There are myriad possible sendmail configurations. In this space we'll illustrate just a few important types of configuration that will be useful in many sendmail installations.
It is sometimes useful to overwrite the From: field of an outgoing mail message. Let's say you have a web-based program that generates email. Normally the mail message would appear to come from the user who owned the web server process. We might want to specify some other source address so that the mail appears to have originated from some other user or address on that machine. sendmail provides a means of specifying which systems users are to be entrusted with the ability to do this.
The use_ct_file feature enables the specification and use of a file that lists the names of trusted users. By default, a small number of system users are trusted by sendmail (root, for example). The default filename for this feature is /etc/mail/trusted-users in systems exploiting the /etc/mail/ configuration directory and /etc/sendmail.ct in those that don't. You can specify the name and location of the file by overriding the confCT_FILE definition.
Add FEATURE(use_ct_file) to your sendmail.mc file to enable the feature.
Mail aliases are a powerful feature that enable mail to be directed to mailboxes that are alternate names for users or processes on a destination host. For example, it is common practice to have feedback or comments relating to a World Wide Web server to be directed to “webmaster.” Often there isn't a user known as “webmaster” on the target machine, instead it is an alias of another system user. Another common use of mail aliases is exploited by mailing list server programs in which an alias directs incoming messages to the list server program for handling.
The /etc/aliases file is where the aliases are stored. The sendmail program consults this file when determining how to handle an incoming mail message. If it finds an entry in this file matching the target user in the mail message, it redirects the message to wherever the entry describes.
Specifically there are three things that aliases allow to happen:
They provide a shorthand or well-known name for mail to be addressed to in order to go to one or more persons.
They can invoke a program with the mail message as the input to the program.
They can send mail to a file.
All systems require aliases for Postmaster and MAILER-DAEMON to be RFC-compliant.
Always be extremely aware of security when defining aliases that invoke programs or write to programs, since sendmail generally runs with root permissions.
Details concerning mail aliases may be found in the aliases(5) manual page. A sample aliases file is shown in Example 18-4.
Example 18-4. Sample aliases File
# # The following two aliases must be present to be RFC-compliant. # It is important to resolve them to 'a person' who reads mail routinely. # postmaster: root # required entry MAILER-DAEMON: postmaster # required entry # # # demonstrate the common types of aliases # usenet: janet # alias for a person admin: joe,janet # alias for several people newspak-users: :include:/usr/lib/lists/newspak # read recipients from file changefeed: |/usr/local/lib/gup # alias that invokes program complaints: /var/log/complaints # alias writes mail to file #
Whenever you update the /etc/aliases file, be sure to run the command:
# /usr/lib/sendmail -bi
Sometimes a host finds mail that it is unable to deliver directly to the desired remote host. It is often convenient to have a single host on a network take on the role of managing transmission of mail to remote hosts that are difficult to reach, rather than have each local host try to do this independently.
There are a few good reasons to have a single host take on mail management. You can simplify management by having only one host with a comprehensive mail configuration that knows how to handle all of the different mail transport types, such as UUCP, Usenet, etc. All other hosts need only a single tranport protocol to send their mail to this central host. Hosts that fill this central mail routing and forwarding role are called smart hosts. If you have a smart host that will accept mail from you, you can send it mail of any sort and it will manage the routing and transmission of that mail to the desired remote destinations.
Another good application for smart host configurations is to manage transmission of mail across a private firewall. An organization may elect to install a private IP network and use their own, unregistered IP addresses. The private network may be connected to the Internet through a firewall. Sending mail to and from hosts in the private network to the outside world using SMTP would not be possible in a conventional configuration because the hosts are not able to accept or establish direct network connections to hosts on the Internet. Instead, the organization could elect to have the firewall provide a mail smart host function. The smart host running on the firewall is able to establish direct network connections with hosts both on the private network and on the Internet. The smart host would accept mail from both hosts on the private network and the Internet, store them in local storage and then manage the retransmission of that mail to the correct host directly.
Smart hosts are usually used when all other methods of delivery have failed. In the case of the organization with the private network, it would be perfectly reasonable to have the hosts attempt to deliver mail directly first, and if that fails then to send it to the smart host. This relieves the smart host of a lot of traffic because other hosts can directly send mail to other hosts on the private network.
sendmail provides a simple method of configuring a smart host using the SMART_HOST feature; when implementing it in the Virtual Brewery configuration, we do exactly this. The relevant portions of our configuration that define the smart host are:
define(`SMART_HOST', `uucp-new:moria') LOCAL_NET_CONFIG # This rule ensures that all local mail is delivered using the # smtp transport, everything else will go via the smart host. R$* < @ $* .$m. > $* $#smtp $@ $2.$m. $: $1 < @ $2.$m. > $3
The SMART_HOST macro allows you to specify the host that should relay all outgoing mail that you are unable to deliver directly, and the mail transport protocol to use to talk to it.
