tl;dr:
res_init() does not correctly initialize the _res struct.
The code:
res_init();
if(_res.options & RES_INIT) {
printf("RES_INIT set.\n");
} else {
printf("RES_INIT not set.\n");
}
outputs "RES_INIT set." correctly, and that bit is set. res_init() does all of
its setting through res_init.c,
https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_init.c;h=66561ffac2ffc039707676ed8b4bf36ee50ee889;hb=HEAD#l151
That's all fine and dandy. -- (The actual resetting happens through the
function __res_vinit())
However, res_init() neglects to set things that the ubuntu-specific eglibc
requires for it to be a valid struct, such as the last modified time of
/etc/resolv.conf -- "last_mtime".
Thus, the first call to res_query() resets the _res struct, effectively making
any changes before it and after the original res_init(), useless.
As we can see in res_data.c,
https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_data.c;h=81c9ae5bfd7ef71ebb986b5c9572c1859684ba39;hb=HEAD#l185
, res_query() calls __res_maybe_init (), which decides whether we need to
re-call res_init() or not.
In the comments for that __res_maybe_init() function:
/* Initialize resp if RES_INIT is not yet set or if res_init in some other
thread requested re-initializing. */
On the first run of __res_maybe_init()[thus, effectively, res_query), it
will always call __res_vinit(), because res_init() does not set the
'last_mtime'. -- That's because the last_mtime is an Ubuntu-specific
feature, likely added for security reasons.
>>>>>> if ((ret == 0) && (last_mtime != statbuf.st_mtime)) {
<<<<<<<
last_mtime = statbuf.st_mtime;
atomicinc (__res_initstamp);
}
__libc_lock_unlock (lock);
if (__res_initstamp != resp->_u._ext.initstamp) {
if (resp->nscount > 0)
__res_iclose (resp, true);
return __res_vinit (resp, 1);
}
even shorter tl;dr:
'last_mtime' is an ubuntu-specific feature added to eglibc's resolv library,
which is only set inside the __res_maybe_init() function.
When calling res_init(), it does not set 'last_mtime', as that uses
__res_vinit(), not __res_maybe_init().
When calling res_query() for the first time, all the changes made to the _res
struct are wiped, with the exception of int retrans, int retry, u_long options.
This is because res_query uses __res_maybe_init(), which will reset _res if
'last_mtime' has not been set(or is old)
A quick fix is to replace __res_vinit()'s usage within the res_query()
function with __res_maybe_init(), which takes the exact same parameters.
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https://bugs.launchpad.net/bugs/1432378
Title:
libresolv res_init() does not correctly inititalize internals
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