Zone File Reference

 

 

Zone File

 

Types of Resource Records and When to Use Them

This section, largely borrowed from RFC 1034, describes the concept of a Resource Record (RR) and explains when each is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

 

Resource Records

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate RRs. The order of RRs in a set is not significant and need not be preserved by nameservers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes, for example, to specify that a particular nearby server be tried first. See Section 6.2.14.10 and Section 6.2.14.11.

The components of a Resource Record are:

TTL the time to live of the RR. This field is a 32 bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded.
owner name the domain name where the RR is found.
type an encoded 16 bit value that specifies the type of the resource in this resource record. Types refer to abstract resources.
class an encoded 16 bit value that identifies a protocol family or instance of a protocol.
RDATA the type and sometimes class-dependent data that describes the resource.

The following are types of valid RRs (some of these listed, although not obsolete, are experimental (x) or historical (h) and no longer in general use):

A a host address.
A6 an IPv6 address.
AAAA Obsolete format of IPv6 address
AFSDB (x) location of AFS database servers. Experimental.
CNAME identifies the canonical name of an alias.
DNAME for delegation of reverse addresses. Replaces the domain name specified with another name to be looked up. Described in RFC 2672.
HINFO identifies the CPU and OS used by a host.
ISDN (x) representation of ISDN addresses. Experimental.
KEY stores a public key associated with a DNS name.
LOC (x) for storing GPS info. See RFC 1876. Experimental.
MX identifies a mail exchange for the domain. See RFC 974 for details.
NS the authoritative nameserver for the domain.
NXT used in DNSSEC to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and indicate what RR types are present for an existing name. See RFC 2535 for details.
PTR a pointer to another part of the domain name space.
RP (x) information on persons responsible for the domain. Experimental.
RT (x) route-through binding for hosts that do not have their own direct wide area network addresses. Experimental.
SIG ("signature") contains data authenticated in the secure DNS. See RFC 2535 for details.
SOA identifies the start of a zone of authority.
SRV information about well known network services (replaces WKS).
WKS (h) information about which well known network services, such as SMTP, that a domain supports. Historical, replaced by newer RR SRV.
X25 (x) representation of X.25 network addresses. Experimental.

The following classes of resource records are currently valid in the DNS:

IN the Internet system.

RDATA is the type-dependent or class-dependent data that describes the resource:

A for the IN class, a 32 bit IP address.
A6 maps a domain name to an IPv6 address, with a provision for indirection for leading "prefix" bits.
CNAME a domain name.
DNAME provides alternate naming to an entire subtree of the domain name space, rather than to a single node. It causes some suffix of a queried name to be substituted with a name from the DNAME record's RDATA.
MX a 16 bit preference value (lower is better) followed by a host name willing to act as a mail exchange for the owner domain.
NS a fully qualified domain name.
PTR a fully qualified domain name.
SOA several fields.

The owner name is often implicit, rather than forming an integral part of the RR. For example, many nameservers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL) which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The meaning of the TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; it is also timed out, but by the refreshing policies for the zone. The TTL is assigned by the administrator for the zone where the data originates. While short TTLs can be used to minimize caching, and a zero TTL prohibits caching, the realities of Internet performance suggest that these times should be on the order of days for the typical host. If a change can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency during the change, and then increased back to its former value following the change.

The data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as "pointers" to other data in the DNS.

 

Textual expression of RRs

RRs are represented in binary form in the packets of the DNS protocol, and are usually represented in highly encoded form when stored in a nameserver or resolver. In the examples provided in RFC 1034, a style similar to that used in master files was employed in order to show the contents of RRs. In this format, most RRs are shown on a single line, although continuation lines are possible using parentheses.

The start of the line gives the owner of the RR. If a line begins with a blank, then the owner is assumed to be the same as that of the previous RR. Blank lines are often included for readability.

Following the owner, we list the TTL, type, and class of the RR. Class and type use the mnemonics defined above, and TTL is an integer before the type field. In order to avoid ambiguity in parsing, type and class mnemonics are disjoint, TTLs are integers, and the type mnemonic is always last. The IN class and TTL values are often omitted from examples in the interests of clarity.

The resource data or RDATA section of the RR are given using knowledge of the typical representation for the data.

For example, we might show the RRs carried in a message as: 

ISI.EDU.
 MX
 10 VENERA.ISI.EDU.
 

 MX
 10 VAXA.ISI.EDU
 
VENERA.ISI.EDU
 A
 128.9.0.32
 

 A
 10.1.0.52
 
VAXA.ISI.EDU
 A
 10.2.0.27
 

 A
 128.9.0.33

The MX RRs have an RDATA section which consists of a 16 bit number followed by a domain name. The address RRs use a standard IP address format to contain a 32 bit internet address.

This example shows six RRs, with two RRs at each of three domain names.

Similarly we might see: 

XX.LCS.MIT.EDU. IN
 A
 www.setgetweb.com4
 
CH
 A
 MIT.EDU. 2420

This example shows two addresses for XX.LCS.MIT.EDU, each of a different class.

 

Discussion of MX Records

As described above, domain servers store information as a series of resource records, each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of a RR is as a typed pair of datum, a domain name matched with relevant data, and stored with some additional type information to help systems determine when the RR is relevant.

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent will fall back to the next largest priority. Priority numbers do not have any absolute meaning . they are relevant only respective to other MX records for that domain name. The domain name given is the machine to which the mail will be delivered. It must have an associated A record . CNAME is not sufficient.

