Hi Mohammad!

If I setup an UA with URL sip:beeplove@mydomain.org my client is going to find its sip server from DNS SRV record, right?

Yes.

If I have these in nameserver _sip._udp SRV 0 0 5060 sip.mydomain.org. _sip._tcp SRV 0 0 5060 sip.mydomain.org.

is that mean, sip.mydomain.org is my registar server? or proxy server or both?

First, it just means it's a SIP Proxy. What it does is a different thing and can't be determined by the SRV record alone. It could be e.g. your entry proxy which passes REGISTERS on to another proxy being the registrar and passing INVITES to a server farm.

And If my UA have an option to specify proxy and registar server, UA is not going to lookup SRV record, right?

Yes, the SRV entries are only looked up by an external UA or proxy if it wants to determine the proxy it'll have to send, e.g., its INVITE to in order to reach your UA. It would strip the host-part of its r-uri and do a SRV lookup for _sip._udp.mydomain.org. It's similar to SMTP: your own mailclient gets the full DNS name (A record) of your SMTP-server, but other SMTPs are looking up the MX-record of your domain (SRV records are a means of providing "MX-records", so to speak, for different services)

Alex Mack