Sidewinder G2 6.1.2 Administration Guide - Glossary of Technical ...

Chapter 11: DNS (Domain Name System)
About mail exchanger records
About mail
exchanger
records
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Listed below are some additional points about running DNS on your
Sidewinder G2:
• Sidewinder G2 uses Berkeley Internet Name Domain (BIND 9).
• The boot files for the unbound and the Internet name servers are
/etc/named.conf.u and /etc/named.conf.i, respectively. The boot files
specify corresponding directories: /etc/namedb.u and
/etc/namedb.i. When you boot your Sidewinder G2, the name server
daemon (named) is started. The /etc/named.conf.u and
/etc/named.conf.i files specify whether the Sidewinder G2 is a master or a
slave name server and list the names of the files that contain the DNS
database records.
• If you choose to configure the Sidewinder G2 as a master name server on
either the unbound (internal) or Internet (external) side, you can modify the
/etc/namedb.u/domain-name.db and /etc/namedb.i/domain-name.db files
(where domain-name = your site’s domain name). You can add the default
information that is being advertised for these zones.
• The Sidewinder G2 contains a non-blocking DNS resolver to support
reverse IP address look-ups in the active proxy rule group, and name-toaddress
look-ups in the http proxy. The relevant resolver library calls are
gethostbyname() and gethostbyaddr(). The non-blocking DNS resolver
provides a small number of DNS resolver daemons (nbresd) that are
handed queries to resolve on behalf of the client.
When you set up Sidewinder hosted DNS services for your site, you need to
create mail exchanger (MX) records. MX records advertise that you are
accepting mail for a specific domain(s). If you do not create an MX record for
your domain, name servers and users on the Internet will not know how to
send e-mail to you. When an e-mail message is sent from a site on the
Internet, a DNS query is made in order to find the correct mail exchange (MX)
host for the destination domain. The sender’s mail process then sends the email
to the MX host. The Sidewinder G2, through the use of mailertables, will
forward the mail to the internal mail process, which in turn will forward it to the
internal mail host. See “Editing the mail configuration files” on page 354 for
more information on mailertables.
Consider the example shown in Figure 140. Someone in the Internet, Lloyd,
wants to send one of your users, Sharon, an e-mail message, but all Lloyd
knows is Sharon’s e-mail address: sharon@foo.com. The mailer at Lloyd’s site
uses DNS to find the MX record of foo.com. Lloyd’s message for Sharon is
then sent to the mailhost listed in the MX record for Sharon’s site.