[PATCH] udp: Fix port and address checks for DNS forwarder - dev

10 Oct 2022

First off, as we swap endianness for source ports in
udp_fill_data_v{4,6}(), we want host endianness, not network
endianness. It doesn't actually matter if we use htons() or ntohs()
here, but the current version is confusing.

In the IPv4 path, when we remap DNS answers, we already swapped the
endianness as needed for the source port: don't swap it again,
otherwise we'll not map DNS answers for IPv4.

In the IPv6 path, when we remap DNS answers, we want to check that
they came from our upstream DNS server, not the one configured via
--dns-forward (which doesn't even need to exist for this
functionality to work).
---
 udp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/udp.c b/udp.c
index cac9c65..4b201d3 100644
--- a/udp.c
+++ b/udp.c
@@ -678,7 +678,7 @@ static void udp_sock_fill_data_v4(const struct ctx *c, int n,
 	b->iph.tot_len = htons(ip_len);
 
 	src = ntohl(b->s_in.sin_addr.s_addr);
-	src_port = htons(b->s_in.sin_port);
+	src_port = ntohs(b->s_in.sin_port);
 
 	if (src >> IN_CLASSA_NSHIFT == IN_LOOPBACKNET ||
 	    src == INADDR_ANY || src == ntohl(c->ip4.addr_seen)) {
@@ -693,7 +693,7 @@ static void udp_sock_fill_data_v4(const struct ctx *c, int n,
 
 		bitmap_set(udp_act[V4][UDP_ACT_TAP], src_port);
 	} else if (c->ip4.dns_fwd &&
-		   src == ntohl(c->ip4.dns[0]) && ntohs(src_port) == 53) {
+		   src == htonl(c->ip4.dns[0]) && src_port == 53) {
 		b->iph.saddr = c->ip4.dns_fwd;
 	} else {
 		b->iph.saddr = b->s_in.sin_addr.s_addr;
@@ -795,7 +795,7 @@ static void udp_sock_fill_data_v6(const struct ctx *c, int n,
 
 		bitmap_set(udp_act[V6][UDP_ACT_TAP], src_port);
 	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_fwd) &&
-		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns_fwd) && src_port == 53) {
+		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns[0]) && src_port == 53) {
 		b->ip6h.daddr = c->ip6.addr_seen;
 		b->ip6h.saddr = c->ip6.dns_fwd;
 	} else {
-- 
2.35.1


    