[PATCH/RFC] netfilter: nf_conntrack_sip: Handle quirky Cisco phones
From: Kevin Cernekee
Date: Sun Nov 14 2010 - 03:51:19 EST
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 issue was seen on a Cisco CP-7965G, firmware 8-5(3). It appears
to be a well-known problem on 7941 and newer:
http://www.voip-info.org/wiki/view/Standalone+Cisco+7941%252F7961+without+a+local+PBX
Search for "Connecting to the outside world"
I contacted Cisco support and they were not amenable to changing the
behavior. It appears to be RFC3261-compliant, as the "Sent-by port"
field in the request specifies 5060:
18.2.2 Sending Responses
The server transport uses the value of the top Via header field in
order to determine where to send a response. It MUST follow the
following process:
...
o Otherwise (for unreliable unicast transports), if the top Via
has a "received" parameter, the response MUST be sent to the
address in the "received" parameter, using the port indicated
in the "sent-by" value, or using port 5060 if none is specified
explicitly. If this fails, for example, elicits an ICMP "port
unreachable" response, the procedures of Section 5 of [4]
SHOULD be used to determine where to send the response.
This patch modifies nf_*_sip to work around this quirk, by rewriting
the response port to 5060 when the following conditions are met:
- User-Agent starts with "Cisco"
- Incoming TTL was exactly 64 (meaning that our system is the phone's
local router, not an intermediate router)
Tested on Linus' latest 2.6.37-rc tree.
Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx>
---
include/linux/netfilter/nf_conntrack_sip.h | 2 ++
net/ipv4/netfilter/nf_nat_sip.c | 12 ++++++++++++
net/netfilter/nf_conntrack_sip.c | 25 +++++++++++++++++++++++++
3 files changed, 39 insertions(+), 0 deletions(-)
diff --git a/include/linux/netfilter/nf_conntrack_sip.h b/include/linux/netfilter/nf_conntrack_sip.h
index 0ce91d5..a6ea620 100644
--- a/include/linux/netfilter/nf_conntrack_sip.h
+++ b/include/linux/netfilter/nf_conntrack_sip.h
@@ -8,6 +8,7 @@
struct nf_ct_sip_master {
unsigned int register_cseq;
unsigned int invite_cseq;
+ unsigned int cisco_port_mangle;
};
enum sip_expectation_classes {
@@ -90,6 +91,7 @@ enum sip_header_types {
SIP_HDR_EXPIRES,
SIP_HDR_CONTENT_LENGTH,
SIP_HDR_CALL_ID,
+ SIP_HDR_USER_AGENT,
};
enum sdp_header_types {
diff --git a/net/ipv4/netfilter/nf_nat_sip.c b/net/ipv4/netfilter/nf_nat_sip.c
index e40cf78..4b9a46d 100644
--- a/net/ipv4/netfilter/nf_nat_sip.c
+++ b/net/ipv4/netfilter/nf_nat_sip.c
@@ -121,6 +121,7 @@ static unsigned int ip_nat_sip(struct sk_buff *skb, unsigned int dataoff,
enum ip_conntrack_info ctinfo;
struct nf_conn *ct = nf_ct_get(skb, &ctinfo);
enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
+ struct nf_conn_help *help = nfct_help(ct);
unsigned int coff, matchoff, matchlen;
enum sip_header_types hdr;
union nf_inet_addr addr;
@@ -225,6 +226,17 @@ next:
return NF_DROP;
}
+ /* Mangle destination port for Cisco phones, then fix up checksums */
+ if (help->help.ct_sip_info.cisco_port_mangle) {
+ struct udphdr *uh;
+
+ uh = (struct udphdr *)(skb->data + ip_hdrlen(skb));
+ uh->dest = htons(SIP_PORT);
+
+ if (!nf_nat_mangle_udp_packet(skb, ct, ctinfo, 0, 0, NULL, 0))
+ return NF_DROP;
+ }
+
if (!map_sip_addr(skb, dataoff, dptr, datalen, SIP_HDR_FROM) ||
!map_sip_addr(skb, dataoff, dptr, datalen, SIP_HDR_TO))
return NF_DROP;
diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index bcf47eb..6042f66 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -18,6 +18,7 @@
#include <linux/udp.h>
#include <linux/tcp.h>
#include <linux/netfilter.h>
+#include <linux/ip.h>
#include <net/netfilter/nf_conntrack.h>
#include <net/netfilter/nf_conntrack_core.h>
@@ -338,6 +339,7 @@ static const struct sip_header ct_sip_hdrs[] = {
[SIP_HDR_EXPIRES] = SIP_HDR("Expires", NULL, NULL, digits_len),
[SIP_HDR_CONTENT_LENGTH] = SIP_HDR("Content-Length", "l", NULL, digits_len),
[SIP_HDR_CALL_ID] = SIP_HDR("Call-Id", "i", NULL, callid_len),
+ [SIP_HDR_USER_AGENT] = SIP_HDR("User-Agent", NULL, NULL, string_len),
};
static const char *sip_follow_continuation(const char *dptr, const char *limit)
@@ -1366,6 +1368,29 @@ static int process_sip_request(struct sk_buff *skb, unsigned int dataoff,
unsigned int matchoff, matchlen;
unsigned int cseq, i;
+ /* 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, flag the connection here so that
+ * responses will be redirected to the correct port.
+ */
+ do {
+ static const char cisco[] = "Cisco";
+ struct iphdr *iph = ip_hdr(skb);
+ struct nf_conn_help *help = nfct_help(ct);
+
+ if (iph->ttl != 63)
+ break;
+ if (ct_sip_get_header(ct, *dptr, 0, *datalen,
+ SIP_HDR_USER_AGENT, &matchoff, &matchlen) <= 0)
+ break;
+ if (matchlen < strlen(cisco))
+ break;
+ if (strnicmp(*dptr + matchoff, cisco, strlen(cisco)) != 0)
+ break;
+
+ help->help.ct_sip_info.cisco_port_mangle = 1;
+ } while (0);
+
for (i = 0; i < ARRAY_SIZE(sip_handlers); i++) {
const struct sip_handler *handler;
--
1.7.0.4
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