Five Tips for a Better sendmail Configuration
Pages: 1, 2
Limit the Number of
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
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.Edit the submit.mc file. Add the
MASQUERADE_ASmacro to the configuration so that replies to mail sent by the local host will go to a server that has an active SMTP port, and add the name of the mail relay host to the MSP
FEATUREcommand. Here are sample lines from a modified 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
daemon, deleting the
-bd flag. For example, change this:
/usr/sbin/sendmail -bd -q15m
Terminate the currently running daemon and re-run
sendmail without the -bd
Modifying the submit.mc file in the manner described above prevents
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
configuration to prevent
sendmail from creating a default MSA configuration.
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
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
DAEMON_OPTIONS macro in the configuration. This feature is not
required when you are only changing MTA values.
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
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.
Addr value used by
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.
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
makemap before they can be used. All are useful files that
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
sendmailconfiguration changes are needed to use this database.
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
sendmailconfiguration changes are needed to use this file.
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
The access database is a powerful and flexible
sendmailconfiguration 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
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
sendmailconfiguration to use this database.
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
sendmailserver can be configured to provide mail service for mail domains that do not have any existence beyond the
sendmailserver 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
sendmailconfiguration to use this database.
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
procmailmailer when it is defined internally to the
sendmailconfiguration 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
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.
For more information, or to order the book, click here.
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