will become I3, I2, and I1.
We can also show what an implementation might store in the ancillary
data object as the Routing header is being built by the sending
process. If we assume a 32-bit architecture where sizeof(struct
cmsghdr) equals 12, with a desired alignment of 4-byte boundaries,
then the call to inet6_rth_space(3) returns 68: 12 bytes for the
cmsghdr structure and 56 bytes for the Routing header (8 + 3*16).
The call to inet6_rth_init() initializes the ancillary data object to
contain a Type 0 Routing header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first call to inet6_rth_add() adds I1 to the list.
draft-ietf-ipngwg-2292bis-01.txt [Page 63]
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
The next call to inet6_rth_add() adds I2 to the list.
draft-ietf-ipngwg-2292bis-01.txt [Page 64]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 52 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[2] = I2 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
The last call to inet6_rth_add() adds I3 to the list.
draft-ietf-ipngwg-2292bis-01.txt [Page 65]
INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_len = 68 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_level = IPPROTO_IPV6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmsg_type = IPV6_RTHDR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=6 | Routing Type=0| Seg Left=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[1] = I1 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[2] = I2 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Address[3] = I3 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
cmsg_len is incremented by 16, and the Segments Left field is
incremented by 1.
23.2. Receiving Routing Headers
This example assumes that the application has enabled IPV6_RECVRTHDR
socket option. The application prints and reverses a source route
and uses that to echo the received data.
struct sockaddr_in6 addr;
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struct msghdr msg;
struct iovec iov;
struct cmsghdr *cmsgptr;
size_t cmsgspace;
void *optptr;
int optlen;
int segments;
int i;
char databuf[8192];
segments = 100; /* Enough */
optlen = inet6_rth_space(IPV6_RTHDR_TYPE_0, segments);
cmsgspace = CMSG_SPACE(optlen);
cmsgptr = malloc(cmsgspace);
if (cmsgptr == NULL) {
perror("malloc");
exit(1);
}
optptr = CMSG_DATA(cmsgptr);
msg.msg_control = (char *)cmsgptr;
msg.msg_controllen = cmsgspace;
msg.msg_name = (struct sockaddr *)&addr;
msg.msg_namelen = sizeof (addr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
iov.iov_base = databuf;
iov.iov_len = sizeof (databuf);
msg.msg_flags = 0;
if (recvmsg(s, &msg, 0) == -1) {
perror("recvmsg");
return;
}
if (msg.msg_controllen != 0 &&
cmsgptr->cmsg_level == IPPROTO_IPV6 &&
cmsgptr->cmsg_type == IPV6_RTHDR) {
struct in6_addr *in6;
char asciiname[INET6_ADDRSTRLEN];
struct ip6_rthdr0 *rthdr;
rthdr = (struct ip6_rthdr0 *)optptr;
segments = inet6_rth_segments(optptr);
printf("route (%d segments, %d left): ",
segments, rthdr->ip6r0_segleft);
for (i = 0; i < segments; i++) {
in6 = inet6_rth_getaddr(optptr, i);
if (in6 == NULL)
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INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
printf("<NULL> ");
else
printf("%s ", inet_ntop(AF_INET6,
(void *)in6->s6_addr,
asciiname, INET6_ADDRSTRLEN));
}
if (inet6_rth_reverse(optptr, optptr) == -1) {
printf("reverse failed");
return;
}
}
iov.iov_base = databuf;
iov.iov_len = strlen(databuf);
if (sendmsg(s, &msg, 0) == -1)
perror("sendmsg");
if (cmsgptr != NULL)
free(cmsgptr);
Note: The above example is a simple illustration. It skips some
error checks involving the MSG_TRUNC and MSG_CTRUNC flags.
24. Appendix C: Examples using the inet6_opt_XXX() functions
This shows how Hop-by-Hop and Destination options can be both built
as well as parsed using the inet6_opt_XXX() functions. This examples
assume that there are defined values for OPT_X and OPT_Y.
