27 Nov
2019
27 Nov
'19
12:56 p.m.
### Description
When relaying an `INVITE` from a Kamailio proxy to an interconnect we are using DNS SRV
records for load-balancing and failover. The proxy is listening on both a private
interface and a public interface with an rfc1918 (ie. `10.0.0.14`) and a public IP address
(ie. `185.0.0.34`) respectively.
The first branch (before DNS SRV failover) is working as expected. The message will be
relayed from the received socket (`185.0.0.34`) to the interconnect. When this branch
results in a timeout, the proxy will try to do DNS SRV failover. This new branch and any
subsequent branches will no longer use the initial received socket as source. In our case
we see that the private address (`10.0.0.14`) is now being used as source address.
### Troubleshooting
#### Reproduction
DNS SRV:
```
_sip._udp.transit.net. SRV 10 10 5060 transit1.net.
_sip._udp.transit.net. SRV 20 10 5060 transit2.net.
transit1.net. A 185.10.20.30
transit2.net. A 185.10.20.31
```
Kamailio:
```
$du = "sip:transit.net;transport=udp";
xinfo("Relaying [$rm] request: [$ru] with Call-ID [$ci]");
t_set_fr(0, 1000);
if (not t_relay()) {
sl_reply_error();
}
```
Network flow:
```
12:30:00 INVITE udp:10.0.0.18:5060 => udp:185.0.0.34:5060 (internal request to proxy)
12:30:00 INVITE udp:185.0.0.34:5060 => udp:185.10.20.30:5060 (relaying from proxy to
interconnect)
^^^^^^^^^^
(request times out after 1 second, proxy will do a failover to the next endpoint)
12:30:01 INVITE udp:10.0.0.14:5060 => udp:185.10.20.31:5060 (relaying to next
interconnect address)
^^^^^^^^^
```
#### Log Messages
Attempt to see where it goes wrong:
```
onsend_route {
xinfo("[$RAut] [$Rut] [$sas]\n");
xinfo("$snd(buf)\n");
}
```
```
INFO: [sip:185.0.0.34:5060;transport=udp] [sip:185.0.0.34:5060;transport=udp]
[udp:10.0.0.18:5060]
INFO: INVITE sip:+1234567890@transit.net;user=phone SIP/2.0#015#012Record-Route:
<sip:185.0.0.34;lr;ftag=tDr7m6erX1N3D>#015#012Via: SIP/2.0/UDP
10.0.0.14;branch=z9hG4bKafe7.7fb590e263fa44677514193a6a1156ce.1#015#012Via: SIP/2.0/UDP
10.0.0.18;received=10.0.0.18;rport=5060;branch=z9hG4bK6t59a17N60FcB ...
```
So the `Record-Route` seems to be correct, but the top most `Via` header shows the private
IP address. The message is being sent from the private IP address as well and never
reaches the second address from the interconnect.
### Possible Solutions
A workaround to add `$fs = "udp:185.0.0.34:5060"` in the `onsend_route` seems to
be effective.
### Additional Information
* **Kamailio Version** - output of `kamailio -v`
```
version: kamailio 5.2.3 (x86_64/linux)
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, 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_URI_SIZE 1024, BUF_SIZE 65535,
DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: unknown
compiled with gcc 7.4.0
```
* **Operating System**:
```
Ubuntu 18.04 LTS
Linux proxy4 4.15.0-64-generic #73-Ubuntu SMP Thu Sep 12 13:16:13 UTC 2019 x86_64 x86_64
x86_64 GNU/Linux
```
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