Five Tips for a Better sendmail Configuration by Craig Hunt, author of the sendmail Cookbook 01/15/2004

Unix vendors bundle sendmail with the operating system and ship it out preconfigured. It is very easy to install the OS and use the sendmail configuration delivered with the system. Don't be fooled by this apparent simplicity. In the long run, it is better to take charge of your fate. Creating a custom sendmail configuration using current software and features selected for your environment will give you better performance, reliability, security, and maintainability. Here are some tips to get you started on building a better sendmail configuration.

Keep sendmail Up to Date

It is important to upgrade to a new sendmail release when the release fixes a security problem. sendmail is a popular target for network intruders. They attack sendmail because it is a large, complex program that runs on a very large number of systems. The weaknesses exposed by these attacks are quickly fixed in new sendmail releases. Subscribing to the sendmail-announce mailing list helps you stay informed about these potential problems and lets you know as soon as the problems are fixed. Subscribe to the sendmail-announce mailing list by sending mail to majordomo@lists.sendmail.org that contains the following line:

subscribe sendmail-announce

Keep track of new sendmail releases in order to determine when upgrading is important to you. When you decide to upgrade, download the sendmail source code distribution from http://www.sendmail.org/current-release.html, from any of the mirror sites, or from ftp://ftp.sendmail.org/pub/sendmail/. Additionally, get the signature file associated with the version of sendmail you downloaded. For example, the signature file for sendmail.8.12.10.tar.gz is sendmail.8.12.10.tar.gz.sig.

If you do not have the current sendmail PGP keys on your key ring, also download the PGPKEYS file; you need it to verify the signature. Add the PGP keys to your key ring. In the following example, gpg (GNU Privacy Guard) is used to import the PGPKEYS file:

$ gpg --import PGPKEYS
gpg: key 16F4CCE9: not changed
gpg: key 396F0789: public key imported
gpg: key 678C0A03: not changed
gpg: key CC374F2D: not changed
gpg: key E35C5635: not changed
gpg: key A39BA655: not changed
gpg: key D432E19D: not changed
gpg: key 12D3461D: not changed 
gpg: key BF7BA421: not changed
gpg: key A00E1563: non exportable signature (class 10) - skipped
gpg: key A00E1563: not changed
gpg: key 22327A01: not changed
gpg: Total number processed: 11
gpg:               imported: 1  (RSA: 1)
gpg:              unchanged: 10

Of the 11 exportable keys in the PGPKEYS file, only one is imported to our key ring. The not changed comments for the other ten keys shows that they were already installed on the key ring. The first time you import PGPKEYS, all 11 keys will be added to the key ring. Before using the new key, verify its fingerprint, as in this gpg example:

$ gpg --fingerprint 396F0789
pub  1024R/396F0789 2003-01-15 Sendmail Signing Key/2003 <sendmail@Sendmail.ORG>
     Key fingerprint = C4 73 DF 4A 97 9C 27 A9  EE 4F B2 BD 55 B5 E0 0F

Related Reading sendmail Cookbook By Craig Hunt

Compare the displayed fingerprint against the valid fingerprints for sendmail signing keys. (Table 1-1 in the sendmail Cookbook lists the valid keys, as does page 41 in Bryan Costales' sendmail book.) If the fingerprint is correct, you can sign, and thus validate, the key using a tool such as gpg.

Remember, it is not necessary to download and import the PGPKEYS file every time you download a new sendmail release. New keys are only added to the PGPKEYS file about once a year.

After the sendmail keys have been added to the key ring and signed, verify the sendmail distribution tarball. Only when the tarball is verified can it be safely restored, compiled, and installed.

Patching sendmail is an alternative to downloading a new source code distribution. Updating sendmail with a source code patch is very similar to installing a completely new sendmail source code distribution. In both cases:

• You download source code from sendmail.org. In one case, it is a large .tar file, and in the other it is a small patch file.
• You download a signature file to verify the source code.
• You use gpg or pgp to verify the downloaded source file.
• You recompile, reinstall, and restart sendmail.

The biggest differences between the two approaches to closing a security hole are:

• In one case, you use tar to create a completely new source tree.
• In the other case, you use patch to change files in an existing source tree.

