[Qemu-devel] [PATCH v2 1/5] sockets: ensure we can bind to both ipv4 & ipv6 separately

["Daniel P. Berrange" <berrange@redhat.com>]

When binding to an IPv6 socket we currently force the
IPV6_V6ONLY flag to off. This means that the IPv6 socket
will accept both IPv4 & IPv6 sockets when QEMU is launched
with something like

  -vnc :::1

While this is good for that case, it is bad for other
cases. For example if an empty hostname is given,
getaddrinfo resolves it to 2 addresses 0.0.0.0 and ::,
in that order. We will thus bind to 0.0.0.0 first, and
then fail to bind to :: on the same port. The same
problem can happen if any other hostname lookup causes
the IPv4 address to be reported before the IPv6 address.

When we get an IPv6 bind failure, we should re-try the
same port, but with IPV6_V6ONLY turned on again, to
avoid clash with any IPv4 listener.

This ensures that

  -vnc :1

will bind successfully to both 0.0.0.0 and ::, and also
avoid

  -vnc :1,to=2

from mistakenly using a 2nd port for the :: listener.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 util/qemu-sockets.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index d8183f7..397212b 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -208,22 +208,37 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
         }
 
         socket_set_fast_reuse(slisten);
-#ifdef IPV6_V6ONLY
-        if (e->ai_family == PF_INET6) {
-            /* listen on both ipv4 and ipv6 */
-            const int off = 0;
-            qemu_setsockopt(slisten, IPPROTO_IPV6, IPV6_V6ONLY, &off,
-                            sizeof(off));
-        }
-#endif
 
         port_min = inet_getport(e);
         port_max = saddr->has_to ? saddr->to + port_offset : port_min;
         for (p = port_min; p <= port_max; p++) {
+#ifdef IPV6_V6ONLY
+            /* listen on both ipv4 and ipv6 */
+            int v6only = 0;
+#endif
             inet_setport(e, p);
+#ifdef IPV6_V6ONLY
+        rebind:
+            if (e->ai_family == PF_INET6) {
+                qemu_setsockopt(slisten, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
+                                sizeof(v6only));
+            }
+#endif
             if (bind(slisten, e->ai_addr, e->ai_addrlen) == 0) {
                 goto listen;
             }
+
+#ifdef IPV6_V6ONLY
+            /* If we got EADDRINUSE from an IPv6 bind & V6ONLY is unset,
+             * it could be that the IPv4 port is already claimed, so retry
+             * with V6ONLY set
+             */
+            if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
+                v6only = 1;
+                goto rebind;
+            }
+#endif
+
             if (p == port_max) {
                 if (!e->ai_next) {
                     error_setg_errno(errp, errno, "Failed to bind socket");
-- 
2.9.3