<h3>Description</h3>
<p>Kamailio is using the wrong source port when relaying INVITE packets with t_relay(). Another interesting thing is that while doing this, Kamailio reports to Homer via HEPv2 that it sent the packet from port 4242, but a tcpdump on the server shows this to be false so maybe homer was just looking at the R-URI port?<br>
There is a somewhat random element to this, so it is very difficult to reproduce. I will provide the scenario which led to this problem.</p>
<h4>Reproduction</h4>
<p>This is the code block which is responsible for relaying INVITEs to phone from our PBX servers, it is also the only instance of port 5062 being inserted into a SIP message.</p>
<pre><code>if(is_method("INVITE"))
{
if(lookup("location", "sip:$rU@location"))
{
remove_hf("Contact");
append_hf("Contact: <sip:$sel(cfg_get.global.publicIP):5062>\r\n"); //A vendor's SIPIS server doesn't work on port 4242, does changing this port break SIP compliance?
rtpproxy_manage("cow");
t_relay();
exit;
}
}
</code></pre>
<ul>
<li>Kamailio listens for UDP+TCP on ports 5060,5062, and 4242.</li>
<li>Phone A registers to tcp:KamailioIP:4242 from tcp:CustomerIP:10042</li>
<li>Phone B registers to tcp:KamailioIP:4242 from tcp:CustomerIP:11042</li>
<li>These phones have been re-registering from and to the same ports for a long time without problems with the same REGISTER callID (same transaction so nothing about the connection has changed).</li>
<li>At this point we have TCP connections <code>tcp:CustomerIP:10042 -> tcp:KamailioIP:4242</code> and <code>tcp:CustomerIP:11042 -> tcp:KamailioIP:4242</code></li>
<li>Kamailio sends INVITEs from tcp:KamailioIP:4242 to the address of the registered phone with a contact header that tells the phones to send responses to tcp:KamailioIP:5062 (as seen in the above code block). This causes a new TCP connection to be made from the customer's router to Kamailio, just for this transaction.</li>
<li>All is well until phone A sends a response and the customer's router, by pure chance, decides to use port 11042 for the new TCP connection. <code>tcp:CustomerIP:11042 -> tcp:KamailioIP:5062</code></li>
<li>From this point on Kamailio begins to route INVITEs, that were meant for phone B, to phone A using this TCP connection <code>tcp:CustomerIP:11042 -> tcp:KamailioIP:5062</code> (source port 5062!) rather than the connection that phone B is actually registered to <code>tcp:CustomerIP:11042 -> tcp:KamailioIP:4242</code>.</li>
<li>The customer's router sends packets from this TCP socket to phone A because the connection identified by the Kamailio source port had been created by phone A.</li>
<li>Kamailio is still routing other types of packets to phone B from port 4242 like NOTIFY in response to a subscription and OPTIONs keep alives, but INVITEs (which are an entirely new SIP transaction) are being sent from the wrong source port.</li>
</ul>
<h3>Possible <del>Solutions</del> Problems</h3>
<p>I have the default tcp_accept_aliases=0, so ;alias in a Via header is not a problem. We are also not using force_send_socket.<br>
I've tried to figure out how Kamailio works, and I am wondering if this function might be adding the new TCP connection from phone A to port 5062 as an alias of the already existing TCP connection from phone B to port 4242.<br>
<a href="https://github.com/kamailio/kamailio/blob/cb7810c939da9c8f4385b530539487528ac8705d/src/core/tcp_main.c#L1287-L1300">https://github.com/kamailio/kamailio/blob/cb7810c939da9c8f4385b530539487528ac8705d/src/core/tcp_main.c#L1287-L1300</a></p>
<p>I also found these comments interesting.<br>
<a href="https://github.com/kamailio/kamailio/blob/52111974b4571e0562e8e731df80f48dbc504915/src/core/forward.c#L288-L290">https://github.com/kamailio/kamailio/blob/52111974b4571e0562e8e731df80f48dbc504915/src/core/forward.c#L288-L290</a><br>
<a href="https://github.com/kamailio/kamailio/blob/52111974b4571e0562e8e731df80f48dbc504915/src/core/forward.c#L76-L81">https://github.com/kamailio/kamailio/blob/52111974b4571e0562e8e731df80f48dbc504915/src/core/forward.c#L76-L81</a></p>
<h3>Additional Information</h3>
<ul>
<li><strong>Kamailio Version</strong> - output of <code>kamailio -v</code></li>
</ul>
<pre><code>version: kamailio 5.0.5 (x86_64/linux)
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: unknown
compiled on 04:38:13 Feb 2 2018 with gcc 4.8.5
</code></pre>
<ul>
<li><strong>Operating System</strong>:</li>
</ul>
<pre><code>Linux dallas-sip1-int 3.10.0-514.16.1.el7.x86_64 #1 SMP Wed Apr 12 15:04:24 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
</code></pre>
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