84d71c8e2c
This commit changes TLS DNS so that partial writes by the SSL_write_ex() function are taken into account properly. Now, before doing encryption, we are flushing the buffers for outgoing encrypted data. The problem is fairly complicated and originates from the fact that it is somewhat hard to understand by reading the documentation if and when partial writes are supported/enabled or not, and one can get a false impression that they are not supported or enabled by default (https://www.openssl.org/docs/man3.1/man3/SSL_write_ex.html). I have added a lengthy comment about that into the code because it will be more useful there. The documentation on this topic is vague and hard to follow. The main point is that when SSL_write_ex() fails with SSL_ERROR_WANT_WRITE, the OpenSSL code tells us that we need to flush the outgoing buffers and then call SSL_write_ex() again with exactly the same arguments in order to continue as partial write could have happened on the previous call to SSL_write_ex() (that is not hard to verify by calling BIO_pending(sock->tls.app_rbio) before and after the call to SSL_write_ex() and comparing the returned values). This aspect was not taken into account in the code. Now, one can wonder how that could have led to the behaviour that we saw in the #4255 bug report. In particular, how could we lose one message and duplicate one twice? That is where things get interesting. One needs to keep two things in mind (that is important): Firstly, the possibility that two (or more) subsequent SSL_write_ex() calls will be done with exactly the same arguments is very high (the code does not guarantee that in any way, but in practice, that happens a lot). Secondly, the dnsperf (the software that helped us to trigger the bug) bombed the test server with messages that contained exactly the same data. The only difference in the responses is message IDs, which can be found closer to the start of a message. So, that is what was going on in the older version of the code: 1. During one of the isc_nm_send() calls, the SSL_write_ex() call fails with SSL_ERROR_WANT_WRITE. Partial writing has happened, though, and we wrote a part of the message with the message ID (e.g. 2014). Nevertheless, we have rescheduled the complete send operation asynchronously by a call to tlsdns_send_enqueue(). 2. While the asynchronous request has not been completed, we try to send the message (e.g. with ID 2015). The next isc_nm_send() or re-queued send happens with a call to SSL_write_ex() with EXACTLY the same arguments as in the case of the previous call. That is, we are acting as if we want to complete the previously failed SSL_write_ex() attempt (according to the OpenSSL documentation: https://www.openssl.org/docs/man3.1/man3/SSL_write_ex.html, the "Warnings" section). This way, we already have a start of the message containing the previous ID (2014 in our case) but complete the write request with the rest of the data given in the current write attempt. However, as responses differ only in message ID, we end up sending a valid (properly structured) DNS message but with the ID of the previous one. This way, we send a message with ID from the previous isc_nm_send() attempt. The message with the ID from the send request from this attempt will never be sent, as the code thinks that it is sending it now (that is how we send the message with ID 2014 instead of 2015, as in our example, thus making the message with ID 2015 never to be sent). 3. At some point later, the asynchronous send request (the rescheduled on the first step) completes without an error, sending a second message with the same ID (2014). It took exhausting SSL write buffers (so that a data encryption attempt cannot be completed in one operation) via long DoT streams in order to exhibit the behaviour described above. The exhaustion happened because we have not been trying to flush the buffers often enough (especially in the case of multiple subsequent writes). In my opinion, the origin of the problem can be described as follows: It happened due to making wrong guesses caused by poorly written documentation.
BIND 9
Contents
- Introduction
- Reporting bugs and getting help
- Contributing to BIND
- Building BIND
- Automated testing
- Documentation
- Change log
- Acknowledgments
Introduction
BIND (Berkeley Internet Name Domain) is a complete, highly portable implementation of the Domain Name System (DNS) protocol.
The BIND name server, named, can act as an authoritative name
server, recursive resolver, DNS forwarder, or all three simultaneously. It
implements views for split-horizon DNS, automatic DNSSEC zone signing and
key management, catalog zones to facilitate provisioning of zone data
throughout a name server constellation, response policy zones (RPZ) to
protect clients from malicious data, response rate limiting (RRL) and
recursive query limits to reduce distributed denial of service attacks,
and many other advanced DNS features. BIND also includes a suite of
administrative tools, including the dig and delv DNS lookup tools,
nsupdate for dynamic DNS zone updates, rndc for remote name server
administration, and more.
BIND 9 began as a complete rewrite of the BIND architecture that was used in versions 4 and 8. Internet Systems Consortium (https://www.isc.org), a 501(c)(3) US public benefit corporation dedicated to providing software and services in support of the Internet infrastructure, developed BIND 9 and is responsible for its ongoing maintenance and improvement. BIND is open source software licensed under the terms of the Mozilla Public License, version 2.0.
For a detailed list of changes made throughout the history of BIND 9, see the file CHANGES. See below for details on the CHANGES file format.
For up-to-date versions and release notes, see https://www.isc.org/download/.
For information about supported platforms, see the "Supported Platforms" section in the BIND 9 Administrator Reference Manual.
Reporting bugs and getting help
To report non-security-sensitive bugs or request new features, you may open an issue in the BIND 9 project on the ISC GitLab server at https://gitlab.isc.org/isc-projects/bind9.
Please note that, unless you explicitly mark the newly created issue as
"confidential," it will be publicly readable. Please do not include any
information in bug reports that you consider to be confidential unless
the issue has been marked as such. In particular, if submitting the
contents of your configuration file in a non-confidential issue, it is
advisable to obscure key secrets; this can be done automatically by
using named-checkconf -px.
