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Kamailio I-CSCF – SRV Lookup Behaviour

Recently I had a strange issue I thought I’d share.

Using Kamailio as an Interrogating-CSCF, Kamailio was getting the S-CSCF details from the User-Authorization-Answer’s “Server-Name” (602) AVP.

The value was set to:

sip:scscf.mnc001.mcc001.3gppnetwork.org:5060

But the I-CSCF was only looking up A-Records for scscf.mnc001.mcc001.3gppnetwork.org, not using DNS-SRV.

The problem? The Server-Name I had configured as a full SIP URI in PyHSS including the port, meant that Kamailio only looks up the A-Record, and did not do a DNS-SRV lookup for the domain.

Dropping the port number saw all those delicious SRV records being queried.

Something to keep in mind if you use S-CSCF pooling with a Kamailio based I-CSCF, if you want to use SRV records for load balancing / traffic sharing, don’t include the port, and if instead you want it to go to the specified host found by an A-record, include the port.

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