The approach you use depends on your personal preferences and the nature of the version of sendmail currently installed on your system. If you use a version of sendmail provided by a vendor that has some special features, patching may be a way to fix a security problem while retaining those features. Of course, there is no guarantee. If the vendor has extensively modified the sendmail source, the patch may not work.

Roll Your Own Binary

There are good reasons to compile your own copy of sendmail instead of using the copy provided to you as part of your Unix distribution. First, the copy of sendmail included in your Unix distribution is probably grossly out of date. sendmail changes frequently, and some of these changes provide critical security fixes. Sticking with an outdated copy of sendmail is just asking for trouble.

You can usually obtain an updated sendmail binary from your Unix vendor for any critical security fix; however, I prefer to have more personal control over the process. I like to decide what fixes are important to me, instead of letting the vendor decide for me. Additionally, I like to decide what features should be compiled into sendmail instead of settling for the features the vendor chooses.

Another advantage to compiling your own sendmail binaries is that you can create consistency across all systems. Sun will compile sendmail with some settings, HP will use others, and Red Hat will use still others. By selecting the features you want compiled in, and using those settings on all of your Unix platforms, you can create a consistent sendmail environment.

Compile sendmail using the Build utility provided by the sendmail developers. Build detects the architecture of the system and builds a basic sendmail binary that should work for most situations. To customize the compile, create a site.config.m4 configuration file in the devtools/Site directory. As the file extension ".m4" implies, the site.config.m4 file contains m4 commands. Three commands are commonly used:

1. define, which stores a value in a variable.
2. APPENDDEF, which appends a value to an existing list of values stored in a variable.
3. PREPENDDEF, which adds values to the beginning of a list of values stored in a variable.

Customizing the sendmail binary to support OpenLDAP is an example of when a site.config.m4 file is needed. To add LDAP support, set LDAP values in the site.config.m4 file, and recompile and reinstall sendmail, as shown in the following example:

# cd /usr/local/src/sendmail-8.12.10/devtools/Site
# cat >> site.config.m4
APPENDDEF('confMAPDEF', '-DLDAPMAP')
APPENDDEF('confLIBS', '-lldap -llber')
Ctrl-D
# cd /usr/local/src/sendmail-8.12.10
# ./Build -c
# ./Build install

In this example, the first APPENDDEF command adds -DLDAPMAP to the list of supported map types stored in the confMAPDEF define. The second APPENDDEF command adds -lldap and -llber to the list of libraries stored in the confLIBS define.

Build is then used to recompile sendmail. Notice the -c option on the Build command line. It is needed to make sure that Build detects the changes made to the site.config.m4 file. After recompiling, sendmail is reinstalled.

Most custom Build configurations are no more complicated than this example. However, there are more than 100 variables that can be set for the Build configuration. See the devtools/README file for a complete list.

Create Custom Configurations

The sendmail configuration provided by your Unix vendor is, out of necessity, a compromise. The vendor cannot anticipate exactly how you will use the system, let alone how you will use sendmail on that system. Vendors create configurations that are useful for a wide variety of situation, which makes it difficult for the vendor to optimize the configuration for any one situation. Create a custom configuration to optimize sendmail for your specific system, which means you must create different configurations for the different types of systems you plan to use.

The first step is to create a sendmail architecture that shows the role each system will play in delivering the mail. This doesn't need to be a complicated plan. For example, you might say that this system will be allowed to receive mail through the firewall, that system will be allowed to send mail through the firewall, and these systems will use a relay for outbound mail and retrieve inbound mail via POP or IMAP. This sample architecture only defines three different sendmail configurations: one for the mail hub that collects inbound mail, one for the relay that handles outbound mail, and one for all of the other systems that use the hub and the relay.

A custom configuration can be based on the vendor configuration, but it is often better to start with the simple generic configurations provided by the sendmail developers. It is easier to add features to a simple configuration than it is to start with a complex configuration, which may need to have many things removed as well as added. The directory structure created by the sendmail source code distribution tarball contains the ingredients needed to build a sendmail configuration, including the generic configuration files.

