### 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:+1234567...@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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