[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Thu, 25 Aug 2022 09:26:13 -0400
From: Rich Felker <dalias@...c.org>
To: Markus Wichmann <nullplan@....net>
Cc: musl@...ts.openwall.com
Subject: Re: IPv4 fallback in __res_msend_rc not functional
On Thu, Aug 25, 2022 at 04:58:59AM +0200, Markus Wichmann wrote:
> On Wed, Aug 24, 2022 at 07:32:28PM -0400, Rich Felker wrote:
> > Does this work?
> >
> > diff --git a/src/network/res_msend.c b/src/network/res_msend.c
> > index 3e018009..105bf598 100644
> > --- a/src/network/res_msend.c
> > +++ b/src/network/res_msend.c
> > @@ -68,14 +68,15 @@ int __res_msend_rc(int nqueries, const unsigned char *const *queries,
> > }
> >
> > /* Get local address and open/bind a socket */
> > - sa.sin.sin_family = family;
> > fd = socket(family, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
> >
> > /* Handle case where system lacks IPv6 support */
> > if (fd < 0 && family == AF_INET6 && errno == EAFNOSUPPORT) {
> > fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
> > family = AF_INET;
> > + sl = sizeof sa.sin;
> > }
> > + sa.sin.sin_family = family;
> > if (fd < 0 || bind(fd, (void *)&sa, sl) < 0) {
> > if (fd >= 0) close(fd);
> > pthread_setcancelstate(cs, 0);
> >
>
> That would have been my proposal as well, although I would have added a
> "if (ns[j].sin.sin_family == family)" in front of the sendto call as
> well.
I'd thought about adapting the loop that converts v4 addresses to v6
to operate in both directions and delete v6 addresses from the list,
but that's a lot more work and more error-prone. I guess we could do
your check but it doesn't really matter, and in some sense the
failures might be "nice to see" in strace to show that the system is
misconfigured (can't send to the requested v6 nameserver). But..
> Another question to think about is if the function should terminate
> early if there are no usable servers in the config. Without the IPv4
> fallback, this could only happen with conf->nns == 0, but with it, it
> can also happen when all configured servers are IPv6 and IPv6 is
> unusable (which the callers can't know). In that case, the function
> would now not send anything (as sendto() fails silently), and wait in
> vain for a response.
Yes.. failing here would require running through the list and checking
that there's at least one matching family, then figuring out what
error code to return when there's not. I guess it would be EAI_SYSTEM
with errno=EAFNOSUPPORT or something like that, or perhaps just
ENOENT. The case where resolv.conf exists but has no nameservers
(nns=0) is also not really handled right now and I'm not sure if the
intent was for it to fail or behave as if resolv.conf was missing.
Probably it should fail with something like ENOENT, or just continuing
to timeout as it does now I guess..?
Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
