Start of Authority Record (SOA)

Defined in RFC 1035. The SOA defines global parameters for the zone (domain). There is only one SOA record allowed in a zone file.

Format

name        ttl class rr    name-server email-addr  (sn ref ret ex min)
example.com.    IN    SOA   ns.example.com. hostmaster.example.com. (
                              2003080800 ; sn = serial number
                              172800     ; ref = refresh = 2d
                              900        ; ret = update retry = 15m
                              1209600    ; ex = expiry = 2w
                              3600       ; min = minimum = 1h
                              )
; the following are also valid using @ and blank 
@               IN    SOA   ns.example.com. hostmaster.example.com. (
                IN    SOA   ns.example.com. hostmaster.example.com. (

The most complex and most critical record in the zone file. The following notes apply:

Field Description
name The 'root name' of the zone. Most commonly written as @ or Origin Value.
ttl Standard TTL values apply (range 0 to 2147483647 clarified by RFC 2181). The slave (Secondary) DNS does not use the the TTL value but various parameters defined within the SOA - see below. For more information about TTL values.
class Defines the class of record and normally takes the value IN = Internet (Defaulted in not present). It may also take the value HS = Hesiod and CH = Chaos both historic MIT protocols.
name-server Any name server that will respond authoritatively for the domain. Called the Primary Master in the context of Dynamic DNS (DDNS). If DDNS is not used this may be any suitable name server either within the zone file or in an external or foreign zone (also called out-of-zone or even out-of-bailiwick). To mimimise confusion this is most commonly written as a Fully-qualified Domain Name (FQDN and ends with a dot). If the record points to an EXTERNAL server (not defined in this zone) it MUST be a FQDN and end with a '.' (dot), for example, ns1.example.net. If the name server is within this domain (in this zone file) it can be written as ns1 (without the dot) which will be expanded to include the $ORIGIN. In the jargon this field is called MNAME field which is why we called it name-server. When to use the dot.
email-addr Email address of the person responsible for this zone and to which email may be sent to report errors or problems. In the jargon this is called the RNAME field which is why we called it email-addr. The email address of a suitable DNS admin but more commonly the technical contact for the domain. By convention (in RFC 2142) it is suggested that the reserved mailbox hostmaster be used for this purpose but any sensible and stable email address will work. NOTE: Format is mailbox-name.domain.com, for example, hostmaster.example.com (uses a dot not the more normal @ sign, since @ has other uses in the zone file) but mail is sent to hostmaster@example.com. Most commonly ending with a '.' (dot) but if the email address lies within this domain you can just use hostmaster (see also example below). when to use the dot.
sn = serial number Unsigned 32 bit value in range 1 to 4294967295 with a maximum increment of 2147483647. In BIND implementations this is defined to be a 10 digit field. This value MUST increment when any resource record in the zone file is updated. A slave (Secondary) DNS server will read the master DNS SOA record periodically, either on expiry of refresh (defined below) or when it receives a NOTIFY and compares arithmetically its current value of sn with that received from the master DNS. If the sn value from the master is arithmetically HIGHER than that currently stored by the slave then a zone transfer (AXFR/IXFR) is initiated. If the value of sn from the master DNS SOA is the same or LOWER then no zone transfer is initiated. The convention is to use a date based sn value to simplify the task of incrementing the sn - the most popular convention being yyyymmddss where yyyy = year, mm = month and dd = day ss = a sequence number in case you update it more than once in the day! Using this date format convention the value 2005021002 indicates the last update was on the 10th February 2005 and it was the third update that day. The date format is just a convention, not a requirement, so BIND (or any other DNS software) will not validate the contents of this field. It is easy to make mistakes and get serial numbers out of sequence. Fix Serial Numbers.

Note: the arithmetic used by the serial number is defined in RFC 1982.