In our configuration we are using the uucp-new transport to UUCP host moria. If we wanted to configure sendmail to use an SMTP-based Smart Host, we would instead use something like:
Can you guess what the LOCAL_NET_CONFIG macro and the rewrite rule might be doing?
The LOCAL_NET_CONFIG macro allows you to add raw sendmail rewrite rules to your configuration that define what mail should stay within the local mail system. In our example, we've used a rule that matches any email address where the host belongs to our domain (.$m.) and rewrite it so that it is sent directly to the SMTP mailer. This ensures that any message for a host on our local domain is directed immediately to the SMTP mailer and forwarded to that host, rather than falling through to our smart host, which is the default treatment.
If you've subscribed to a mailing list, published your email address on a web site, or posted an article to UseNet, you will most likely have begun to receive unsolicited advertising email. It is commonplace now for people to scour the net in search of email addresses to add to mailing lists that they then sell to companies seeking to advertise their products. This sort of mass-mailing behavior is commonly called spamming.
The Free On-line Dictionary of Computing offers a mail-specific definition of spam as:
2. (A narrowing of sense 1, above) To indiscrimately send large amounts of unsolicited e-mail meant to promote a product or service. Spam in this sense is sort of like the electronic equivalent of junk mail sent to "Occupant."
In the 1990s, with the rise in commercial awareness of the net, there are actually scumbags who offer spamming as a "service" to companies wishing to advertise on the net. They do this by mailing to collections of e-mail addresses, Usenet news, or mailing lists. Such practises have caused outrage and aggressive reaction by many net users against the individuals concerned.
Fortunately, sendmail includes some support for mechanisms that can help you deal with unsolicited mail.
The Real-time Blackhole List is a public facility provided to help reduce the volume of unsolicited advertising you have to contend with. Known email sources and hosts are listed in a queryable database on the Internet. They're entered there by people who have received unsolicited advertising from some email address. Major domains sometimes find themselves on the list because of slip-ups in shutting down spam. While some people complain about particular choices made by the maintainers of the list, it remains very popular and disagreements are usually worked out quickly. Full details on how the service is operated may be found from the home site of the Mail Abuse Protection System at http://maps.vix.com/rbl/.
If you enable this sendmail feature, it will test the source address of each incoming mail message against the Real-time Blackhole List to determine whether to accept the message. If you run a large site with many users, this feature could save a considerable volume of disk space. This feature accepts a parameter to specify the name of the server to use. The default is the main server at rbl.maps.vix.com.
To configure the Real-time Blackhole List feature, add the following macro declaration to your sendmail.mc file:
Should you wish to specify some other RBL server, you would use a declaration that looks like:
An alternative system that offers greater flexibility and control at the cost of manual configuration is the sendmail access_db feature. The access database allows you to configure which hosts or users you will accept mail from and which you will relay mail for.
Managing who you will relay mail for is important, as it is another technique commonly employed by spamming hosts to circumvent systems such as the Real-time Blackhole List just described. Instead of sending the mail to you directly, spammers will relay the mail via some other unsuspecting host who allows it. The incoming SMTP connection then doesn't come from the known spamming host, it instead comes from the relay host. To ensure that your own mail hosts aren't used in this way, you should relay mail only for known hosts. Versions of sendmail that are 8.9.0 or newer have relaying disabled by default, so for those you'll need to use the access database to enable individual hosts to relay.
The general idea is simple. When a new incoming SMTP connection is received, sendmail retrieves the message header information and then consults the access database to see whether it should proceed to accept the body of the message itself.
The access database is a collection of rules that describe what action should be taken for messages received from nominated hosts. The default access control file is called /etc/mail/access. The table has a simple format. Each line of the table contains an access rule. The lefthand side of each rule is a pattern used to match the sender of an incoming mail message. It may be a complete email address, a hostname, or an IP address. The righthand side is the action to take. There are five types of action you may configure. These are:
Accept the mail message.
Accept messages from this host or user even if they are not destined for our host; that is, accept messages for relaying to other hosts from this host.
Reject the mail with a generic message.
Discard the message using the $#discard mailer.
Return an error message using ### as the error code (which should be RFC-821 compliant) and “any text” as the message.
