Before this update, BIND would attempt to do a full recursive resolution
process for each query received if the requested rrset had its ttl
expired. If the resolution fails for any reason, only then BIND would
check for stale rrset in cache (if 'stale-cache-enable' and
'stale-answer-enable' is on).
The problem with this approach is that if an authoritative server is
unreachable or is failing to respond, it is very unlikely that the
problem will be fixed in the next seconds.
A better approach to improve performance in those cases, is to mark the
moment in which a resolution failed, and if new queries arrive for that
same rrset, try to respond directly from the stale cache, and do that
for a window of time configured via 'stale-refresh-time'.
Only when this interval expires we then try to do a normal refresh of
the rrset.
The logic behind this commit is as following:
- In query.c / query_gotanswer(), if the test of 'result' variable falls
to the default case, an error is assumed to have happened, and a call
to 'query_usestale()' is made to check if serving of stale rrset is
enabled in configuration.
- If serving of stale answers is enabled, a flag will be turned on in
the query context to look for stale records:
query.c:6839
qctx->client->query.dboptions |= DNS_DBFIND_STALEOK;
- A call to query_lookup() will be made again, inside it a call to
'dns_db_findext()' is made, which in turn will invoke rbdb.c /
cache_find().
- In rbtdb.c / cache_find() the important bits of this change is the
call to 'check_stale_header()', which is a function that yields true
if we should skip the stale entry, or false if we should consider it.
- In check_stale_header() we now check if the DNS_DBFIND_STALEOK option
is set, if that is the case we know that this new search for stale
records was made due to a failure in a normal resolution, so we keep
track of the time in which the failured occured in rbtdb.c:4559:
header->last_refresh_fail_ts = search->now;
- In check_stale_header(), if DNS_DBFIND_STALEOK is not set, then we
know this is a normal lookup, if the record is stale and the query
time is between last failure time + stale-refresh-time window, then
we return false so cache_find() knows it can consider this stale
rrset entry to return as a response.
The last additions are two new methods to the database interface:
- setservestale_refresh
- getservestale_refresh
Those were added so rbtdb can be aware of the value set in configuration
option, since in that level we have no access to the view object.
There were several problems with rbt hashtable implementation:
1. Our internal hashing function returns uint64_t value, but it was
silently truncated to unsigned int in dns_name_hash() and
dns_name_fullhash() functions. As the SipHash 2-4 higher bits are
more random, we need to use the upper half of the return value.
2. The hashtable implementation in rbt.c was using modulo to pick the
slot number for the hash table. This has several problems because
modulo is: a) slow, b) oblivious to patterns in the input data. This
could lead to very uneven distribution of the hashed data in the
hashtable. Combined with the single-linked lists we use, it could
really hog-down the lookup and removal of the nodes from the rbt
tree[a]. The Fibonacci Hashing is much better fit for the hashtable
function here. For longer description, read "Fibonacci Hashing: The
Optimization that the World Forgot"[b] or just look at the Linux
kernel. Also this will make Diego very happy :).
3. The hashtable would rehash every time the number of nodes in the rbt
tree would exceed 3 * (hashtable size). The overcommit will make the
uneven distribution in the hashtable even worse, but the main problem
lies in the rehashing - every time the database grows beyond the
limit, each subsequent rehashing will be much slower. The mitigation
here is letting the rbt know how big the cache can grown and
pre-allocate the hashtable to be big enough to actually never need to
rehash. This will consume more memory at the start, but since the
size of the hashtable is capped to `1 << 32` (e.g. 4 mio entries), it
will only consume maximum of 32GB of memory for hashtable in the
worst case (and max-cache-size would need to be set to more than
4TB). Calling the dns_db_adjusthashsize() will also cap the maximum
size of the hashtable to the pre-computed number of bits, so it won't
try to consume more gigabytes of memory than available for the
database.
FIXME: What is the average size of the rbt node that gets hashed? I
chose the pagesize (4k) as initial value to precompute the size of
the hashtable, but the value is based on feeling and not any real
data.
For future work, there are more places where we use result of the hash
value modulo some small number and that would benefit from Fibonacci
Hashing to get better distribution.
Notes:
a. A doubly linked list should be used here to speedup the removal of
the entries from the hashtable.
b. https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/
(cherry picked from commit e24bc324b4)
Originally, every library and binaries got linked to everything, which
creates unnecessary overlinking. This wasn't as straightforward as it
should be as we still support configuration without libtool for 9.16.
Couple of smaller issues related to include headers and an issue where
sanitizer overload dlopen and dlclose symbols, so we were getting false
negatives in the autoconf test.
