SRV Records Explained

## SRV Records Explained An SRV Record (Service record) is a DNS record that points to a specific server and port for a given service and protocol. Unlike A or MX records that map hostnames to IP addresses, SRV records encode connection parameters — including port numbers — directly in DNS. Applications use SRV records to discover where a service lives without hardcoding server addresses or ports into clients. ## Why SRV Records Exist Standard DNS records answer "what IP address is `mail.example.com`?" but they cannot answer "what port does the XMPP service run on for `example.com`?" SRV records solve this by encoding service name, protocol, priority, weight, port, and target hostname into a single record. This makes service discovery automatic: a SIP phone client, XMPP client, or game server can query DNS and learn exactly where and how to connect — no manual configuration needed. ## SRV Record Format SRV records use a special naming convention: ``` _service._protocol.name TTL IN SRV priority weight port target ``` Example: ``` _xmpp-client._tcp.example.com. 3600 IN SRV 10 50 5222 chat.example.com. ``` Breaking this down: | Field | Value | Meaning | |-------|-------|---------| | Name | `_xmpp-client._tcp.example.com` | Service + protocol + domain | | Priority | `10` | Lower = preferred (like MX) | | Weight | `50` | Load-balancing within same priority | | Port | `5222` | TCP port to connect to | | Target | `chat.example.com` | Hostname of the server | ### Service and Protocol Labels The `_service` prefix uses standardized names from the IANA Service Name Registry. Common values: | Service | Protocol | Meaning | |---------|---------|---------| | `_sip` | `_tcp` or `_udp` | VoIP/SIP | | `_xmpp-client` | `_tcp` | XMPP client connections | | `_xmpp-server` | `_tcp` | XMPP server-to-server | | `_autodiscover` | `_tcp` | Outlook/Exchange autodiscover | | `_imaps` | `_tcp` | IMAP over SSL | | `_submission` | `_tcp` | SMTP email submission | | `_minecraft` | `_tcp` | Minecraft Java Edition | | `_stun` | `_udp` | STUN (VoIP NAT traversal) | | `_turn` | `_tcp` | TURN (media relay) | ## Priority and Weight **Priority** works identically to MX record priority: lower number = higher preference. Clients try the lowest-priority SRV target first. **Weight** is used for load distribution among records with the *same* priority. Clients pick targets proportionally by weight: ``` _sip._tcp.example.com. SRV 10 70 5060 sip1.example.com. _sip._tcp.example.com. SRV 10 30 5060 sip2.example.com. ``` Here both have priority 10. `sip1` gets ~70% of connections, `sip2` gets ~30%. This is weighted round-robin load balancing via DNS. If all SRV records in a priority group are unreachable, clients fall back to the next priority group. ## Microsoft 365 / Skype for Business SRV Records Microsoft 365 requires SRV records for Skype for Business and Teams autodiscover: ``` _sip._tls.example.com. SRV 100 1 443 sipdir.online.lync.com. _sipfederationtls._tcp.example.com. SRV 100 1 5061 sipfed.online.lync.com. ``` These values are provided in the Microsoft 365 admin center under Domains and must be added exactly as shown. ## Autodiscover for Outlook (Exchange) Exchange and Office 365 use both CNAME and SRV records for autodiscover: ``` ; Preferred: CNAME approach autodiscover.example.com. CNAME autodiscover.outlook.com. ; Fallback: SRV approach _autodiscover._tcp.example.com. SRV 10 10 443 autodiscover.outlook.com. ``` Outlook clients first try the CNAME; if that fails, they fall back to the SRV record. ## Minecraft Java Edition Minecraft servers can use SRV records so players connect using a clean domain name without specifying a port: ``` _minecraft._tcp.play.example.com. SRV 0 5 25565 mc.example.com. ``` Players connect to `play.example.com` and the Minecraft client resolves the SRV record to find the actual server and port. Without SRV, players would need to type `mc.example.com:25565`. ## XMPP / Jabber ``` _xmpp-client._tcp.example.com. SRV 5 0 5222 xmpp.example.com. _xmpp-server._tcp.example.com. SRV 5 0 5269 xmpp.example.com. ``` Two records are needed: one for client connections (port 5222) and one for server-to-server federation (port 5269). ## SIP / VoIP ``` _sip._udp.example.com. SRV 10 10 5060 sip.example.com. _sip._tcp.example.com. SRV 10 10 5060 sip.example.com. _sips._tcp.example.com. SRV 10 10 5061 sip.example.com. ``` Both UDP and TCP variants are common. `_sips` uses TLS (port 5061). ## The Target Hostname Must Have an A Record Like MX records, SRV targets must be hostnames that resolve to IP addresses — not IP addresses directly. Always ensure `chat.example.com`, `sip.example.com`, or whatever hostname you use has a corresponding A Record. ## Disabling a Service A SRV record with a period (`.`) as the target and `0` for port signals that the service is explicitly not available: ``` _sip._tcp.example.com. SRV 0 0 0 . ``` This prevents clients from searching further or timing out — they immediately know the service is unsupported. ## Verifying SRV Records ```bash dig _xmpp-client._tcp.example.com SRV dig _sip._tcp.example.com SRV ``` Use the DNS Record Helper tool to verify SRV records are visible globally and contain the correct port and target values. ## Next Steps - Setting Up DNS for Your Domain — full DNS configuration walkthrough - A Record vs CNAME: When to Use Each — how to handle the A records that SRV targets point to - MX Records: Setting Up Email for Your Domain — email routing, conceptually similar to SRV priority/weight