SRV records

Module 5: Other records · Reading time: 6 minutes

Learning objective: After this lesson, you'll be able to read an SRV record and fill in the fields correctly.

Key concepts: SRV, name structure _service._proto, priority, weight, port, target

What's an SRV record for?

An A record tells you where a name lives. An MX record tells you where mail for a domain should go. But how does a programme for telephony, chat or calendars know on which server and which port the service for your domain runs? That's what the SRV record (from service) is for, described in RFC 2782.

You'll mainly come across SRV records with:

• business telephony and chat (SIP, and older setups of Microsoft Teams and Skype for Business with _sip._tls and _sipfederationtls._tcp),
• email programmes that automatically look for settings (_imaps._tcp, _submission._tcp, _autodiscover._tcp),
• calendars and contacts (_caldavs._tcp, _carddavs._tcp),
• game servers such as Minecraft (_minecraft._tcp).

Usually you'll get the exact values from the service. You don't need to work them out yourself, but you do need to copy them correctly.

What an SRV record looks like

_sip._tls.example.com.  3600  IN  SRV  100 1 443 sipdir.provider.example.

The name consists of three parts:

The value consists of four parts, always in this order:

The target must be a name with its own A or AAAA record, not an IP address, and according to the standard, not a CNAME either. A target of just a dot (.) means: "this service is explicitly unavailable here".

Multiple servers

Just as with MX, you can create multiple SRV records for the same service:

_imaps._tcp.example.com.  3600 IN SRV  10 60 993 imap1.example.com.
_imaps._tcp.example.com.  3600 IN SRV  10 40 993 imap2.example.com.
_imaps._tcp.example.com.  3600 IN SRV  20  0 993 reserve.example.com.

Read this as: try the two servers with priority 10 first, where imap1 gets roughly 60% of connections and imap2 gets 40%. If neither is reachable, try reserve.

How control panels show the fields

The SRV record is the record where control panels differ most from each other. Some panels have separate fields for service, protocol, priority, weight, port and target. Others have a single name field and a single value field. Three things often go wrong:

1. The domain name appearing twice in the name. If you enter _sip._tls.example.com as the name, while the panel adds your domain name itself afterwards, you end up with _sip._tls.example.com.example.com. In that case, only enter _sip._tls.
2. Forgetting the underscores. sip.tls is a regular subdomain name and won't be found by any programme.
3. The fields in the wrong order in a value field. The order is always priority, weight, port, target.

After saving, always check with a lookup tool (lesson 8.2) that the record is listed under exactly the right name.

In practice: what it looks like in a control panel

As an example, we'll use the DNS screen in My WebOké, which has a help window for SRV records. In DirectAdmin, cPanel, Cloudflare and most registrars, you fill in the same six pieces of information, sometimes on a single line.

1. Click Add Record and choose SRV.
2. Enter the service (_sip) and choose the protocol (_tls).
3. Enter the priority, weight and port as specified by the service.
4. For the target, enter the server name, with a dot at the end.
5. Save.

Summary

• An SRV record tells you where and on which port a particular service for your domain runs.
• The name is _service._proto.name, with underscores.
• The value is always: priority, weight, port, target.
• Lowest priority first; with equal priority, the weight distributes the traffic.
• Copy the values from the service exactly, and check that the name hasn't been duplicated.

Quiz

Exercise

Exercise Z5.1: Looking up an SRV record. Gmail publishes an SRV record that lets email programmes find the IMAP server. Look up the SRV record for _imaps._tcp.gmail.com and fill in the port and target.

Exercise O5.1: Setting it up yourself in your practice zone. Create two records in your practice zone:

• an A record sip with the value 192.0.2.50 (the "server" for the service);
• an SRV record for the service _sip, protocol _tls, with priority 100, weight 1, port 443 and target sip.<code>.dnspractice.eu. (your own practice zone, with the dot at the end).

In the practice zone, the SRV record points to a fake server in your own zone; for your real domain, enter the target that the real service specifies. Then click Check.

Further reading

This article describes the steps in the WebOké panel; it works similarly in other panels.

• WebOké manual 5.5: DNS helpers: MX, SPF, DMARC, CAA and SRV
• DirectAdmin guide 5.1: What is DNS and when do you change it?