WKS - Well Known Services

Name Server Operations Guide for BIND Release 4.9.5 : Files : Standard Resource Record Format : WKS - Well Known Services
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6.5.7. WKS - Well Known Services


{name}	 {ttl}	 addr-class   WKS   address	  protocol   list of services
		 IN	      WKS   128.32.0.10   UDP	     who route timed domain
		 IN	      WKS   128.32.0.10   TCP	     ( echo telnet
							     discard sunrpc sftp
							     uucp-path systat daytime
							     netstat qotd nntp
							     link chargen ftp
							     auth time whois mtp
							     pop rje finger smtp
							     supdup hostnames
							     domain
							     nameserver )

The Well Known Services record, WKS, describes the well known services supported by a particular protocol at a specified address. The list of services and port numbers come from the list of services specified in /etc/services. There should be only one WKS record per protocol per address. Note that RFC1123 says of WKS records:

2.2 Using Domain Name Service ... An application SHOULD NOT rely on the ability to locate a WKS record containing an accurate listing of all services at a particular host address, since the WKS RR type is not often used by Internet sites. To confirm that a service is present, simply attempt to use it. ... 5.2.12 WKS Use in MX Processing: RFC-974, p. 5

RFC-974 recommended that the domain system be queried for WKS ("Well-Known Service") records, to verify that each proposed mail target does support SMTP. Later experience has shown that WKS is not widely supported, so the WKS step in MX processing SHOULD NOT be used. ... 6.1.3.6 Status of RR Types ... The TXT and WKS RR types have not been widely used by Internet sites; as a result, an application cannot rely on the existence of a TXT or WKS RR in most domains.