14.6 Resource Records

14.6 Resource Records

We've seen a few different types of resource records (RRs) so far: an IP address has a type of A, and PTR means a pointer query. We've also seen that RRs are what a name server returns: answer RRs, authority RRs, and additional information RRs. There are about 20 different types of resource records, some of which we'll now describe. Also, more RR types are being added over time.

A An A record defines an IP address. It is stored as a 32-bit binary value.
PTR This is the pointer record used for pointer queries. The IP address is represented as a domain name (a sequence of labels) in the in-addr.arpa domain.
CNAME

This stands for "canonical name." It is represented as a domain name (a sequence of labels). The domain name that has a canonical name is often called an alias. These are used by some FTP sites to provide an easy to remember alias for some other system.

For example, the gated server (mentioned in Section 10.3) is available through anonymous FTP from the server gated.cornell.edu. But there is no system named gated, this is an alias for some other system. That other system is the canonical name for gated.cornell.edu:

 sun %  host -t cname gated.cornell.edu  gated.cornell.edu       CNAME   COMET.CIT.CORNELL.EDU 

Here we use the -t option to specify one particular query type.

HINFO

Host information: two arbitrary character strings specifying the CPU and operating system. Not all sites provide HINFO records for all their systems, and the information provided may not be up to date.

 sun %  host -t hinfo sun  sun.tuc.noao.edu         HINFO   Sun-4/25       Sun4.1.3 
MX

Mail exchange records, which are used in the following scenarios: (1) A site that is not connected to the Internet can get an Internet-connected site to be its mail exchanger . The two sites then work out an alternative way to exchange any mail that arrives, often using the UUCP protocol. (2) MX records provide a way to deliver mail to an alternative host when the destination host is not available. (3) MX records allow organizations to provide virtual hosts that one can send mail to, such as cs.university.edu, even if a host with that name doesn't exist. (4) Organizations with firewall gateways can use MX records to limit connectivity to internal systems.

Many sites that are not connected to the Internet have a UUCP link with an Internet connected site such as UUNET. MX records are then provided so that electronic mail can be sent to the site using the standard user @host notation. For example, a fictitious domain foo.com might have the following MX records:

 sun %  host -t mx foo.com  foo.com            MX     relay1.UU.NET    foo.com            MX     relay2.UU.NET 

MX records are used by mailers on hosts connected to the Internet. In this example the other mailers are told "if you have mail to send to user@foo.com , send the mail to relay1.uu.net or relay2.uu.net ."

MX records have 16-bit integers assigned to them, called preference values. If multiple MX records exist for a destination, they're used in order, starting with the smallest preference value.

Another example of MX records handles the case when a host is down or unavailable. In that case the mailer uses the MX records only if it can't connect to the destination using TCP. In the case of the author's primary system, which is connected to the Internet by a SLIP connection, which is down most of the time, we have:

 sun %  host -tv mx sun  Query about sun for record types MX    Trying sun within tuc.noao.edu ...    Query done, 2 answers, authoritative status: no error    sun.tuc.noao.edu    86400  IN    MX    0  sun.tuc.noao.edu    sun.tuc.noao.edu    86400  IN    MX    10 noao.edu 

We also specified the -v option, to see the preference values. (This option also causes other fields to be output.) The second field, 86400, is the time-to-live value in seconds. This TTL is 24 hours (24 —60 —60). The third column, IN, is the class (Internet). We see that direct delivery to the host itself, the first MX record, has the lowest preference value of 0. If that doesn't work (i.e., the SLIP link is down), the next higher preference is used (10) and delivery is attempted to the host noao.edu. If that doesn't work, the sender will time out and retry at a later time.

In Section 28.3 we show examples of SMTP mail delivery using MX records.

NS Name server record. These specify the authoritative name server for a domain. They are represented as domain names (a sequence of labels). We'll see examples of these records in the next section.

These are the common types of RRs. We'll encounter many of them in later examples.



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

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