An example /etc/mail/access might look like:
email@example.com REJECT aol.com REJECT 184.108.40.206 REJECT firstname.lastname@example.org OK linux.org.au RELAY
This example would reject any email received from email@example.com, any host in the domain aol.com and the host 220.127.116.11. The next rule would accept email from firstname.lastname@example.org despite the fact that the domain itself has a reject rule. The last rule allows relaying of mail from any host in the linux.org.au domain.
To enable the access database feature, use the following declaration in your sendmail.mc file:
The default definition builds the database using hash -o /etc/mail/access, which generates a simple hashed database from the plain text file. This is perfectly adequate in most installations. There are other options that you should consider if you intend to have a large access database. Consult the sendmail book or other sendmail documentation for details.
If you have users or automated processes that send mail but will never need to receive it, it is sometimes useful to refuse to accept mail destined for them. This saves wasted disk-space storing mail that will never be read. The blacklist_recipients feature, when used in combination with the access_db feature, allows you to disable the receipt of mail for local users.
To enable the feature, you add the following lines to your sendmail.mc file, if they're not already there:
To disable receipt of mail for a local user, simply add his details into the access database. Usually you would use the ### entry style that would return a meaningful error message to the sender so they know why the mail is not being delivered. This feature applies equally well to users in virtual mail domains, and you must include the virtual mail domain in the access database specification. Some sample /etc/mail/access entries might look like:
daemon 550 Daemon does not accept or read mail. flacco 550 Mail for this user has been administratively disabled. email@example.com 550 Mail disabled for this recipient.
Virtual email hosting provides a host the capability of accepting and delivering mail on behalf of a number of different domains as though it were a number of separate mail hosts. Most commonly, virtual hosting is exploited by Internet Application Providers in combination with virtual web hosting, but it's simple to configure and you never know when you might be in a position to virtual host a mailing list for your favorite Linux project, so we'll describe it here.
When sendmail receives an email message, it compares the destination host in the message headers to the local host name. If they match, sendmail accepts the message for local delivery; if they differ, sendmail may decide to accept the message and attempt to forward it on to the final destination (See the section called The access database” earlier in this chapter for details on how to configure sendmail to accept mail for forwarding ).
If we wish to configure virtual email hosting, the first thing we need to do is to convince sendmail that it should also accept mail for the domains that we are hosting. Fortunately, this is a very simple thing to do.
The sendmail use_cw_file feature allows us to specify the name of a file where we store domain names for which sendmail accepts mail. To configure the feature, add the feature declaration to your sendmail.mc file:
The default name of the file will be /etc/mail/local-host-names for distributions using the /etc/mail/ configuration directory or /etc/sendmail.cw for those that don't. Alternatively, you can specify the name and location of the file by overriding the confCW_FILE macro using a variation on:
To stick with the default filename, if we wished to offer virtual hosting to the bovine.net, dairy.org, and artist.org domains, we would create a /etc/mail/local-host-names that looks like:
bovine.net dairy.org artist.org
When this is done, and assuming appropriate DNS records exist that point those domain names to our host, sendmail will accept mail messages for those domains as though they were destined for our real domain name.
The sendmail virtusertable feature configures support for the virtual user table, where we configure virtual email hosting. The virtual user table maps incoming mail destined for some user@host to some otheruser@otherhost. You can think of this as an advanced mail alias feature, one that operates using not just the destination user, but also the destination domain.
To configure the virtusertable feature, add the feature to your sendmail.mc configuration as shown:
By default, the file containing the rules to perform translations will be /etc/mail/virtusertable. You can override this by supplying an argument to the macro definition; consult a detailed sendmail reference to learn about what options are available.
The format of the virtual user table is very simple. The lefthand side of each line contains a pattern representing the original destination mail address; the righthand side has a pattern representing the mail address the virtual hosted address will be mapped to.
The following example shows three possible types of entries:
firstname.lastname@example.org colin email@example.com firstname.lastname@example.org @dairy.org email@example.com @artist.org $firstname.lastname@example.org
The first entry redirects mail sent to a user in the bovine.net virtual domain to a local user on the machine. The second entry redirects mail to a user in the same virtual domain to a user in another domain. The third example redirects all mail addressed to any user in the dairly.org virtual domain to a single remote mail address. Finally, the last entry redirects any mail to a user in the artist.org virtual domain to the same user in another domain; for example, email@example.com would be redirected to firstname.lastname@example.org.
The Free On-Line Dictionary of Computing can be found packaged in many Linux distributions, or online at its home page at http://wombat.doc.ic.ac.uk/foldoc/.