The DNS-SRV allows administrators to use several servers for a single domain, to move services from host to host with little fuss, and to designate some hosts as primary servers for a service and others as backups. For some reason the “host” with the highest priority cannot be reached, the SIP phone or proxy trying to reach the user within the domain will attempt to reach the next host defined within the SRV record.
In short, DNS-SRV allows users to have uninterrupted phone service when one of the call servers is temporary unavailable, for example sometimes the server is needed to be taken offline for maintenance.
If DNS-SRV is selected for transport type by R2X, it will be sending NAPTR lookup to obtain SIP server IP address, port, and transport type. It will follow the following procedure.
1. Sending NAPTR lookup
2. If the server does not respond with NAPRT record, it will then send out DNS SRV to check if the any UDP, TCP, or TLD exists.
3. When receive respond from SRV request, the top priority address will be selected.
How to configure in R27:
1. Go to Account-Basic.
2. Enable a SIP account.
3. Fill in the account settings.
4. Fill in the SIP server domain.
5. Select the transport type as DNS-SRV.
Users can refer to RFC3263 for more information about DNS-SRV.