For a given domain, if there is both a CNAME record and an MX record, the MX record is in error, and will be ignored. Instead, the mail will be delivered to the server specified in the MX record pointed to by the CNAME. 

example.com.
 IN
 MX
 10
 mail.example.com.
 

 IN
 MX
 10
 mail2.example.com.
 

 IN
 MX
 20
 mail.backup.org.
 
mail.example.com.
 IN
 A
 10.0.0.1
 
 
mail2.example.com.
 IN
 A
 10.0.0.2

For example:

Mail delivery will be attempted to mail.example.com and mail2.example.com (in any order), and if neither of those succeed, delivery to mail.backup.org will be attempted.

 

Setting TTLs

The time to live of the RR field is a 32 bit integer represented in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded. The following three types of TTL are currently used in a zone file.

SOA The last field in the SOA is the negative caching TTL. This controls how long other servers will cache no-such-domain (NXDOMAIN) responses from you. The maximum time for negative caching is 3 hours (3h).
$TTL The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.
RR TTLs Each RR can have a TTL as the second field in the RR, which will control how long other servers can cache the it.

All of these TTLs default to units of seconds, though units can be explicitly specified, for example, 1h30m.

 

Inverse Mapping in IPv4

Reverse name resolution (that is, translation from IP address to name) is achieved by means of the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, read left to right. This is the opposite order to the way IP addresses are usually written. Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the machine or, optionally, multiple PTR records if the machine has more than one name. For example, in the [example.com] domain: 

$ORIGIN
 2.1.10.in-addr.arpa
 
3
 IN PTR foo.example.com.

Note: The $ORIGIN lines in the examples are for providing context to the examples only-they do not necessarily appear in the actual usage. They are only used here to indicate that the example is relative to the listed origin.

 

Other Zone File Directives

The Master File Format was initially defined in RFC 1035 and has subsequently been extended. While the Master File Format itself is class independent all records in a Master File must be of the same class.

Master File Directives include $ORIGIN, $INCLUDE, and $TTL.

 

The $ORIGIN Directive

Syntax: $ORIGIN domain-name [ comment]

$ORIGIN sets the domain name that will be appended to any unqualified records. When a zone is first read in there is an implicit $ORIGIN . The current $ORIGIN is appended to the domain specified in the $ORIGIN argument if it is not absolute. 

$ORIGIN example.com.
WWW     CNAME   MAIN-SERVER
is equivalent to 

WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM.

 

The $INCLUDE Directive

Syntax: $INCLUDE filename [ origin ] [ comment ]

Read and process the file filename as if it were included into the file at this point. If origin is specified the file is processed with $ORIGIN set to that value, otherwise the current $ORIGIN is used.

The origin and the current domain name revert to the values they had prior to the $INCLUDE once the file has been read.

Note: RFC 1035 specifies that the current origin should be restored after an $INCLUDE, but it is silent on whether the current domain name should also be restored. BIND 9 restores both of them. This could be construed as a deviation from RFC 1035, a feature, or both.

 

The $TTL Directive

Syntax: $TTL default-ttl [ comment ]

Set the default Time To Live (TTL) for subsequent records with undefined TTLs. Valid TTLs are of the range 0-2147483647 seconds.

$TTL is defined in RFC 2308.

 

BIND Master File Extension: the $GENERATE Directive

Syntax: $GENERATE range hs type rhs [ comment ]

$GENERATE is used to create a series of resource records that only differ from each other by an iterator. $GENERATE can be used to easily generate the sets of records required to support sub /24 reverse delegations described in RFC 2317: Classless IN-ADDR.ARPA delegation. 

$ORIGIN 0.0.192.IN-ADDR.ARPA.
$GENERATE 1-2 0 NS SERVER$.EXAMPLE.
$GENERATE 1-127 $ CNAME $.0

is equivalent to 

0.0.0.192.IN-ADDR.ARPA NS SERVER1.EXAMPLE.
0.0.0.192.IN-ADDR.ARPA NS SERVER2.EXAMPLE.
1.0.0.192.IN-ADDR.ARPA CNAME 1.0.0.0.192.IN-ADDR.ARPA
2.0.0.192.IN-ADDR.ARPA CNAME 2.0.0.0.192.IN-ADDR.ARPA
...
127.0.0.192.IN-ADDR.ARPA CNAME 127.0.0.0.192.IN-ADDR.ARPA
.

range This can be one of two forms: start-stop or start-stop/step. If the first form is used then step is set to 1. All of start, stop and step must be positive.
lhs lhs describes the owner name of the resource records to be created. Any single $ symbols within the lhs side are replaced by the iterator value. To get a $ in the output you need to escape the $ using a backslash \, e.g. \$. The $ may optionally be followed by modifiers which change the offset from the interator, field width and base. Modifiers are introduced by a { immediately following the $ as ${offset[,width[,base]]}. e.g. ${-20,3,d} which subtracts 20 from the current value, prints the result as a decimal in a zero padded field of with 3. Available output forms are decimal (d), octal (o) and hexadecimal (x or X for uppercase). The default modifier is ${0,0,d}. If the lhs is not absolute, the current $ORIGIN is appended to the name. For compatability with earlier versions $$ is still recognised a indicating a literal $ in the output.
type At present the only supported types are PTR, CNAME and NS.
rhs rhs is a domain name. It is processed similarly to lhs.
The $GENERATE directive is a BIND extension and not part of the standard zone file format.