IPv4 Address Record (A)
Defined in RFC 1035. Forward maps a host name to IPv4 address. The only
parameter is an ipv4 field which is a single IPv4 address in dotted decimal
format. The ipv4 field is an address not a label (name) and therefore is not
terminated with a '.' (dot). Valid owner-name format (a.k.a 'label' in the DNS
jargon). If host name is BLANK (or space) then the last valid name (or label) is
substituted.
Format
owner-name ttl class rr ipv4
joe IN A 192.168.254.3
If multiple addresses are defined with the same name then BIND will respond
to queries with all all the addresses defined but the order may change.
Cyclic order is the BIND default but you can control it with the BIND
named.conf statement rrset-order. The same IP may be defined with different
names (Beware: in this case a reverse lookup may not give the result you
want). IP addresses do not have to be in the same class or range. IPv4
addresses are reverse-mapped using the PTR RR.
Note:
A RRs translate a name to an IPv4 address - the owner-name does not have to be
the real host name of the PC or server (as it appears in /etc/hosts or in
response to a hostname command). It can be, or not, as you choose. Thus, if a
servers's name is alice.example.com with an IP address of 192.168.2.1 and it
provides, say, a DNS service then you can either define an NS RR for
alice.example.com or define an NS RR for, say, ns1.example.com and an A RR for
ns1.example.com A 192.168.2.1 (the same IP address as host alice.example.com)
as illustrated below:
; zone file snippet
$ORIGIN example.com.span>
...
IN NS alice.example.com.
...
alice IN A 192.168.2.1 ; real host name
; equally valid configuration
IN NS ns1.example.com.
...
ns1 IN A 192.168.2.1 ; service name
alice IN A 192.168.2.1 ; host name (same IPv4)
Examples & Variations
; zone fragment for example.com
$TTL 2d ; zone default = 2 days or 172800 seconds
joe IN A 192.168.0.3 ; joe & www = same ip
www IN A 192.168.0.3
; functionally the same as the record above
www.example.com. A 192.168.0.3
fred 3600 IN A 192.168.0.4 ; ttl overrides $TTL default
ftp IN A 192.168.0.24 ; round robin with next
IN A 192.168.0.7
mail IN A 192.168.0.15 ; mail = round robin
mail IN A 192.168.0.32
mail IN A 192.168.0.3
squat IN A 10.0.14.13 ; address in another range & class
In the above example BIND will respond to queries for mail.example.com as
follows (assuming you are using the default rrset-order order cyclic):
1st query 192.168.0.15, 192.168.0.32, 192.168.0.3
2nd query 192.168.0.3, 192.168.0.15, 192.168.0.32
3rd query 192.168.0.32, 192.168.0.3, 192.168.0.15
4th query 192.168.0.15, 192.168.0.32, 192.168.0.3