udp: Translate offender addresses for ICMP messages

We've recently added support for propagating ICMP errors related to a UDP
flow from the host to the guest, by handling the extended UDP error on the
socket and synthesizing a suitable ICMP on the tap interface.

Currently we create that ICMP with a source address of the "offender" from
the extended error information - the source of the ICMP error received on
the host.  However, we don't translate this address for cases where we NAT
between host and guest.  This means (amongst other things) that we won't
get a "Connection refused" error as expected if send data from the guest to
the --map-host-loopback address.  The error comes from 127.0.0.1 on the
host, which doesn't make sense on the tap interface and will be discarded
by the guest.

Because ICMP errors can be sent by an intermediate host, not just by the
endpoints of the flow, we can't handle this translation purely with the
information in the flow table entry.  We need to explicitly translate this
address by our NAT rules, which we can do with the nat_inbound() helper.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

3 files changed, 19 insertions, 6 deletions

diff --git a/fwd.c b/fwd.c
index 5c70e83..b73c2c8 100644
--- a/fwd.c
+++ b/fwd.c
@@ -450,8 +450,8 @@ uint8_t fwd_nat_from_splice(const struct ctx *c, uint8_t proto,
  * Only handles translations that depend *only* on the address.  Anything
  * related to specific ports or flows is handled elsewhere.
  */
-static bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
-			 union inany_addr *translated)
+bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
+		 union inany_addr *translated)
 {
 	if (!IN4_IS_ADDR_UNSPECIFIED(&c->ip4.map_host_loopback) &&
 	    inany_equals4(addr, &in4addr_loopback)) {
diff --git a/fwd.h b/fwd.h
index 3562f3c..0458a3c 100644
--- a/fwd.h
+++ b/fwd.h
@@ -7,6 +7,7 @@
 #ifndef FWD_H
 #define FWD_H
 
+union inany_addr;
 struct flowside;
 
 /* Number of ports for both TCP and UDP */
@@ -47,6 +48,8 @@ void fwd_scan_ports_udp(struct fwd_ports *fwd, const struct fwd_ports *rev,
 			const struct fwd_ports *tcp_rev);
 void fwd_scan_ports_init(struct ctx *c);
 
+bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
+		 union inany_addr *translated);
 uint8_t fwd_nat_from_tap(const struct ctx *c, uint8_t proto,
 			 const struct flowside *ini, struct flowside *tgt);
 uint8_t fwd_nat_from_splice(const struct ctx *c, uint8_t proto,
diff --git a/udp.c b/udp.c
index d09b3eb..f5a5cd1 100644
--- a/udp.c
+++ b/udp.c
@@ -539,10 +539,10 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx,
 		.msg_control = buf,
 		.msg_controllen = sizeof(buf),
 	};
-	const struct flowside *toside;
+	const struct flowside *fromside, *toside;
+	union inany_addr offender, otap;
 	char astr[INANY_ADDRSTRLEN];
 	char sastr[SOCKADDR_STRLEN];
-	union inany_addr offender;
 	const struct in_addr *o4;
 	in_port_t offender_port;
 	struct udp_flow *uflow;
@@ -602,6 +602,7 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx,
 
 	uflow = udp_at_sidx(sidx);
 	ASSERT(uflow);
+	fromside = &uflow->f.side[sidx.sidei];
 	toside = &uflow->f.side[!sidx.sidei];
 	topif = uflow->f.pif[!sidx.sidei];
 	dlen = rc;
@@ -614,15 +615,24 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx,
 		/* XXX Can we support any other cases? */
 		goto fail;
 
+	/* If the offender *is* the endpoint, make sure our translation is
+	 * consistent with the flow's translation.  This matters if the flow
+	 * endpoint has a port specific translation (like --dns-match).
+	 */
+	if (inany_equals(&offender, &fromside->eaddr))
+		otap = toside->oaddr;
+	else if (!nat_inbound(c, &offender, &otap))
+		goto fail;
+
 	if (hdr->cmsg_level == IPPROTO_IP &&
-	    (o4 = inany_v4(&offender)) && inany_v4(&toside->eaddr)) {
+	    (o4 = inany_v4(&otap)) && inany_v4(&toside->eaddr)) {
 		dlen = MIN(dlen, ICMP4_MAX_DLEN);
 		udp_send_tap_icmp4(c, ee, toside, *o4, data, dlen);
 		return 1;
 	}
 
 	if (hdr->cmsg_level == IPPROTO_IPV6 && !inany_v4(&toside->eaddr)) {
-		udp_send_tap_icmp6(c, ee, toside, &offender.a6, data, dlen,
+		udp_send_tap_icmp6(c, ee, toside, &otap.a6, data, dlen,
 				   FLOW_IDX(uflow));
 		return 1;
 	}