Книга: Linux Network Administrator Guide, Second Edition

Mail Routing on the Internet

Mail Routing on the Internet

On the Internet, the destination host's configuration determines whether any specific mail routing is performed. The default is to deliver the message to the destination by first determining what host the message should be sent to and then delivering it directly to that host. Most Internet sites want to direct all inbound mail to a highly available mail server that is capable of handling all this traffic and have it distribute the mail locally. To announce this service, the site publishes a so-called MX record for its local domain in its DNS database. MX stands for Mail Exchanger and basically states that the server host is willing to act as a mail forwarder for all mail addresses in the domain. MX records can also be used to handle traffic for hosts that are not connected to the Internet themselves, like UUCP networks or FidoNet hosts that must have their mail passed through a gateway.

MX records are always assigned a preference. This is a positive integer. If several mail exchangers exist for one host, the mail transport agent will try to transfer the message to the exchanger with the lowest preference value, and only if this fails will it try a host with a higher value. If the local host is itself a mail exchanger for the destination address, it is allowed to forward messages only to MX hosts with a lower preference than its own; this is a safe way of avoiding mail loops. If there is no MX record for a domain, or no MX records left that are suitable, the mail transport agent is permitted to see if the domain has an IP address associated with it and attempt delivery directly to that host.

Suppose that an organization, say Foobar, Inc., wants all its mail handled by its machine mailhub. It will then have MX records like this in the DNS database:

green.foobar.com. IN MX 5 mailhub.foobar.com.

This announces mailhub.foobar.com as a mail exchanger for green.foobar.com with a preference of 5. A host that wishes to deliver a message to [email protected] checks DNS and finds the MX record pointing at mailhub. If there's no MX with a preference smaller than 5, the message is delivered to mailhub, which then dispatches it to green.

This is a very simple description of how MX records work. For more information on mail routing on the Internet, refer to RFC-821, RFC-974, and RFC-1123.

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