DNS Records Types
The list below covers all of the supported types, and explains what they are used for.
* Address (A) An Address records associates an IP address with a hostname. Any system that you want to be able to connect to via HTTP, telnet or some other protocol using its hostname must have an address record so that clients can look up its IP. A single hostname can have more than one Address record, which is often done to spread the load for a website across multiple servers. It is also valid to create multiple records of this type with different names but the same IP, such as when setting up name-based Apache virtual servers. When creating or editing an Address record, the field Address is displayed for entering the IP associated with the hostname. A field labelled Update reverse? also appears, which controls the automatic creation and modification of a Reverse Address record in the appropriate reverse zone. See the Adding and editing records section above for more details.
* Name Sever (NS) Records of this type defines a name server that is responsible for a zone. Every zone must have at least one Name Server record for itself, and may have additional records that specify the DNS servers responsible for subdomains. If you set up a secondary DNS server for a zone, be sure to add a Name Server record for the zone on the master server. In this case, the name of the record will be the canonical name of the zone, such as example.com.. When creating or editing a record of this type, a field labelled Name Server will be displayed. This must be filled in with the IP address or hostname of the DNS server that is responsible for the zone. If you enter a hostname, it must have an IP address set by an Address record in some zone on your server.
* Name Alias (CNAME) This type of record creates an additional name for an existing Address or Reverse Address record. When a DNS client requests the IP address of a record of this type, it will get the IP of the record that the Name Alias points to instead. This kind of record can be useful if you have a single host that needs to be accessible under several different names, such as a web server doing name-based virtual hosting. Even though this could also be done by creating multiple Address records, creating just a single Address and multiple Name Aliases is more flexible as it allows easier updating if the IP address of the host ever changes. The forms for editing and creating Name Alias records contains a field labelled Real Name. This must be filled in with either the canonical name of the record that the alias points to (such as webserver.example.com.), or with a short name that is relative to the zone that the Name Alias record is in.
* Mail Server (MX) Mail Server records tell mail delivery programs like Sendmail and Qmail which system to contact when delivering mail to a domain or host. Without a record of this type, mail for a domain will be delivered to the system whose IP is specified in the Address record for the zone itself. This is not always desirable, as you may want that IP to be the address of a webserver, so that web browsers can connect to http://example.com/ as well as http://www.example.com/. A Mail Server record can solve this problem by having only email for example.com sent to another hosts, and all other traffic to the webserver. Each Mail Server record has a priority, which tells mail delivery programs which mail server should be tried first. The record with the lowest priority should point to the system that actually receives and stores email for the domain, while those with higher priorities generally point to systems that will simply relay mail. Delivery programs will try each in turn starting with the lowest, so that if the primary mail server is down email will still be sent to a relay that can hold it until the primary comes back up.
When adding or editing a Mail Server record, two additional fields are displayed. The first is labelled Mail Server, and must be filled in with the canonical or relative hostname of a system that can accept mail for the domain or hostname entered in the Name field. The second is labelled Priority, and must be used to specify a numerical priority for this particular mail server. Normally a priority of 5 is used for the primary mail server, and 10 for backup relays. If you only have one mail server for your domain, it doesn’t really matter what number is entered into this field. It is possible for two servers to have the same priority, in which case one will be chosen randomly to deliver to. A Mail Server record can use the * wildcard in its name, which indicates to mail programs that a particular mailserver is responsible for all hosts in a domain. For example, a record named like *.example.com would match the hostname pc1.example.com and any other hosts in the zone. This can be useful if you want to force mail that would otherwise be delivered directly to workstations in your domain to go through a central mailserver instead. Webmin will not let you use wildcards unless the Allow wildcards module configuration option is set to Yes though, as explained in the *Configuring the BIND DNS Server module* section.
* Host Information (HINFO) Records of this type are used to record information about the hardware and operating system of a particular host. For example, you might create one that says that server1.example.com is an x86 PC running Linux. However, they are very rarely used and are in fact considered a security risk, as they give out information to potential attackers that could be used to take over a server. When creating or editing a Host Information record, the fields Hardware and Operating System are displayed for entering the architecture and operating system type of a host. The values you enter must not contain any spaces – typically, they are replaced in the hardware type and operating system strings with _ characters.
* Text (TXT) A Text record associates an arbitrary message of some kind with a name. Although they are hardly ever used, they can be useful for attaching comments to hostnames. Be aware though that any such comments will be available to anyone on the Internet that can look up records in your domain, and so should not contain sensitive information. The field Message is displayed when entering or editing a Text record. You can enter any text that you like, including spaces.
