Extended DNS Records – SVR, DNAME and AAAA records

We continue looking at less commonly used DNS records, but we also look at the soon to be common AAAA record which is merely the IPV6 version of an A record. Before that, we will first examine the SRV and the rare DNAME records.

The SRV extended DNS record

SRV records are DNS entries that define a service ie the hostname and port number of a server for a specific service. Certain protocols such as SIP and XMPP often need SRV records to function properly. A SRV record looks like this:

_service._proto.name. TTL class SRV priority weight port target.

This can be broken down as follows:

• service: the symbolic name of the desired service.
• proto: the transport protocol of the desired service; this is usually either TCP or UDP.
• name: the domain name for which this record is valid, ending in a dot.
• TTL: standard DNS time to live field.
• class: standard DNS class field (this is always IN).
• priority: the priority of the target host, lower value means more preferred.
• weight: A relative weight for records with the same priority, higher value means more preferred.
• port: the TCP or UDP port on which the service is to be found.
• target: the canonical hostname of the machine providing the service, ending in a dot.

An example of a zone file entry for a SIP server may look as follows:

_sip._tcp.mydomain.co.ke. 86400 IN SRV 0 5 5060 sipserver.voipprovider.co.ke.

The TARGET hostname (in this case sipserver.voipprovider.co.ke must resolve via an A or AAAA record and may not be a CNAME. This is the same as an MX record.

The priority field is used to determine which server is used first if there are multiple entries for the same server. The weight is used in case multiple priorities with the same value are used to effect load-balancing.

Query of the SRV can be done by requesting the type SRV – unlike SPF, DKIM or DMARC which are all TXT type records.

The DNAME extended DNS record

Unlike CNAME which is an alias for a single record ie www.domain.com is an alias for domain.com, DNAME redirects all records below it to the new domain. This is useful if you want to redirect an entire domain and all the subdomain requests associated with it to another domain as follows:

; zone fragment for example.com
; uses DNAME RR to re-direct to new domain, example.net
$TTL 2d ; zone default = 2 days or 172800 seconds
$ORIGIN example.com.
; zone apex RRs
....
           IN      DNAME example.net.
....
; www and ftp RRs (or any other) are not required since they are
; BELOW DNAME (which is at the apex)
; any normal zone RRs from this point are IGNORED 
; due to the presence of the DNAME
; they can be left in the zone file 
; or they could be deleted

Any records below the DNAME entry are ignored.

The AAAA Record

The technical explanation of the AAAA record is that it a Resource Record (RR) type 28 which returns a 128-bit IPV6 address, most commonly used to map a hostname to an IPV6 address.  An example:

The AAAA record is thus exactly the same as an A record, only for IPV6.

DNS is not as complicated as it is sometimes made out to be. Here is a list of the currently valid record types, courtesy of Wikipedia:

Resource records

Happy Hosting!