RFC 2782

Q: What does the Introduction section cover in RFC 2782 — DNS SRV?

The Introduction section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 2782 — DNS SRV. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the SRV Record Format section cover in RFC 2782 — DNS SRV?

The SRV Record Format section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 2782 — DNS SRV. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the 3GPP Usage section cover in RFC 2782 — DNS SRV?

The 3GPP Usage section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 2782 — DNS SRV. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the Selection Algorithm section cover in RFC 2782 — DNS SRV?

The Selection Algorithm section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 2782 — DNS SRV. This content is part of the BLACKLABS RFC Reference Library.

BLACKLABS

A DNS RR for specifying the location of services (DNS SRV)

Standards Track Obsoletes: RFC 2052

February 2000

Abstract: DNS SRV records specify the location (hostname and port) of servers for specific services and protocols. SRV records are used in 3GPP for Diameter peer discovery, SIP server location, and NRF-based NF discovery supplementation.

Canonical source: https://www.rfc-editor.org/rfc/rfc2782 | IETF Datatracker

1. Introduction

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SRV records extend DNS to provide service discovery. Unlike A/AAAA records that map hostnames to IPs, SRV records specify which host and port handle a particular service, with priority and weight for load balancing.

In 3GPP, SRV records are used for Diameter peer discovery (CER/CEA connection setup), SIP/IMS server location (RFC 3263), and as a fallback for NF discovery when NRF is unavailable.

2. SRV Record Format

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Field	Description
_service	Service name (e.g., _diameter, _sip, _http)
_protocol	Transport protocol (_tcp, _sctp, _udp)
priority	Lower value = higher priority (0-65535)
weight	Load balancing weight among same-priority servers
port	TCP/UDP/SCTP port number
target	FQDN of the server (must have A/AAAA records)

  _service._protocol.name  TTL  IN  SRV  priority weight port target
Examples:
_diameter._sctp.epc.mnc001.mcc208.3gppnetwork.org. 300 IN SRV 10 50 3868 dra1.epc...
_diameter._sctp.epc.mnc001.mcc208.3gppnetwork.org. 300 IN SRV 10 50 3868 dra2.epc...
_diameter._sctp.epc.mnc001.mcc208.3gppnetwork.org. 300 IN SRV 20 0  3868 dra3.epc...
_sip._tcp.ims.mnc001.mcc208.3gppnetwork.org. 300 IN SRV 10 100 5060 pcscf1.ims...

3. 3GPP Usage

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Diameter peer discovery: _diameter._sctp.realm for finding DRA/DEA peers
SIP/IMS: _sip._tcp, _sip._udp, _sips._tcp for P-CSCF/S-CSCF discovery
NAPTR + SRV: 3GPP uses NAPTR records to select transport, then SRV for specific servers
FQDN conventions: 3GPP defines canonical FQDN formats (TS 23.003 §19) for DNS entries

4. Selection Algorithm

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Group SRV records by priority (ascending — lowest first)
Within same priority, select by weighted random (higher weight = more likely)
If selected server fails, try next in weight order, then next priority group
Cache TTL should be respected — stale SRV records can cause misrouting