netfilter: nf_ct_sip: support Cisco 7941/7945 IP phones

Most SIP devices use a source port of 5060/udp on SIP requests, so the
response automatically comes back to port 5060:

    phone_ip:5060 -> proxy_ip:5060   REGISTER
    proxy_ip:5060 -> phone_ip:5060   100 Trying

The newer Cisco IP phones, however, use a randomly chosen high source
port for the SIP request but expect the response on port 5060:

    phone_ip:49173 -> proxy_ip:5060  REGISTER
    proxy_ip:5060 -> phone_ip:5060   100 Trying

Standard Linux NAT, with or without nf_nat_sip, will send the reply back
to port 49173, not 5060:

    phone_ip:49173 -> proxy_ip:5060  REGISTER
    proxy_ip:5060 -> phone_ip:49173  100 Trying

But the phone is not listening on 49173, so it will never see the reply.

This patch modifies nf_*_sip to work around this quirk by extracting
the SIP response port from the Via: header, iff the source IP in the
packet header matches the source IP in the SIP request.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/nf_conntrack_sip.h

@@ -4,12 +4,15 @@
 
 #include <net/netfilter/nf_conntrack_expect.h>
 
+#include <linux/types.h>
+
 #define SIP_PORT	5060
 #define SIP_TIMEOUT	3600
 
 struct nf_ct_sip_master {
 	unsigned int	register_cseq;
 	unsigned int	invite_cseq;
+	__be16		forced_dport;
 };
 
 enum sip_expectation_classes {

net/netfilter/nf_conntrack_sip.c

@@ -1440,8 +1440,25 @@ static int process_sip_request(struct sk_buff *skb, unsigned int protoff,
 {
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
+	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);
+	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
 	unsigned int matchoff, matchlen;
 	unsigned int cseq, i;
+	union nf_inet_addr addr;
+	__be16 port;
+
+	/* Many Cisco IP phones use a high source port for SIP requests, but
+	 * listen for the response on port 5060.  If we are the local
+	 * router for one of these phones, save the port number from the
+	 * Via: header so that nf_nat_sip can redirect the responses to
+	 * the correct port.
+	 */
+	if (ct_sip_parse_header_uri(ct, *dptr, NULL, *datalen,
+				    SIP_HDR_VIA_UDP, NULL, &matchoff,
+				    &matchlen, &addr, &port) > 0 &&
+	    port != ct->tuplehash[dir].tuple.src.u.udp.port &&
+	    nf_inet_addr_cmp(&addr, &ct->tuplehash[dir].tuple.src.u3))
+		ct_sip_info->forced_dport = port;
 
 	for (i = 0; i < ARRAY_SIZE(sip_handlers); i++) {
 		const struct sip_handler *handler;

net/netfilter/nf_nat_sip.c

@@ -95,6 +95,7 @@ static int map_addr(struct sk_buff *skb, unsigned int protoff,
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
 	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);
 	char buffer[INET6_ADDRSTRLEN + sizeof("[]:nnnnn")];
 	unsigned int buflen;
 	union nf_inet_addr newaddr;
@@ -107,7 +108,8 @@ static int map_addr(struct sk_buff *skb, unsigned int protoff,
 	} else if (nf_inet_addr_cmp(&ct->tuplehash[dir].tuple.dst.u3, addr) &&
 		   ct->tuplehash[dir].tuple.dst.u.udp.port == port) {
 		newaddr = ct->tuplehash[!dir].tuple.src.u3;
-		newport = ct->tuplehash[!dir].tuple.src.u.udp.port;
+		newport = ct_sip_info->forced_dport ? :
+			  ct->tuplehash[!dir].tuple.src.u.udp.port;
 	} else
 		return 1;
 
@@ -144,6 +146,7 @@ static unsigned int nf_nat_sip(struct sk_buff *skb, unsigned int protoff,
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
 	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);
 	unsigned int coff, matchoff, matchlen;
 	enum sip_header_types hdr;
 	union nf_inet_addr addr;
@@ -258,6 +261,21 @@ next:
 	    !map_sip_addr(skb, protoff, dataoff, dptr, datalen, SIP_HDR_TO))
 		return NF_DROP;
 
+	/* Mangle destination port for Cisco phones, then fix up checksums */
+	if (dir == IP_CT_DIR_REPLY && ct_sip_info->forced_dport) {
+		struct udphdr *uh;
+
+		if (!skb_make_writable(skb, skb->len))
+			return NF_DROP;
+
+		uh = (void *)skb->data + protoff;
+		uh->dest = ct_sip_info->forced_dport;
+
+		if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo, protoff,
+					      0, 0, NULL, 0))
+			return NF_DROP;
+	}
+
 	return NF_ACCEPT;
 }
 
@@ -311,8 +329,10 @@ static unsigned int nf_nat_sip_expect(struct sk_buff *skb, unsigned int protoff,
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
 	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+	struct nf_ct_sip_master *ct_sip_info = nfct_help_data(ct);
 	union nf_inet_addr newaddr;
 	u_int16_t port;
+	__be16 srcport;
 	char buffer[INET6_ADDRSTRLEN + sizeof("[]:nnnnn")];
 	unsigned int buflen;
 
@@ -326,8 +346,9 @@ static unsigned int nf_nat_sip_expect(struct sk_buff *skb, unsigned int protoff,
 	/* If the signalling port matches the connection's source port in the
 	 * original direction, try to use the destination port in the opposite
 	 * direction. */
-	if (exp->tuple.dst.u.udp.port ==
-	    ct->tuplehash[dir].tuple.src.u.udp.port)
+	srcport = ct_sip_info->forced_dport ? :
+		  ct->tuplehash[dir].tuple.src.u.udp.port;
+	if (exp->tuple.dst.u.udp.port == srcport)
 		port = ntohs(ct->tuplehash[!dir].tuple.dst.u.udp.port);
 	else
 		port = ntohs(exp->tuple.dst.u.udp.port);