[Dnsmasq-discuss] [PATCH] Fix parsing of IPv6 addresses with peer from netlink

[Beniamino Galvani]

In the most common case, an IPv6 address doesn't have a peer and the
IFA_ADDRESS netlink attribute contains the address itself.

But if the address has a peer (typically for point to point links),
then IFA_ADDRESS contains the peer address and IFA_LOCAL contains the
address [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/addrconf.c?h=v5.17#n5030

Fix the parsing of IPv6 addresses with peers, as currently dnsmasq
unsuccessfully tries to bind on the peer address.

A simple reproducer is:

  dnsmasq --conf-file=/dev/null -i dummy1 -d --bind-dynamic &
  sleep 2
  ip link add dummy1 type dummy
  ip link set dummy1 up
  ip addr add dev dummy1 fd01::1/64 peer fd01::2/64
  ip addr add dev dummy1 fd01::42/64
  sleep 2
  ss -lnp | grep dnsmasq | grep fd01

Before the patch:
  dnsmasq: failed to create listening socket for fd01::2: Cannot assign requested address
  dnsmasq: failed to create listening socket for fd01::2: Cannot assign requested address
  udp   UNCONN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23947,fd=14))
  tcp   LISTEN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23947,fd=15

After:
  udp   UNCONN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=16))
  udp   UNCONN 0    [fd01::1]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=14))
  tcp   LISTEN 0   [fd01::42]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=17))
  tcp   LISTEN 0    [fd01::1]:53   [::]:*    users:(("dnsmasq",pid=23973,fd=15))
---
 src/netlink.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/netlink.c b/src/netlink.c
index da82943..a6d1972 100644
--- a/src/netlink.c
+++ b/src/netlink.c
@@ -258,7 +258,9 @@ int iface_enumerate(int family, void *parm, int (*callback)())
 		    
 		    while (RTA_OK(rta, len1))
 		      {
-			if (rta->rta_type == IFA_ADDRESS)
+			if (rta->rta_type == IFA_LOCAL)
+			  addrp = ((struct in6_addr *)(rta+1));
+			else if (rta->rta_type == IFA_ADDRESS && !addrp)
 			  addrp = ((struct in6_addr *)(rta+1)); 
 			else if (rta->rta_type == IFA_CACHEINFO)
 			  {
-- 
2.34.1