When --with-zlib is passed to ./configure (or when the latter
autodetects zlib's presence), libisc uses certain zlib functions and
thus libisc's users should be linked against zlib in that case. Adjust
Makefile variables appropriately to prevent shared build failures caused
by underlinking.
(cherry picked from commit fc967ba092)
Start enforcing the clang-format rules on changed files
Closes#46
See merge request isc-projects/bind9!3063
(cherry picked from commit a04cdde45d)
d2b5853b Start enforcing the clang-format rules on changed files
618947c6 Switch AlwaysBreakAfterReturnType from TopLevelDefinitions to All
654927c8 Add separate .clang-format files for headers
5777c44a Reformat using the new rules
60d29f69 Don't enforce copyrights on .clang-format
adjust clang-format options to get closer to ISC style
See merge request isc-projects/bind9!3061
(cherry picked from commit d3b49b6675)
0255a974 revise .clang-format and add a C formatting script in util
e851ed0b apply the modified style
Add curly braces using uncrustify and then reformat with clang-format back
Closes#46
See merge request isc-projects/bind9!3057
(cherry picked from commit 67b68e06ad)
36c6105e Use coccinelle to add braces to nested single line statement
d14bb713 Add copy of run-clang-tidy that can fixup the filepaths
056e133c Use clang-tidy to add curly braces around one-line statements
Reformat source code with clang-format
Closes#46
See merge request isc-projects/bind9!2156
(cherry picked from commit 7099e79a9b)
4c3b063e Import Linux kernel .clang-format with small modifications
f50b1e06 Use clang-format to reformat the source files
11341c76 Update the definition files for Windows
df6c1f76 Remove tkey_test (which is no-op anyway)
Also disable the semantic patch as the code needs tweaks here and there because
some destroy functions might not destroy the object and return early if the
object is still in use.
122 cleanup:
CID 1452696 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking s suggests that it may be
null, but it has already been dereferenced on all paths
leading to the check.
123 if (s != NULL)
124 isc_mem_free(mctx, s);
This second commit uses second semantic patch to replace the calls to
dns_name_copy() with NULL as third argument where the result was stored in a
isc_result_t variable. As the dns_name_copy(..., NULL) cannot fail gracefully
when the third argument is NULL, it was just a bunch of dead code.
Couple of manual tweaks (removing dead labels and unused variables) were
manually applied on top of the semantic patch.
isc_event_allocate() calls isc_mem_get() to allocate the event structure. As
isc_mem_get() cannot fail softly (e.g. it never returns NULL), the
isc_event_allocate() cannot return NULL, hence we remove the (ret == NULL)
handling blocks using the semantic patch from the previous commit.
- removed some dead code
- dns_zone_setdbtype is now void as it could no longer return
anything but ISC_R_SUCCESS; calls to it no longer check for a result
- controlkeylist_fromconfig() is also now void
- fixed a whitespace error
The ax_check_openssl m4 macro used OPENSSL_INCLUDES. Rename the
subst variable to OPENSSL_CFLAGS and wrap AX_CHECK_OPENSSL() in
action-if-not-found part of PKG_CHECK_MODULE check for libcrypto.
The json-c have previously leaked into the global namespace leading
to forced -I<include_path> for every compilation unit using isc/xml.h
header. This MR fixes the usage making the caller object opaque.
The libxml2 have previously leaked into the global namespace leading
to forced -I<include_path> for every compilation unit using isc/xml.h
header. This MR fixes the usage making the caller object opaque.
This properly orders clearing the freed pointer and calling isc_refcount_destroy
as early as possible to have ability to put proper memory barrier when cleaning
up reference counting.
- Replace external -DOPENSSL/-DPKCS11CRYPTO with properly AC_DEFINEd
HAVE_OPENSSL/HAVE_PKCS11
- Don't enforce the crypto provider from platform.h, just from dst_api.c
and configure scripts
Replace dns_fixedname_init() calls followed by dns_fixedname_name()
calls with calls to dns_fixedname_initname() where it is possible
without affecting current behavior and/or performance.
This patch was mostly prepared using Coccinelle and the following
semantic patch:
@@
expression fixedname, name;
@@
- dns_fixedname_init(&fixedname);
...
- name = dns_fixedname_name(&fixedname);
+ name = dns_fixedname_initname(&fixedname);
The resulting set of changes was then manually reviewed to exclude false
positives and apply minor tweaks.
It is likely that more occurrences of this pattern can be refactored in
an identical way. This commit only takes care of the low-hanging fruit.
allows named to provide stale cached answers when
the authoritative server is under attack.
See max-stale-ttl, stale-answer-enable,
stale-answer-ttl. [RT #44790]