* Well Known Service (WKS) A record of this type associates a hostname, port and protocol with a name. It can be thought of as a generalized variant of the Mail Server record, which tells clients which host provides a particular service for some domain or hostname. However, almost no programs actually look up WKS records, so in practice they are pretty much useless. When adding or editing one of these records, the fields Address, Protocol and Services are available. The first is for entering the IP address of a host that provides the services for the host or domain entered into the Name field. The second is for selecting the network protocol that the services use, either TCP or UDP. The last is for entering a list of port numbers or names (from the /etc/services file) for services that the host provides.
* Responsible Person (PR) This type of record is used for specifying the person or group responsible for a particular host. Each of these records has two values associated with it – an email address, and the name of Text record containing the person’s name. Responsible Person records are rarely seen, and are not used by any mail delivery program or Internet client. The Email Address field shown when editing or adding one of these records is for entering the complete address (like jcameron@example.com) of the person responsible for the host whose name is entered into the Name field. The Text Record Name field is for entering the relative or canonical name of a Text record that contains the person’s real name.
* Location (LOC) Location records are used to specify the physical location in latitude and longitude of a host. They are hardly ever seen, and thus not used by many programs. However, they can be useful in large organizations that have hosts in many countries. When adding or editing a Location record, the field Latitude and Longitude is displayed for entering the location of the host in the Name field. It must be formatted like 42 21 43.528 N 71 05 06.284 W 12.00m 30.00m 10000.00m 10.00m.
* Service Address (SRV) Records of this type are used to associate a domain name, service name and protocol with a particular host. They allow you to specify which server a client should contact for a particular service and hostname, instead of just connecting to the host. In a way, they are like Mail Server records but far more flexible. For example, you can specify that the POP3 server for example.com is mail.example.com, but the webserver is www.example.com. At the time of writing, SRV records are mostly used by Windows client systems.
When adding or editing a Service Address record, the fields Protocol and Service name are displayed near the Name text box. For the protocol, you must select either TCP or UDP from the menu. For the service name, you must enter a well-known name from the /etc/services file, such as pop3 or telnet. To look up an SRV record, a client combines the service name, protocol and name to get a record name like ___telnet.___tcp.example.com. Webmin does this for you automatically when editing or adding a Service Address record, but you can see the combined name on the page listing records of this type. Webmin also automatically added the _s before the service and protocol, but hides them when a SRV record is being displayed or edited. This means that there is no need to enter then manually when creating or editing a record of this type. The Priority field must be used to enter a numeric priority for this server, which has the same meaning as the priority in a Mail Server record. The Weight field must contain a weighing for this particular server, or zero if there is only one record with the same name, protocol and service name. A higher weighting tells clients to try this server more often than one with a lower weight. The Port field must contain a port number for clients to connect to on the server, which does not necessarily have to be the standard port for the service. In the Server field, you must enter the hostname or IP address of the system that actually provides the service, and that clients actually connect to.
The record types support by Webmin in reverse zones are :
* Reverse Address (PTR) A reverse address record associates a hostname with an IP address in a reverse zone. For DNS clients to be able to lookup hostnames from IP addresses in your network, you will need to create one record of this type for each host. However, most of the time this is done automatically by Webmin when adding and editing Address records. If you create your own Reverse Address records, make sure that they are synchronized with the matching Address records. When adding or editing a record of this type, the fields Address and Hostname are displayed. The first is for entering a complete IP address, like 192.168.1.10. This will be automatically converted by Webmin to the in-addr.arpa format used internally by the DNS system for reverse addresses. The second field is for entering a hostname in canonical form, such as pc1.example.com., be sure to always put a dot at the end, or else the hostname will be relative to the reverse zone, which is definitely not what you want.
* Name Server (NS) Name Server records in a reverse zone have an identical purpose to those in a forward domain – they tell other DNS servers the IP address or hostname of a server responsible for the zone or a sub-domain. This means that one must be added for each primary or secondary DNS server for the zone. The Zone Name field that appears when adding or editing a record of this type is for entering the name of the zone that the server is responsible for, which will typically be the zone that contains the record. However, unlike Reverse Address records this field is not automatically converted to in-addr.arpa format. Instead, you must enter it in fully qualified form like 1.168.192.in-addr.arpa. if defining an nameserver for the 192.168.1 network. In the Name Server field, you must enter an IP address or canonical form hostname for the DNS server, such as ns1.example.com..
* Name Alias (CNAME) Records of this type behave exactly the same in reverse zones as they do in forward domains. However, you must fill in the Name and Real Name fields with reverse names in in-addr.arpa format, as Webmin will not convert them for you. Name Alias fields are most useful in reverse zones for doing partial subnet delegation, as covered in the Partial reverse delegation section below.
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