netfilter: nf_nat_sip: fix RTP/RTCP source port translations

Each media stream negotiation between 2 SIP peers will trigger creation
of 4 different expectations (2 RTP and 2 RTCP):
 - INVITE will create expectations for the media packets sent by the
   called peer
 - reply to the INVITE will create expectations for media packets sent
   by the caller

The dport used by these expectations usually match the ones selected
by the SIP peers, but they might get translated due to conflicts with
another expectation. When such event occur, it is important to do
this translation in both directions, dport translation on the receiving
path and sport translation on the sending path.

This commit fixes the sport translation when the peer requiring it is
also the one that starts the media stream. In this scenario, first media
stream packet is forwarded from LAN to WAN and will rely on
nf_nat_sip_expected() to do the necessary sport translation. However, the
expectation matched by this packet does not contain the necessary information
for doing SNAT, this data being stored in the paired expectation created by
the sender's SIP message (INVITE or reply to it).

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_sip.c

@@ -18,6 +18,7 @@
 
 #include <net/netfilter/nf_nat.h>
 #include <net/netfilter/nf_nat_helper.h>
+#include <net/netfilter/nf_conntrack_core.h>
 #include <net/netfilter/nf_conntrack_helper.h>
 #include <net/netfilter/nf_conntrack_expect.h>
 #include <net/netfilter/nf_conntrack_seqadj.h>
@@ -316,6 +317,9 @@ static void nf_nat_sip_seq_adjust(struct sk_buff *skb, unsigned int protoff,
 static void nf_nat_sip_expected(struct nf_conn *ct,
 				struct nf_conntrack_expect *exp)
 {
+	struct nf_conn_help *help = nfct_help(ct->master);
+	struct nf_conntrack_expect *pair_exp;
+	int range_set_for_snat = 0;
 	struct nf_nat_range2 range;
 
 	/* This must be a fresh one. */
@@ -327,15 +331,42 @@ static void nf_nat_sip_expected(struct nf_conn *ct,
 	range.min_addr = range.max_addr = exp->saved_addr;
 	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
 
-	/* Change src to where master sends to, but only if the connection
-	 * actually came from the same source. */
-	if (nf_inet_addr_cmp(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3,
+	/* Do media streams SRC manip according with the parameters
+	 * found in the paired expectation.
+	 */
+	if (exp->class != SIP_EXPECT_SIGNALLING) {
+		spin_lock_bh(&nf_conntrack_expect_lock);
+		hlist_for_each_entry(pair_exp, &help->expectations, lnode) {
+			if (pair_exp->tuple.src.l3num == nf_ct_l3num(ct) &&
+			    pair_exp->tuple.dst.protonum == ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.dst.protonum &&
+			    nf_inet_addr_cmp(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3, &pair_exp->saved_addr) &&
+			    ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u.all == pair_exp->saved_proto.all) {
+				range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
+				range.min_proto.all = range.max_proto.all = pair_exp->tuple.dst.u.all;
+				range.min_addr = range.max_addr = pair_exp->tuple.dst.u3;
+				range_set_for_snat = 1;
+				break;
+			}
+		}
+		spin_unlock_bh(&nf_conntrack_expect_lock);
+	}
+
+	/* When no paired expectation has been found, change src to
+	 * where master sends to, but only if the connection actually came
+	 * from the same source.
+	 */
+	if (!range_set_for_snat &&
+	    nf_inet_addr_cmp(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3,
 			     &ct->master->tuplehash[exp->dir].tuple.src.u3)) {
 		range.flags = NF_NAT_RANGE_MAP_IPS;
 		range.min_addr = range.max_addr
 			= ct->master->tuplehash[!exp->dir].tuple.dst.u3;
-		nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);
+		range_set_for_snat = 1;
 	}
+
+	/* Perform SRC manip. */
+	if (range_set_for_snat)
+		nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);
 }
 
 static unsigned int nf_nat_sip_expect(struct sk_buff *skb, unsigned int protoff,