Otherwise, requests initially sent to a failing nameserver would
stay there indefinitely, even if other nameservers would work.
Fix for sourceforge bug 3518439
When a nameserver is down, we periodically try sending a "probe"
message to that nameserver to see if it has come back up. If a
nameserver comes up, we cancel any pending probe messages.
Cancelling a probe message while handling the probe's response would
result in a access-after-free or a double-free, so when we notice that
we're about to call a nameserver up because of having received a probe
from it, we need to check whether current response is the response
from the probe.
There was a case where we didn't to that, though: when the resolver
gave us an unusual error response to our request that it resolve
google.com. This is pretty rare, but apparently it can happen with
some weird cacheing nameservers -- the one on the mikrotik router, for
example. Without this patch, we would crash with a NULL pointer
derefernce.
Thanks to Hannes Sowa for finding this issue and helping me track it
down.
Sometimes DNS reply has nothing but query section. It does not look like
error, so it should be treated as NODATA with TTL=0 as soon as there is
no SOA record to deduce negative TTL from.
Previously, once the callback was scheduled, it was unsafe to cancel
a request, but there was no way to tell that. Now it is safe to
cancel a request until the callback is invoked, at which point it
isn't.
Found and diagnosed by Denis Bilenko.
Remember, the code
int is_less_than(int a, unsigned b) {
return a < b;
}
is buggy, since the C integer promotion rules basically turn it into
int is_less_than(int a, unsigned b) {
return ((unsigned)a) < b;
}
and we really want something closer to
int is_less_than(int a, unsigned b) {
return a < 0 || ((unsigned)a) < b;
}
.
Suggested by an example from Ralph Castain
Remember that in a fit of ANSI C compliance, Microsoft decided to
screw portability by renaming basically all the functions in unistd.h to
get prefixed with an understore.
For some reason, mingw didn't seem to mind, but at least some people's
compilers did: see bug 3044490.
The old logging code was littered with places where we stored messages in
static char[] fields. This is fine in a single-threaded program, but if you
ever tried to log evdns messages from two threads at once, you'd hit a race.
This patch also refactors evdns's debug_ntop function into a more useful
evutil_sockaddr_port_format() function, with unit tests.
