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The resolver in the GNU C Library | |
********************************* | |
Starting with version 2.2, the resolver in the GNU C Library comes | |
from BIND 8. Only a subset of the src/lib/resolv part of libbind is | |
included here; basically the parts that are needed to provide the | |
functionality present in the resolver from BIND 4.9.7 that was | |
included in the previous release of the GNU C Library, augmented by | |
the parts needed to provide thread-safety. This means that support | |
for things as dynamic DNS updates and TSIG keys isn't included. If | |
you need those facilities, please take a look at the full BIND | |
distribution. | |
Differences | |
=========== | |
The resolver in the GNU C Library still differs from what's in BIND | |
8.2.3-T5B: | |
* The resolver in glibc strictly adheres to the recommendations in RFC | |
1535. BIND 8.2.3-T5B seems to relax those rules a bit (see the code | |
that's wrapped in `#ifndef RFC1535'). | |
* The RES_DEBUG option (`options debug' in /etc/resolv.conf) has been | |
disabled. | |
* The resolver in glibc allows underscores in domain names. | |
* The <resolv.h> header in glibc includes <netinet/in.h> and | |
<arpa/nameser.h> to make it self-contained. | |
* The `res_close' function in glibc only tries to close open files | |
referenced through `_res' if the RES_INIT bit is set in | |
`_res.options'. This fixes a potential security bug with programs | |
that bogusly call `res_close' without initialising the resolver | |
state first. Note that the thread-safe `res_nclose' still doesn't | |
check the RES_INIT bit. By the way, you're not really supposed to | |
call `res_close/res_nclose' directly. | |
* The resolver in glibc can connect to a nameserver over IPv6. Just | |
specify the IPv6 address in /etc/resolv.conf. You cannot change the | |
address of an IPv6 nameserver dynamically in your program though. | |
Using the resolver in multi-threaded code | |
========================================= | |
The traditional resolver interfaces `res_query', `res_search', | |
`res_mkquery', `res_send' and `res_init', used a static (global) | |
resolver state stored in the `_res' structure. Therefore, these | |
interfaces are not thread-safe. Therefore, BIND 8.2 introduced a set | |
of "new" interfaces `res_nquery', `res_nsearch', `res_nmkquery', | |
`res_nsend' and `res_ninit' that take a `res_state' as their first | |
argument, so you can use a per-thread resolver state. In glibc, when | |
you link with -lpthread, such a per-thread resolver state is already | |
present. It can be accessed using `_res', which has been redefined as | |
a macro, in a similar way to what has been done for the `errno' and | |
`h_errno' variables. This per-thread resolver state is also used for | |
the `gethostby*' family of functions, which means that for example | |
`gethostbyname_r' is now fully thread-safe and re-entrant. The | |
traditional resolver interfaces however, continue to use a single | |
resolver state and are therefore still thread-unsafe. The resolver | |
state is the same resolver state that is used for the initial ("main") | |
thread. | |
This has the following consequences for existing binaries and source | |
code: | |
* Single-threaded programs will continue to work. There should be no | |
user-visible changes when you recompile them. | |
* Multi-threaded programs that use the traditional resolver interfaces | |
in the "main" thread should continue to work, except that they no | |
longer see any changes in the global resolver state caused by calls | |
to, for example, `gethostbyname' in other threads. Again there | |
should be no user-visible changes when you recompile these programs. | |
* Multi-threaded programs that use the traditional resolver interfaces | |
in more than one thread should be just as buggy as before (there are | |
no problems if you use proper locking of course). If you recompile | |
these programs, manipulating the _res structure in threads other | |
than the "main" thread will seem to have no effect though. | |
* In Multi-threaded that manipulate the _res structure, calls to | |
functions like `gethostbyname' in threads other than the "main" | |
thread won't be influenced by the those changes anymore. So if you | |
set RES_USE_INET6, a call to `gethostbyname' won't return any IPv6 | |
hosts anymore. If you recompile such programs, manipulating the | |
_res structure will affect the thread in which you do so instead of | |
the "main" thread. | |
We recommend to use the new thread-safe interfaces in new code, since | |
the traditional interfaces have been deprecated by the BIND folks. | |
For compatibility with other (older) systems you might want to | |
continue to use those interfaces though. | |
Using the resolver in C++ code | |
============================== | |
There resolver contains some hooks which will allow the user to | |
install some callback functions that make it possible to filter DNS | |
requests and responses. Although we do not encourage you to make use | |
of this facility at all, C++ developers should realise that it isn't | |
safe to throw exceptions from such callback functions. | |
Source code | |
=========== | |
The following files come from the BIND distribution (currently version | |
8.2.3-T5B): | |
src/include/ | |
arpa/nameser.h | |
arpa/nameser_compat.h | |
resolv.h | |
src/lib/resolv/ | |
herror.c | |
res_comp.c | |
res_data.c | |
res_debug.c | |
res_debug.h | |
res_init.c | |
res_mkquery.c | |
res_query.c | |
res_send.c | |
src/lib/nameser/ | |
ns_name.c | |
ns_netint.c | |
ns_parse.c | |
ns_print.c | |
ns_samedomain.c | |
ns_ttl.c | |
src/lib/inet/ | |
inet_addr.c | |
inet_net_ntop.c | |
inet_net_pton.c | |
inet_neta.c | |
inet_ntop.c | |
inet_pton.c | |
nsap_addr.c | |
src/lib/isc/ | |
base64.c | |
Some of these files have been optimised a bit, and adaptations have | |
been made to make them fit in with the rest of glibc. The more | |
non-obvious changes are wrapped in something like `#ifdef _LIBC'. | |
res_libc.c is home-brewn, although parts of it are taken from res_data.c. | |
res_hconf.c and res_hconf.h were contributed by David Mosberger, and | |
do not come from BIND. | |
The files gethnamaddr.c, mapv4v6addr.h and mapv4v6hostent.h are | |
leftovers from BIND 4.9.7. |