conf: Split the notions of read DNS addresses and offered ones

With --dns-forward, if the host has a loopback address configured as
DNS server, we should actually use it to forward queries, but, if
--no-map-gw is passed, we shouldn't offer the same address via DHCP,
NDP and DHCPv6, because it's not going to be reachable.

Problematic configuration:

* systemd-resolved configuring the usual 127.0.0.53 on the host: we
  read that from /etc/resolv.conf

* --dns-forward specified with an unrelated address, for example
  198.51.100.1

We still want to forward queries to 127.0.0.53, if we receive one
directed to 198.51.100.1, so we can't drop 127.0.0.53 from our list:
we want to use it for forwarding. At the same time, we shouldn't
offer 127.0.0.53 to the guest or container either.

With this change, I'm only covering the case of automatically
configured DNS servers from /etc/resolv.conf. We could extend this to
addresses configured with command-line options, but I don't really
see a likely use case at this point.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/dhcpv6.c b/dhcpv6.c
index e763aed..67262e6 100644
--- a/dhcpv6.c
+++ b/dhcpv6.c
@@ -379,7 +379,7 @@ static size_t dhcpv6_dns_fill(const struct ctx *c, char *buf, int offset)
 	if (c->no_dhcp_dns)
 		goto search;
 
-	for (i = 0; !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns[i]); i++) {
+	for (i = 0; !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_send[i]); i++) {
 		if (!i) {
 			srv = (struct opt_dns_servers *)(buf + offset);
 			offset += sizeof(struct opt_hdr);
@@ -387,7 +387,8 @@ static size_t dhcpv6_dns_fill(const struct ctx *c, char *buf, int offset)
 			srv->hdr.l = 0;
 		}
 
-		memcpy(&srv->addr[i], &c->ip6.dns[i], sizeof(srv->addr[i]));
+		memcpy(&srv->addr[i], &c->ip6.dns_send[i],
+		       sizeof(srv->addr[i]));
 		srv->hdr.l += sizeof(srv->addr[i]);
 		offset += sizeof(srv->addr[i]);
 	}