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Date: Wed, 24 Aug 2022 19:32:28 -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 Wed, Aug 24, 2022 at 07:26:58PM -0400, Rich Felker wrote:
> On Wed, Aug 24, 2022 at 09:03:49PM +0200, Markus Wichmann wrote:
> > Hi all,
> >
> > I noticed something while reading some code: There is a fallback in
> > __res_msend_rc(), in case an IPv6 socket is requested but cannot be
> > allocated. In that case, the function tries to create an IPv4 socket
> > instead. However, I do not think this code can work that way. For
> > reference, this is the code:
> >
> > /* 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;
> > }
> > if (fd < 0 || bind(fd, (void *)&sa, sl) < 0) {
> > if (fd >= 0) close(fd);
> > pthread_setcancelstate(cs, 0);
> > return -1;
> > }
> >
> > The problem is, if the fallback is triggered, the local address is still
> > set to be an IPv6 address, and so the bind() must necessarily fail with
> > EINVAL.
> >
> > The fix depends on whether the fallback is still intended functionality
> > or not. If not, then the easiest would be to just get rid of the entire
> > fallback block. If the fallback is still intended to work, then the
> > fallback block must reset sl to the length of an IPv4 socket, and the
> > setting of sa.sin.sin_family must be delayed until after that block.
> >
> > There is also the issue of the sendto() loop further down in the
> > function. If it is intended that the socket can be an IPv4 socket but
> > there can be IPv6 addresses in the list, then it might be prudent to
> > prevent sendto() from sending to the wrong address family. Or not, I
> > mean, you do not test for errors from sendto(), and the sends to the
> > wrong address family are just going to fail. So they would only waste
> > time and change errno, but not much of a visible side effect.
>
> Thanks for reporting this! It's intended to be functional, but it's
> probably of little consequence whether it works since the issue seems
> to arise only when resolv.conf requested IPv6 nameservers but the
> system doesn't support IPv6. I'll look at what it'll take to fix it...
> hopefully it won't be too bad.
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);
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