Support Library
SRV Records (Enterprise Use Cases)
How SRV records work: priority, weight, port and target, the enterprise SRV names you will meet, and the mistakes that break service discovery.
The short answer
An SRV record, defined in RFC 2782, tells a client which host and port provide a service for a domain. Clients that support it, such as SIP phones, XMPP chat, LDAP and Kerberos clients and Microsoft Active Directory, look it up instead of needing a server name typed in.
$ dig +short SRV _sips._tcp.example.com
10 60 5061 sip1.example.com.
10 40 5061 sip2.example.com.
20 0 5061 sip-backup.example.net.The name is _service._protocol.domain, and each answer gives a priority, a weight, a port and a target host.
Reading the fields
| Field | Meaning | In the example |
|---|---|---|
| Priority | Lower values are tried first. Higher values are fallbacks. | sip1 and sip2 first, sip-backup only if both fail |
| Weight | Shares load among targets with the same priority | About 60% of clients pick sip1, 40% sip2 |
| Port | The TCP or UDP port of the service | 5061, SIP over TLS |
| Target | The host name to connect to | Must have A or AAAA records; must not be a CNAME |
A target of . means the service is deliberately not offered at this domain.
Enterprise records you will meet
| Name | Used by |
|---|---|
| _ldap._tcp.dc._msdcs.example.com | Windows clients finding Active Directory domain controllers |
| _kerberos._tcp.example.com, _kerberos._udp.example.com | Kerberos clients finding a KDC |
| _sip._tls.example.com, _sipfederationtls._tcp.example.com | Microsoft-specific SIP client and federation discovery, including older Skype for Business setups; standard SIPS discovery uses _sips._tcp.example.com (RFC 3263) |
| _autodiscover._tcp.example.com | Outlook finding an Exchange Autodiscover endpoint when the usual host names are not available |
| _xmpp-client._tcp.example.com | XMPP chat clients |
| _caldavs._tcp.example.com, _carddavs._tcp.example.com | Calendar and contacts clients (RFC 6764) |
Records for Active Directory usually belong in internal DNS only. Publishing them in public DNS reveals domain controller names and does nothing useful for outside clients.
Common mistakes
- A target that is a CNAME or an IP address. RFC 2782 requires a host name with address records.
- A target that no longer exists after a server was retired.
- Wrong service or transport label. Standard SIP over TLS uses _sips._tcp; _sip._tls is a Microsoft-specific convention, not the standard TLS transport label.
- All targets at the same priority with weight 0 when you meant to send most traffic to one server.
How DNS Panopticon helps
Domain Visualizer's Advanced scan looks up five SRV names: _sip._tcp, _sip._tls, _autodiscover._tcp, _ldap._tcp and _kerberos._tcp, and shows any it finds with the rest of the records. Other service names, including standard _sips._tcp, are not queried; test those separately with dig. In a zone export, Zone Analyzer can filter on the SRV type to list them all.