For information about ISC's Security Vulnerability Disclosure Policy and
information about reporting potential security issues, please see
SECURITY.md.
Professional support and training for BIND are available from ISC. Contact us at https://www.isc.org/contact for more information.
To join the BIND Users mailing list, or view the archives, visit https://lists.isc.org/mailman/listinfo/bind-users.
If you're planning on making changes to the BIND 9 source code, you may also want to join the BIND Workers mailing list, at https://lists.isc.org/mailman/listinfo/bind-workers.
Contributing to BIND
ISC maintains a public git repository for BIND; details can be found at https://www.isc.org/sourceaccess/.
Information for BIND contributors can be found in the following files:
- General information: CONTRIBUTING.md
- Code of Conduct: CODE_OF_CONDUCT.md
- BIND 9 code style: doc/dev/style.md
- BIND architecture and developer guide: doc/dev/dev.md
Patches for BIND may be submitted as merge requests on the ISC GitLab server.
By default, external contributors do not have the ability to fork BIND on the GitLab server; if you wish to contribute code to BIND, you may request permission to do so. Thereafter, you can create git branches and directly submit requests that they be reviewed and merged.
If you prefer, you may also submit code by opening a
GitLab issue and
including your patch as an attachment, preferably generated by
git format-patch.
Building BIND 9
For information about building BIND 9, see the "Building BIND 9" section in the BIND 9 Administrator Reference Manual.
Automated testing
A system test suite can be run with make check. The system tests require
you to configure a set of virtual IP addresses on your system (this allows
multiple servers to run locally and communicate with each other). These
IP addresses can be configured by running the command
bin/tests/system/ifconfig.sh up as root.
Some tests require Perl and the Net::DNS and/or IO::Socket::INET6 modules,
and are skipped if these are not available. Some tests require Python
and the dnspython module and are skipped if these are not available.
See bin/tests/system/README for further details.
Unit tests are implemented using the CMocka unit testing framework. To build
them, use configure --with-cmocka. Execution of tests is done by the automake
parallel test driver; unit tests are also run by make check.
Documentation
The BIND 9 Administrator Reference Manual (ARM) is included with the source
distribution, and in .rst format, in the doc/arm
directory. HTML and PDF versions are automatically generated and can
be viewed at https://bind9.readthedocs.io/en/latest/index.html.
Man pages for some of the programs in the BIND 9 distribution are also included in the BIND ARM.
Frequently (and not-so-frequently) asked questions and their answers can be found in the ISC Knowledgebase at https://kb.isc.org.
Additional information on various subjects can be found in other
README files throughout the source tree.
Change log
A detailed list of all changes that have been made throughout the development of BIND 9 is included in the file CHANGES, with the most recent changes listed first. Change notes include tags indicating the category of the change that was made; these categories are:
| Category | Description |
|---|---|
| [func] | New feature |
| [bug] | General bug fix |
| [security] | Fix for a significant security flaw |
| [experimental] | Used for new features when the syntax or other aspects of the design are still in flux and may change |
| [port] | Portability enhancement |
| [maint] | Updates to built-in data such as root server addresses and keys |
| [tuning] | Changes to built-in configuration defaults and constants to improve performance |
| [performance] | Other changes to improve server performance |
| [protocol] | Updates to the DNS protocol such as new RR types |
| [test] | Changes to the automatic tests, not affecting server functionality |
| [cleanup] | Minor corrections and refactoring |
| [doc] | Documentation |
| [contrib] | Changes to the contributed tools and libraries in the 'contrib' subdirectory |
| [placeholder] | Used in the main development branch to reserve change numbers for use in other branches, e.g., when fixing a bug that only exists in older releases |
In general, [func] and [experimental] tags only appear in new-feature releases (i.e., those with version numbers ending in zero). Some new functionality may be backported to older releases on a case-by-case basis. All other change types may be applied to all currently supported releases.
Bug report identifiers
Most notes in the CHANGES file include a reference to a bug report or
issue number. Prior to 2018, these were usually of the form [RT #NNN]
and referred to entries in the "bind9-bugs" RT database, which was not open
to the public. More recent entries use the form [GL #NNN] or, less often,
[GL !NNN], which, respectively, refer to issues or merge requests in the
GitLab database. Most of these are publicly readable, unless they include
information which is confidential or security-sensitive.
To look up a GitLab issue by its number, use the URL https://gitlab.isc.org/isc-projects/bind9/issues/NNN. To look up a merge request, use https://gitlab.isc.org/isc-projects/bind9/merge_requests/NNN.
In rare cases, an issue or merge request number may be followed with the letter "P". This indicates that the information is in the private ISC GitLab instance, which is not visible to the public.
Acknowledgments
-
The original development of BIND 9 was underwritten by the following organizations:
Sun Microsystems, Inc. Hewlett Packard Compaq Computer Corporation IBM Process Software Corporation Silicon Graphics, Inc. Network Associates, Inc. U.S. Defense Information Systems Agency USENIX Association Stichting NLnet - NLnet Foundation Nominum, Inc. -
This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit. https://www.OpenSSL.org/
-
This product includes cryptographic software written by Eric Young (eay@cryptsoft.com).
-
This product includes software written by Tim Hudson (tjh@cryptsoft.com).