Many service providers rely on DNS SRV records for failover and resiliency purposes.
Using DNS SRV records, compatible equipment (such as SIP phones and SIP PBX servers) can map several SIP servers to a single SIP domain. Each SIP server can be assigned a priority, and if the server 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.
For example, the SIP domain outboundproxy.com uses DNS SRV records to point to geographically diverse SIP servers located in New York and Dallas. If the New York servers is unavailable, then compatible equipment will automatically failover to the Dallas server. More information about DNS SRV records is available at voip-info.org.
As an end user, it may not be obvious that a SIP domain is using SRV records.
At one time, performing a DNS SRV lookup required knowledge of the Unix command dig, a tool used to interrogate DNS name servers, performs DNS lookups, and display the answers that are returned from the name server(s) that were queried. A typical dig query might appear as follows:
Because dig is a Linux tool, performing a DNS SRV lookup was not easy for Windows users and users unfamiliar with Unix commands. However, it has become easier to perform a DNS SRV lookup using any of several available online portals. Although there are many online DNS lookup tools, there are not many that will allow you to search for DNS SRV records. Two of the portals that I rely on for this purpose are http://centralops.net/co/nslookup.aspx and http://www.digwebinterface.com.
Using a DNS lookup tool allows you to enter a SIP domain name into the web search form, and see all of the SIP servers that are configured. The results are often quite interesting (if you are curious about that sort of thing).
This is a two-step process. First you must lookup the DNS pointer (PTR) record for the domain. The result of this lookup will tell you if the domain is advertising any SIP services. Then you must lookup the service domain that was contained in the pointer record.
For example, looking up all the DNS services for sip.outboundproxy.com will yeild a name pointer (PTR) record showing that there is an SRV record available. The result shown below (taken from http://centralops.net/co/nslookup.aspx) tells us that a SIP service record exists.
Not all SIP service providers bother to publish a pointer (PTR) record since it is just as easy to search for the service (SRV) record straight away. To search for a SIP SRV record, just use the prefix ‘_sip._udp.’ or ‘_sip._tcp.’ (depending on whether you are interested in using UDP or TCP for the SIP connection) before the domain name.
For ample, to search for the SRV records of sip.outboundproxy.com, you would need to enter _sip._udp.sip.outboundproxy.com. The results from three different DNS online lookup tools are shown in the images below.