Книга: Linux Network Administrator Guide, Second Edition

The DNS Database Files

The DNS Database Files

Master files included with named, like named.hosts, always have a domain associated with them, which is called the origin. This is the domain name specified with the cache and primary options. Within a master file, you are allowed to specify domain and host names relative to this domain. A name given in a configuration file is considered absolute if it ends in a single dot, otherwise it is considered relative to the origin. The origin by itself may be referred to using (@).

The data contained in a master file is split up in resource records(RRs). RRs are the smallest units of information available through DNS. Each resource record has a type. A records, for instance, map a hostname to an IP address, and a CNAME record associates an alias for a host with its official hostname. To see an example, look at Example 6.11, which shows the named.hosts master file for the Virtual Brewery.

Resource record representations in master files share a common format:

[domain] [ttl] [class] type rdata

Fields are separated by spaces or tabs. An entry may be continued across several lines if an opening brace occurs before the first newline and the last field is followed by a closing brace. Anything between a semicolon and a newline is ignored. A description of the format terms follows:

domain

This term is the domain name to which the entry applies. If no domain name is given, the RR is assumed to apply to the domain of the previous RR.

ttl

In order to force resolvers to discard information after a certain time, each RR is associated a time to live (ttl). The ttl field specifies the time in seconds that the information is valid after it has been retrieved from the server. It is a decimal number with at most eight digits.

If no ttl value is given, the field value defaults to that of the minimum field of the preceding SOA record.

class

This is an address class, like IN for IP addresses or HS for objects in the Hesiod class. For TCP/IP networking, you have to specify IN.

If no class field is given, the class of the preceding RR is assumed.

type

This describes the type of the RR. The most common types are A, SOA, PTR, and NS. The following sections describe the various types of RRs.

rdata

This holds the data associated with the RR. The format of this field depends on the type of RR. In the following discussion, it will be described for each RR separately.

The following is partial list of RRs to be used in DNS master files. There are a couple more of them that we will not explain; they are experimental and of little use, generally.

SOA

This RR describes a zone of authority (SOA means "Start of Authority"). It signals that the records following the SOA RR contain authoritative information for the domain. Every master file included by a primary statement must contain an SOA record for this zone. The resource data contains the following fields:

origin

This field is the canonical hostname of the primary name server for this domain. It is usually given as an absolute name.

contact

This field is the email address of the person responsible for maintaining the domain, with the "@" sign replaced by a dot. For instance, if the responsible person at the Virtual Brewery were janet, this field would contain janet.vbrew.com.

serial

This field is the version number of the zone file, expressed as a single decimal number. Whenever data is changed in the zone file, this number should be incremented. A common convention is to use a number that reflects the date of the last update, with a version number appended to it to cover the case of multiple updates occurring on a single day, e.g., 2000012600 being update 00 that occurred on January 26, 2000.

The serial number is used by secondary name servers to recognize zone information changes. To stay up to date, secondary servers request the primary server's SOA record at certain intervals and compare the serial number to that of the cached SOA record. If the number has changed, the secondary servers transfer the whole zone database from the primary server.

refresh

This field specifies the interval in seconds that the secondary servers should wait between checking the SOA record of the primary server. Again, this is a decimal number with at most eight digits.

Generally, the network topology doesn't change too often, so this number should specify an interval of roughly a day for larger networks, and even more for smaller ones.

retry

This number determines the intervals at which a secondary server should retry contacting the primary server if a request or a zone refresh fails. It must not be too low, or a temporary failure of the server or a network problem could cause the secondary server to waste network resources. One hour, or perhaps one-half hour, might be a good choice.

expire

This field specifies the time in seconds after which a secondary server should finally discard all zone data if it hasn't been able to contact the primary server. You should normally set this field to at least a week (604,800 seconds), but increasing it to a month or more is also reasonable.

minimum

This field is the default ttl value for resource records that do not explicitly contain one. The ttl value specifies the maximum amount of time other name servers may keep the RR in their cache. This time applies only to normal lookups, and has nothing to do with the time after which a secondary server should try to update the zone information.

If the topology of your network does not change frequently, a week or even more is probably a good choice. If single RRs change more frequently, you could still assign them smaller ttls individually. If your network changes frequently, you may want to set minimum to one day (86,400 seconds).

A

This record associates an IP address with a hostname. The resource data field contains the address in dotted quad notation.

For each hostname, there must be only one A record. The hostname used in this A record is considered the official or canonical hostname. All other hostnames are aliases and must be mapped onto the canonical hostname using a CNAME record. If the canonical name of our host were vlager, we'd have an A record that associated that hostname with its IP address. Since we may also want another name associated with that address, say news, we'd create a CNAME record that associates this alternate name with the canonical name. We'll talk more about CNAME records shortly.

NS records are used to specify a zone's primary server and all its secondary servers. An NS record points to a master name server of the given zone, with the resource data field containing the hostname of the name server.

You will meet NS records in two situations: The first situation is when you delegate authority to a subordinate zone; the second is within the master zone database of the subordinate zone itself. The sets of servers specified in both the parent and delegated zones should match.

The NS record specifies the name of the primary and secondary name servers for a zone. These names must be resolved to an address so they can be used. Sometimes the servers belong to the domain they are serving, which causes a "chicken and egg" problem; we can't resolve the address until the name server is reachable, but we can't reach the name server until we resolve its address. To solve this dilemma, we can configure special A records directly into the name server of the parent zone. The A records allow the name servers of the parent domain to resolve the IP address of the delegated zone name servers. These records are commonly called glue records because they provide the "glue" that binds a delegated zone to its parent.

CNAME

This record associates an alias with a host's canonical hostname. It provides an alternate name by which users can refer to the host whose canonical name is supplied as a parameter. The canonical hostname is the one the master file provides an A record for; aliases are simply linked to that name by a CNAME record, but don't have any other records of their own.

PTR

This type of record is used to associate names in the in-addr.arpa domain with hostnames. It is used for reverse mapping of IP addresses to hostnames. The hostname given must be the canonical hostname.

MX

This RR announces a mail exchanger for a domain. Mail exchangers are discussed in "Mail Routing on the Internet". The syntax of an MX record is:

[domain] [ttl] [class] MX preference host

host names the mail exchanger for domain. Every mail exchanger has an integer preference associated with it. A mail transport agent that wants to deliver mail to domain tries all hosts who have an MX record for this domain until it succeeds. The one with the lowest preference value is tried first, then the others, in order of increasing preference value.

HINFO

This record provides information on the system's hardware and software. Its syntax is:

[domain] [ttl] [class] HINFO hardware software

The hardware field identifies the hardware used by this host. Special conventions are used to specify this. A list of valid "machine names" is given in the Assigned Numbers RFC (RFC-1700). If the field contains any blanks, it must be enclosed in double quotes. The software field names the operating system software used by the system. Again, a valid name from the Assigned Numbers RFC should be chosen.

An HINFO record to describe an Intel-based Linux machine should look something like:

tao 36500 IN HINFO IBM-PC LINUX2.2

and HINFO records for Linux running on Motorola 68000-based machines might look like:

cevad 36500 IN HINFO ATARI-104ST LINUX2.0
jedd 36500 IN HINFO AMIGA-3000 LINUX2.0

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