24.1. Building options
We now provide an example that builds two Hop-by-Hop options using
the example in Appendix B of [RFC-2460].
void *extbuf;
size_t extlen;
int currentlen;
void *databuf;
size_t offset;
uint8_t value1;
uint16_t value2;
uint32_t value4;
uint64_t value8;
/* Estimate the length */
currentlen = inet6_opt_init(NULL, 0);
if (currentlen == -1)
return (-1);
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currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_X, 12, 8, NULL);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_append(NULL, 0, currentlen, OPT_Y, 7, 4, NULL);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_finish(NULL, 0, currentlen);
if (currentlen == -1)
return (-1);
extlen = currentlen;
extbuf = malloc(extlen);
if (extbuf == NULL) {
perror("malloc");
return (-1);
}
currentlen = inet6_opt_init(extbuf, extlen);
if (currentlen == -1)
return (-1);
currentlen = inet6_opt_append(extbuf, extlen, currentlen,
OPT_X, 12, 8, &databuf);
if (currentlen == -1)
return (-1);
/* Insert value 0x12345678 for 4-octet field */
offset = 0;
value4 = 0x12345678;
offset = inet6_opt_set_val(databuf, offset, &value4, sizeof (value4));
/* Insert value 0x0102030405060708 for 8-octet field */
value8 = 0x0102030405060708;
offset = inet6_opt_set_val(databuf, offset, &value8, sizeof (value8));
currentlen = inet6_opt_append(extbuf, extlen, currentlen,
OPT_Y, 7, 4, &databuf);
if (currentlen == -1)
return (-1);
/* Insert value 0x01 for 1-octet field */
offset = 0;
value1 = 0x01;
offset = inet6_opt_set_val(databuf, offset, &value1, sizeof (value1));
/* Insert value 0x1331 for 2-octet field */
value2 = 0x1331;
offset = inet6_opt_set_val(databuf, offset, &value2, sizeof (value2));
/* Insert value 0x01020304 for 4-octet field */
value4 = 0x01020304;
offset = inet6_opt_set_val(databuf, offset, &value4, sizeof (value4));
currentlen = inet6_opt_finish(extbuf, extlen, currentlen);
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if (currentlen == -1)
return (-1);
/* extbuf and extlen are now completely formatted */
24.2. Parsing received options
This example parses and prints the content of the two options in the
previous example.
int
print_opt(void *extbuf, size_t extlen)
{
ip6_dest_t *ext;
int currentlen;
uint8_t type;
size_t len;
void *databuf;
size_t offset;
uint8_t value1;
uint16_t value2;
uint32_t value4;
uint64_t value8;
ext = (ip6_dest_t *)extbuf;
printf("nxt %u, len %u (bytes %d)\n", ext->ip6d_nxt,
ext->ip6d_len, (ext->ip6d_len + 1) * 8);
currentlen = 0;
while (1) {
currentlen = inet6_opt_next(extbuf, extlen, currentlen,
&type, &len, &databuf);
if (currentlen == -1)
break;
printf("Received opt %u len %u\n",
type, len);
switch (type) {
case OPT_X:
offset = 0;
offset = inet6_opt_get_val(databuf, offset,
&value4, sizeof (value4));
printf("X 4-byte field %x\n", value4);
offset = inet6_opt_get_val(databuf, offset,
&value8, sizeof (value8));
printf("X 8-byte field %llx\n", value8);
break;
case OPT_Y:
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INTERNET-DRAFT Advanced Sockets API for IPv6 Oct 22, 1999
offset = 0;
offset = inet6_opt_get_val(databuf, offset,
&value1, sizeof (value1));
printf("Y 1-byte field %x\n", value1);
offset = inet6_opt_get_val(databuf, offset,
&value2, sizeof (value2));
printf("Y 2-byte field %x\n", value2);
offset = inet6_opt_get_val(databuf, offset,
&value4, sizeof (value4));
printf("Y 4-byte field %x\n", value4);
break;
default:
printf("Unknown option %u\n", type);
break;
}
}
return (0);
}
draft-ietf-ipngwg-2292bis-01.txt [Page 71]
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