Hello,

I use DNS failover feature for routing some calls, and I wonder about its limitations.

I noticed that INVITE and ReINVITE requests are correctly routed based on SRV priority/weight in case of failure, but ACK requests do not use them.

Let’s say I have Kamailio A relaying requests to a pool of Kamailios B1 and B2 in TLS.

DNS records are the below ones.

kamailiob.net.            60 NAPTR 20   100  S SIPS+D2T _sips._tcp.kamailiob.net.

_sips._tcp.kamailiob.net.   60 SRV 1    10   5061       kamailiob-1.net.    

_sips._tcp.kamailiob.net.   60 SRV 2    10   5061       kamailiob-2.net.

If B1 is down, I expect requests being relayed to B2 (because of priority 2 in the SRV record).

• For initial INVITEs: R-URI is sip:kamailiob.net
  • If B1 is down, the request is retried to B2 => OK
• For in-dialog requests: Route header in incoming request is sip:kamailiob.net so it is set as next destination with loose_route function
  • If B1 is down, the request is retried to B2 for ReINVITE => OK
  • But once the ACK comes in, with the same Route header (sip:kamailiob.net), Kamailio tries to send it to B1 only (no retries to B2, and even no retransmissions to B1 either).
  • When B1 is back UP again a few seconds later, Kamailio does not try to relay the ACK again. The ACK request is attempted to be retransmitted only once to B1. It fails and no more retries after that.

Is it the expected behavior? Or is it something misconfigured on Kamailio A? Or is it a TLS connection issue?

Here is the DNS configuration on Kamailio A (TM module is enabled):

dns_try_naptr=yes

use_dns_failover=yes

use_dns_cache=yes

dns_srv_lb=yes

enable_tls=yes

tcp_max_connections=4096

tls_max_connections=4096

tcp_reuse_port=yes

tcp_connect_timeout=10

tcp_send_timeout=10

tcp_connection_lifetime=3600

I had a look into the DNS tutorial (https://github.com/kamailio/kamailio/blob/master/doc/tutorials/dns.txt) without finding any hint about this.

If anyone has already played with SIP DNS failover in Kamailio, your help would be appreciated, thanks!

Julien