Some application clients use SRV lookups, a few (to their embarrassment) do not.

SRV record support hasn't even made it into web browsers yet, let alone clients of less-common protocols.

This is the Frequently Given Answer to such statements.

In fact, it is the other way around. The clients of the less-common protocols sometimes make good use of SRV lookups. Indeed, SRV lookups are in widespread real-world successful use, in systems such as Microsoft Windows NT Server networking.

It is the HTTP clients (web browsers and proxy HTTP servers) that are, to their embarrassment and shame, and despite the efforts of Rick van Rein and others, lagging behind here.

Application clients that use SRV lookups

Clients of many services do make use of SRV lookups.

NICNAME clients that use SRV lookups

Old NICNAME clients come with lengthy configuration files that associate domain names with NICNAME servers, which eventually become out of date. In contrast, Modern NICNAME (a.k.a. "whois") clients make (_nicname._tcp) SRV resource record set lookups to find the correct NICNAME server, and need no such configuration files.

The modern mechanism for the location of NICNAME servers is supported by a wide range of registries, who publish SRV resource records that give the locations of their NICNAME servers. The registries that support modern NICNAME clients include, amongst others:

• Nominet ( co.uk., ltd.uk., me.uk., net.uk., nic.uk., org.uk., plc.uk., sch.uk., and uk.)

• AFNIC (fr. and re.)

• ARNES (si.)

• AUNIC (au.)

• DENIC (de.)

• DK-Hostmaster (dk.)

• FreeDNS (eu.org.)

• IEDR (ie.)

• ISNIC (is.)

• NASK (pl.)

• NIC-SE (se.)

• NICAT (at.)

• NORID (no.)

• RESTENA (lu.)

• SIDN (nl.)

• SWITCH (ch. and li.)

LDAP clients that use SRV lookups

The LDAP clients in Microsoft and Linux softwares perform (_ldap._tcp) SRV resource record set lookups to locate LDAP servers. Microsoft Windows NT Server networking is one of the most widespread, but yet little known, examples of the successful use of SRV resource records.

SMTP clients that use SRV lookups

Several modern SMTP Relay and SMTP Submission clients use (_smtp._tcp and _submission._tcp) SRV resource record set lookups to locate servers. These include:

• the SMTP Relay clients (smtprc and etrn) in The Internet Utilities for OS/2

• the SMTP Submission client (sendmail) in The Internet Utilities for OS/2

• exim (as per its documentation)

HTTP clients that use SRV lookups

A few modern HTTP clients use (_http._tcp) SRV resource record set lookups to locate HTTP servers. These include:

• the various HTTP clients (wikiput, wikilogin, wikimove, and so forth) in The Internet Utilities for OS/2

NNTP clients that use SRV lookups

A few modern NNTP clients use (_nntp._tcp) SRV resource record set lookups to locate NNTP servers. These include:

• the NNTP clients (nnfeedg and nnfeedn) in The Internet Utilities for OS/2

The SRV Lookup Laggards' Hall of Shame

It is amazingly hard to convince the authors of certain particular application client softwares to use SRV lookups, even those authors that give examples of such lookups in their own documentation.

HTTP Laggards' Hall of Shame

To the shame and embarrassment of their authors, web browsers and proxy HTTP servers still do not perform SRV lookups. Web browser software authors have still, 6 years after the idea was first proposed, to add SRV lookup to their web browsers.

• The work item for having SRV lookups added to Mozilla has languished for over 5 years.

• Squid doesn't yet employ SRV lookups.

• Whilst Microsoft Windows Server networking is an exemplar that many point to when discussing the use of SRV resource records, whilst Microsoft's Windows 2000 Resource Kit even gives examples of _http._tcp SRV resource records, and whilst the suggestion having been on Microsoft's suggestions list for a couple of years now, Microsoft's own Internet Explorer still does not perform SRV lookups.