On Thursday 09 June 2011 12:44:11 Iñaki Baz Castillo wrote:
According to NAPTR:
~$ host -t naptr oversip.net oversip.net has NAPTR record 5 50 "S" "SIPS+D2T" "" _sips._tcp.oversip.net. oversip.net has NAPTR record 10 50 "S" "SIP+D2T" "" _sip._tcp.oversip.net. oversip.net has NAPTR record 20 50 "S" "SIP+D2U" "" _sip._udp.oversip.net. oversip.net has NAPTR record 40 50 "S" "SIP+D2S" "" _sip._sctp.oversip.net. oversip.net has NAPTR record 50 50 "S" "SIPS+D2S" "" _sips._sctp.oversip.net.
So it should try TLS over TCP first, if it fails try TCP and if it fails try UDP.
However it just uses UDP, why?? Even if I set a minor value to dns_tls_preference (so higher priority I expect) it still uses UDP.
The way I read rfc2915, there is no failover mechanism. The application pick the first target that it supports and uses that. There is no mention of trying other records afterwards. Matching/finding NAPTR records stops once the first match is completed. All other records are discarded. From section 2:
Order A 16-bit unsigned integer specifying the order in which the NAPTR records MUST be processed to ensure the correct ordering of rules. Low numbers are processed before high numbers, and once a NAPTR is found whose rule "matches" the target, the client MUST NOT consider any NAPTRs with a higher value for order (except as noted below for the Flags field).
Note the last sentence.