The top-level directory created by the tarball is assigned a name that identifies the sendmail version number. Thus, the tarball for sendmail 8.12.10 creates a top- level directory named sendmail-8.12.10. Within this directory structure, the m4 source files and libraries used to build the sendmail configuration are located in the cf directory. The cf directory contains the following 11 items:

$ ls sendmail-8.12.10/cf
cf      feature  m4      ostype  sendmail.schema  siteconfig
domain  hack     mailer  README  sh

The cf directory contains two files, README and sendmail.schema, and nine subdirectories. The cf/cf directory is the working directory of the sendmail configuration. The directory contains more than forty files. Most of these files -- more than 30 of them -- are sample master configuration files. You can identify a master configuration file by the .mc extension. Particularly interesting are the generic files designed for use with different operating systems. These are easily identified by names such as generic-linux.mc and generic-solaris.mc.

Copy the generic file for your operating system to a working file. Make the changes required for your configuration to the working file, and then use Build to create the sendmail.cf file for sendmail.

Limit the Number of sendmail Servers

Limit the number of systems running a sendmail listener to reduce security vulnerability and to simplify maintenance. Every network service that accepts inbound connections is a potential target of security attacks. Limiting the SMTP listener to servers increases security. Because SMTP servers must accept connections and data from unknown hosts via the SMTP port, intruders scan for systems that respond to SMTP connections and target attacks against those systems. Mail servers must run the SMTP listener, but on other systems the listener is an unnecessary risk. Controlling the SMTP ports at the firewall and limiting the number of systems listening to those ports provide defense in depth. If an administrator fails to disable the ports on a host, the firewall should stop an attack. If the firewall fails, limiting the number of systems listening to the SMTP ports limits the number of targets.

Limiting the number of systems that run the SMTP listener also reduces the security administrator's workload. If only servers listen on the SMTP port, only servers are in critical need of a security fix for SMTP vulnerabilities. A site with thousands of desktop workstations might have only a handful of valid sendmail servers. Fixing a few servers is much easier than fixing thousands of desktops. Not only that, the skill level of the server administrators is generally high. Most of those administrators can handle the fix by themselves. Desktop users, on the other hand, require much more support. If anyone fails to correctly apply a critical security fix, the entire network remains in danger. Reducing the number of systems that require a critical fix is clearly a security and a maintenance win.

When planning your sendmail architecture, select a limited number of hosts to act as mail exchangers and mail relay hosts. Configure those selected systems to accept inbound mail. Configure the other sendmail systems using one of the two techniques described here.

One technique for limiting the number of SMTP listeners requires modifications to the submit.mc file. Before making any changes, make a backup copy of the original submit.mc file.

MASQUERADE_AS(`mail.example.com')
FEATURE('msp', 'mail.example.com')

Rebuild the submit.cf file and restart the MSP daemon.

Edit the system startup script. Change the command that starts the sendmail daemon, deleting the -bd flag. For example, change this:

/usr/sbin/sendmail -bd -q15m

to this:

/usr/sbin/sendmail -q15m

Terminate the currently running daemon and re-run sendmail without the -bd flag.

Modifying the submit.mc file in the manner described above prevents sendmail from accepting any SMTP connections -- even from the local host. This technique is popular with security types because even someone with login access to the local host cannot attack sendmail through the SMTP ports. The increased security of modifying submit.mc comes at the cost of increased complexity, because this approach requires changes to the MSP configuration and to the system startup files. A simpler alternative is described next.

In this alternative, all changes take place in the master configuration file you use to build the sendmail.cf file. First add the no_default_msa feature to the sendmail configuration to prevent sendmail from creating a default MSA configuration. Then add DAEMON_OPTIONS macros to create your own configuration that limits inbound mail connections to the loopback address. Finally, add the MASQUERADE_AS macro to the configuration so that replies to mail sent by the local host will go to a server that has an active SMTP port. Then rebuild and reinstall sendmail.cf, and restart sendmail. Here are sample lines that could be added to the sendmail configuration:

dnl Don't create a default MSA configuration
FEATURE('no_default_msa')
dnl Limit the MSA to the loopback address
DAEMON_OPTIONS('Name=MSA, Port=587, Addr=127.0.0.1, M=E')
dnl Limit the MTA to the 127.0.0.1 interface
DAEMON_OPTIONS('Name=MTA, Addr=127.0.0.1')
dnl Make sure replies go to the mail host
MASQUERADE_AS('mail.example.com')

Note that the no_default_msa feature must be used before you can change the DaemonPortOptions settings of the MSA. The FEATURE macro must precede the DAEMON_OPTIONS macro in the configuration. This feature is not required when you are only changing MTA values.