refresh Signed 32 bit time value in seconds. Indicates the time when the slave will try to refresh the zone from the master (by reading the master DNS SOA RR). RFC 1912 recommends 1200 to 43200 seconds, low (1200) if the data is volatile or 43200 (12 hours) if it's not. If you are using NOTIFY you can set for much higher values, for instance, 1 or more days (> 86400 seconds). BIND Time format.
retry Signed 32 bit value in seconds. Defines the time between retries if the slave (secondary) fails to contact the master when refresh (above) has expired. Typical values would be 180 (3 minutes) to 900 (15 minutes) or higher. BIND Time format.
expiry Signed 32 bit value in seconds. Indicates when the zone data is no longer authoritative. Used by Slave or (Secondary) servers only. BIND9 slaves stop responding to queries for the zone when this time has expired and no contact has been made with the master. Thus every time the refresh values expires the slave will attempt to read the SOA record from the zone master - and request a zone transfer AXFR/IXFR if sn is HIGHER. If contact is made the expiry and refresh values are reset and the cycle starts again. If the slave fails to contact the master it will retry every retry period but continue to supply authoritative data for the zone until the expiry value is reached at which point it will stop answering queries for the domain. RFC 1912 recommends 1209600 to 2419200 seconds (2-4 weeks) to allow for major outages of the zone master. BIND Time format.
min = minimum Signed 32 bit value in seconds. RFC 2308 (implemented by BIND 9) redefined this value to be the negative caching time - the time a NAME ERROR = NXDOMAIN result may be cached by any resolver. The maximum value allowed by RFC 2308 for this parameter is 3 hours (10800 seconds). This value was historically (in BIND 4 and 8) used to hold the default TTL value for any RR from the zone that did not specify an explicit TTL. RFC 2308 (and BIND 9) uses the $TTL directive as the zone default TTL (and which was also standardized in RFC 2308). You may find older documentation or zone file configurations which reflect the old usage (there there are still a lot of BIND 4/8 operational sites). BIND Time format.

NOTES:

  1. BIND and most other DNS software will accept time values in a number of formats, for example, 1h 3m etc.. Saves all that work with the calculator.
  2. The opening '(' (bracket) MUST appear on the same line as the SOA record. This is defined in RFC 1035 and BIND will reject the whole zone if this rule is broken. The terminating ')' (bracket) can appear on any line as shown in the examples.
  3. The name server defined in the SOA record will ALWAYS have a NS record (there may be more than one NS record). For the sake of completeness the NS record is shown in the example fragments.

Examples

; zone file fragment for example.com
; both name servers are out-of-zone (or out-of-bailiwick)
$TTL 2d ; zone TTL default = 2 days or 172800 seconds
$ORIGIN example.com.
@      IN      SOA   ns.example.net. hostmaster.example.com. (
               2003080800 ; serial number
               1d12h      ; refresh =  1 day 12 hours
               15M        ; update retry = 15 minutes
               3W12h      ; expiry = 3 weeks + 12 hours
               2h20M      ; minimum = 2 hours + 20 minutes
               )
        IN     NS    ns.example.net. name server
        IN     NS    ns.example.org. name server
; since both name servers are out-of-zone (or out-of-bailiwick)
; no A RRs are required (and will be rejected if present)

The email address in the SOA lies within the domain and can be written as an unqualified (no-dot) name:

; zone file fragment for example.com
; both name servers are out-of-zone (or out-of-bailiwick)
$TTL 2d ; zone TTL default = 2 days or 172800 seconds
$ORIGIN example.com.
@      IN      SOA   ns.example.net. hostmaster (
               2003080800 ; serial number
               1d12h      ; refresh =  1 day 12 hours
               15M        ; update retry = 15 minutes
               3W12h      ; expiry = 3 weeks + 12 hours
               2h20M      ; minimum = 2 hours + 20 minutes
               )
        IN     NS    ns.example.net. name server
        IN     NS    ns.example.org. name server
; since both name servers are out-of-zone (or out-of-bailiwick)
; no A RRs are required (and will be rejected if present)

The master DNS is within the domain. The second name server is not:

; zone file fragment for example.com 
$TTL 2d ; zone TTL default = 2 days or 172800 seconds
$ORIGIN example.com.
@      IN      SOA   ns.example.com. hostmaster.example.com. (
               2003080800 ; serial number
               1d12h      ; refresh =  1 day 12 hours
               15M        ; update retry = 15 minutes
               3W12h      ; expiry = 3 weeks + 12 hours
               2h20M      ; minimum = 2 hours + 20 minutes
               )
       IN      NS  ns.example.com.
       IN      NS  ns.example.net.
...
; A record for ns.example.com. RR above
ns     IN      A   192.168.2.1
; the above could have been written as
; ns.example.com. IN  A   192.168.2.1
; ns.example.net is out-of-zone (or out-of-bailiwick)
; an A RRs is not required (and will be rejected if present)

The above rewritten to use unqualified names:

$TTL 2d ; zone TTL default = 2 days or 172800 seconds
$ORIGIN example.com.
@      IN      SOA   ns hostmaster (
               2003080800 ; serial number
               1d12h      ; refresh =  1 day 12 hours
               15M        ; update retry = 15 minutes
               3W12h      ; expiry = 3 weeks + 12 hours
               2h20M      ; minimum = 2 hours + 20 minutes
               )
       IN      NS  ns
       IN      NS  ns.example.net.
...
; A record for the NS RR above
ns     IN      A   192.168.2.1
; the above could have been written as
; ns.example.com. IN  A   192.168.2.1
; ns.example.net is out-of-zone (or out-of-bailiwick)
; an A RRs is not required (and will be rejected if present)

Pro DNS and BIND by Ron Aitchison

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