24.6 Advanced Topics

24.6.1 Basic Configuration

Out of the box, you should be able to send email to external hosts as long as you have set up /etc/resolv.conf or are running your own name server. If you would like to have mail for your host delivered to the MTA (e.g., sendmail) on your own FreeBSD host, there are two methods:

• Run your own name server and have your own domain. For example, FreeBSD.org

• Get mail delivered directly to your host. This is done by delivering mail directly to the current DNS name for your machine. For example, example.FreeBSD.org.

Regardless of which of the above you choose, in order to have mail delivered directly to your host, it must have a permanent static IP address (not a dynamic address, as with most PPP dial-up configurations). If you are behind a firewall, it must pass SMTP traffic on to you. If you want to receive mail directly at your host, you need to be sure of either of two things:

• Make sure that the (lowest-numbered) MX record in your DNS points to your host's IP address.

• Make sure there is no MX entry in your DNS for your host.

Either of the above will allow you to receive mail directly at your host.

Try this:

# hostname
example.FreeBSD.org
# host example.FreeBSD.org
example.FreeBSD.org has address 204.216.27.XX

If that is what you see, mail directly to <[email protected]> should work without problems (assuming sendmail is running correctly on example.FreeBSD.org).

If instead you see something like this:

# host example.FreeBSD.org
example.FreeBSD.org has address 204.216.27.XX
example.FreeBSD.org mail is handled (pri=10) by hub.FreeBSD.org

All mail sent to your host (example.FreeBSD.org) will end up being collected on hub under the same username instead of being sent directly to your host.

The above information is handled by your DNS server. The DNS record that carries mail routing information is the Mail eXchange entry. If no MX record exists, mail will be delivered directly to the host by way of its IP address.

The MX entry for freefall.FreeBSD.org at one time looked like this:

freefall       MX  30  mail.crl.net
freefall        MX  40  agora.rdrop.com
freefall        MX  10  freefall.FreeBSD.org
freefall        MX  20  who.cdrom.com

As you can see, freefall had many MX entries. The lowest MX number is the host that receives mail directly if available; if it is not accessible for some reason, the others (sometimes called “backup MXes”) accept messages temporarily, and pass it along when a lower-numbered host becomes available, eventually to the lowest-numbered host.

Alternate MX sites should have separate Internet connections from your own in order to be most useful. Your ISP or another friendly site should have no problem providing this service for you.

24.6.2 Mail for Your Domain

In order to set up a “mailhost” (a.k.a. mail server) you need to have any mail sent to various workstations directed to it. Basically, you want to “claim” any mail for any hostname in your domain (in this case *.FreeBSD.org) and divert it to your mail server so your users can receive their mail on the master mail server.

To make life easiest, a user account with the same username should exist on both machines. Use adduser(8) to do this.

The mailhost you will be using must be the designated mail exchanger for each workstation on the network. This is done in your DNS configuration like so:

example.FreeBSD.org    A   204.216.27.XX       ; Workstation
            MX  10 hub.FreeBSD.org  ; Mailhost

This will redirect mail for the workstation to the mailhost no matter where the A record points. The mail is sent to the MX host.

You cannot do this yourself unless you are running a DNS server. If you are not, or cannot run your own DNS server, talk to your ISP or whoever provides your DNS.

If you are doing virtual email hosting, the following information will come in handy. For this example, we will assume you have a customer with his own domain, in this case customer1.org, and you want all the mail for customer1.org sent to your mailhost, mail.myhost.com. The entry in your DNS should look like this:

customer1.org      MX  10  mail.myhost.com

You do not need an A record for customer1.org if you only want to handle email for that domain.

The last thing that you must do is tell sendmail on your mailhost what domains and/or hostnames it should be accepting mail for. There are a few different ways this can be done. Either of the following will work:

• Add the hosts to your /etc/mail/local-host-names file if you are using the FEATURE(use_cw_file). If you are using a version of sendmail earlier than 8.10, the file is /etc/sendmail.cw.

• Add a Cwyour.host.com line to your /etc/sendmail.cf or /etc/mail/sendmail.cf if you are using sendmail 8.10 or higher.