The DAEMON_OPTIONS macro sets values for the sendmail.cf DaemonPortOptions statements. A basic sendmail configuration has two DaemonPortOptions statements -- one for the MTA mode of the sendmail daemon and one for the MSA mode. Both port 25, used by the MTA, and port 587, used by the MSA, are accessible from the network, and thus are potentially vulnerable to network attacks.

The default Addr value used by DaemonPortOptions is INADDR_ANY, which means that the daemon accepts connections from any address. Identifying a specific address with the Addr value limits incoming connection to that specific address. Thus, setting Addr=127.0.0.1 means that only connections from the local host that come through the loopback interface will be accepted, which eliminates any inbound connections from the network. Inbound network connections are not accepted on either port 25 or port 587. However, mail can still be sent from the local host. Both of the techniques described here are covered in more detail in Recipe 10.1 and 10.2 of sendmail Cookbook.

Use sendmail Databases

Databases are a key component of sendmail configuration. While the configuration files get most of the attention in print, the databases often get most of the attention in practice. Use the sendmail databases for day-to-day configuration changes and to control the way in which sendmail processes mail.

I'm using the term "database" loosely. Some of the files described here are flat files used to load the sendmail.cf classes. Others are true databases that must be processed through makemap before they can be used. All are useful files that simplify sendmail configuration.

• aliases
  This database specifies how mail is forwarded for the entire system. (Individual users define personal forwarding for their own mail in the .forward file.) The aliases database defines mailing lists, forwards mail to other hosts, and forwards mail to the correct individual users while providing for a flexible mail address structure. No sendmail configuration changes are needed to use this database.

• relay-domains
  This file lists the hosts that are allowed to relay mail. To enable relaying for a host, add the hostname to this file and pass sendmail the SIGHUP signal. No sendmail configuration changes are needed to use this file.

• local-host-names
  This file lists hostnames that are treated as hostname aliases for the local host. Mail addressed to any of these hostnames is accepted for local delivery. To use this file, add FEATURE(use_cw_file) to the sendmail configuration.

• access
  The access database is a powerful and flexible sendmail configuration tool. It provides fine-grained control over relaying, spam control, authentication, and other services. Each line in the database contains two fields: a conditional test and an action taken when the condition is met. A wide variety of conditions and actions are available to control a wide range of services. To use the database, add FEATURE(access_db) to the sendmail configuration.

• genericstable
  The genericstable database rewrites sender addresses. The original sender address is the database key, and the rewritten sender address is the return value. Use the genericstable to present consistent sender addresses to the outside world. Add FEATURE(genericstable) to the sendmail configuration to use this database.

• virtusertable
  This database routes mail for virtual mail domains. A virtual mail domain is similar to a virtual host in the Apache web server. In the same way that a web server can be configured to serve web pages for host computers that do not physically exist, the sendmail server can be configured to provide mail service for mail domains that do not have any existence beyond the sendmail server itself. This database can be used to ease the transition when changing domain names, to provide an alternate identity when the products you sell are not clearly associated with your official domain name, or to support a large number of client domains if you provide a centralized mail service. It is particularly useful if you run an ISP that provides service to a large number of customers or you run an e-business site. Add FEATURE(virtusertable) to the sendmail configuration to use this database.

• mailertable
  The mailertable maps domain names to the internal mailer that should handle mail bound for that domain. Use this database to control mail routing, particularly to handle exceptional circumstances, such as a remote server that cannot handle the standard mail protocol, or to access the procmail mailer when it is defined internally to the sendmail configuration. Add FEATURE(mailertable) to the sendmail configuration to use this database.

There are other sendmail files and databases, but the ones listed above are those I consider most useful for simplifying sendmail configuration. All of the files and databases listed above can be centrally maintained on an LDAP server and accessed by sendmail using the sendmail schema. Maintaining the files centrally further simplifies configuration by providing consistency across an enterprise or department, and by limiting the number of copies of the files that must be maintained. The sendmail Cookbook show how all of these file and databases can be read from an LDAP server.

Craig Hunt has worked with computer systems for the last thirty years.

O'Reilly & Associates recently released (December 2003) sendmail Cookbook.

• Sample Chapter 2, "Delivery and Forwarding," is available free online.

• You can also look at the Table of Contents, the Index, and the full description of the book.

• For more information, or to order the book, click here.

Return to ONLamp.com.

Copyright © 2009 O'Reilly Media, Inc.