[PATCH 2/7] udp: Simplify setting od destination IPv6 address for inbound packets

[David Gibson <david@gibson.dropbear.id.au>]

Depending on several possible NAT situations, udp_update_hdr6() has a few
different paths for setting the destination IPv6 address.  However, this
isn't really separate from the selection of the IPv6 source address:
if the adjusted source address is link-local then we need to use a link
local destination, otherwise we need the global destination address.

This fixes a small bug: it's theoretically possible, although unlikely, for
the dns_match address to be link-local, in which case the previous code
would have used the wrong destination.

This does do slightly more work in the case of NATting packets originating
on the host.  Currently we always NAT those to a link-local address so we
could statically set a link-local destination address, but we now recheck.
However, as well as being simpler, the new approach is more robust if we
want to allow non-link-local NAT-to-host addresses, which we do plan to in
future.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 udp.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/udp.c b/udp.c
index 361f24c..9c96c0f 100644
--- a/udp.c
+++ b/udp.c
@@ -640,18 +640,13 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 
 	b->ip6h.payload_len = htons(udp6_l2_mh_sock[n].msg_len + sizeof(b->uh));
 
-	if (IN6_IS_ADDR_LINKLOCAL(src)) {
-		b->ip6h.daddr = c->ip6.addr_ll_seen;
-	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match) &&
+	if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match) &&
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns_host) &&
 		   src_port == 53) {
-		b->ip6h.daddr = c->ip6.addr_seen;
 		src = &c->ip6.dns_match;
 	} else if (IN6_IS_ADDR_LOOPBACK(src)			||
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr_seen)	||
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr)) {
-		b->ip6h.daddr = c->ip6.addr_ll_seen;
-
 		udp_tap_map[V6][src_port].ts = now->tv_sec;
 		udp_tap_map[V6][src_port].flags |= PORT_LOCAL;
 
@@ -671,11 +666,13 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 			src = &c->ip6.gw;
 		else
 			src = &c->ip6.addr_ll;
-	} else {
-		b->ip6h.daddr = c->ip6.addr_seen;
 	}
 
 	b->ip6h.saddr = *src;
+	if (IN6_IS_ADDR_LINKLOCAL(src))
+		b->ip6h.daddr = c->ip6.addr_ll_seen;
+	else
+		b->ip6h.daddr = c->ip6.addr_seen;
 
 	b->uh.source = b->s_in6.sin6_port;
 	b->uh.dest = htons(dstport);
-- 
@@ -640,18 +640,13 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 
 	b->ip6h.payload_len = htons(udp6_l2_mh_sock[n].msg_len + sizeof(b->uh));
 
-	if (IN6_IS_ADDR_LINKLOCAL(src)) {
-		b->ip6h.daddr = c->ip6.addr_ll_seen;
-	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match) &&
+	if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match) &&
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns_host) &&
 		   src_port == 53) {
-		b->ip6h.daddr = c->ip6.addr_seen;
 		src = &c->ip6.dns_match;
 	} else if (IN6_IS_ADDR_LOOPBACK(src)			||
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr_seen)	||
 		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr)) {
-		b->ip6h.daddr = c->ip6.addr_ll_seen;
-
 		udp_tap_map[V6][src_port].ts = now->tv_sec;
 		udp_tap_map[V6][src_port].flags |= PORT_LOCAL;
 
@@ -671,11 +666,13 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 			src = &c->ip6.gw;
 		else
 			src = &c->ip6.addr_ll;
-	} else {
-		b->ip6h.daddr = c->ip6.addr_seen;
 	}
 
 	b->ip6h.saddr = *src;
+	if (IN6_IS_ADDR_LINKLOCAL(src))
+		b->ip6h.daddr = c->ip6.addr_ll_seen;
+	else
+		b->ip6h.daddr = c->ip6.addr_seen;
 
 	b->uh.source = b->s_in6.sin6_port;
 	b->uh.dest = htons(dstport);
-- 
2.40.1