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Restore localhost fallback in bin/dig/dighost.c

Browse source 
In BIND 9.11 and earlier, dig and similar tools used liblwres for
parsing /etc/resolv.conf.  After getting a list of servers from
liblwres, a tool would check the address family of each server found and
reject those unusable.  When the resulting list of usable servers was
empty, localhost addresses were queried as a fallback.

When liblwres was removed in BIND 9.12, dig and similar tools were
updated to parse /etc/resolv.conf using libirs instead.  As part of that
process, the localhost fallback was removed from bin/dig/dighost.c since
the localhost fallback built into libirs was deemed to be sufficient.
However, libirs only falls back to localhost if it does not find any
name servers at all; if it does find any valid nameserver entry in
/etc/resolv.conf, it just returns it to the caller because it is
oblivious to whether the caller supports IPv4 and/or IPv6 or not.  The
code in bin/dig/dighost.c subsequently filters the returned list of
servers in get_server_list() according to the requested address family
restrictions.  This may result in none of the addresses returned by
libirs being usable, in which case a tool will attempt to work with an
empty server list, causing a hang and subsequently a crash upon user
interruption.

Restore the localhost fallback in bin/dig/dighost.c to prevent the
aforementioned hangs and crashes and ensure recent BIND versions behave
identically to the older ones in the circumstances described above.

(cherry picked from commit 18758392da)
This commit is contained in:
Michał Kępień 2018-11-13 14:31:18 +01:00
parent 14b8bf9d87
commit 8181ffefe7
1 changed files with 23 additions and 0 deletions
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23
bin/dig/dighost.c
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@ -1270,6 +1270,19 @@ create_search_list(irs_resconf_t *resconf) {
}
}
/*%
* Append 'addr' to the list of servers to be queried. This function is only
* called when no server addresses are explicitly specified and either libirs
* returns an empty list of servers to use or none of the addresses returned by
* libirs are usable due to the specified address family restrictions.
*/
static void
add_fallback_nameserver(const char *addr) {
dig_server_t *server = make_server(addr, addr);
ISC_LINK_INIT(server, link);
ISC_LIST_APPEND(server_list, server, link);
}
/*%
* Setup the system as a whole, reading key information and resolv.conf
* settings.
@ -1315,6 +1328,16 @@ setup_system(bool ipv4only, bool ipv6only) {
get_server_list(resconf);
}
/* If we don't find a nameserver fall back to localhost */
if (ISC_LIST_EMPTY(server_list)) {
if (have_ipv6) {
add_fallback_nameserver("::1");
}
if (have_ipv4) {
add_fallback_nameserver("127.0.0.1");
}
}
irs_resconf_destroy(&resconf);
#ifdef HAVE_SETLOCALE