Some implementations of Unix recognize that system information can be found in a variety of places. On Solaris 8, for example, hostnames can be obtained from the /etc/hosts file, from nis, from nisplus, or from DNS. Solaris allows the system administrator to choose the order in which these services are searched with a "service-switch" file. Other systems, such as Ultrix and DEC OSF/1, have a similar concept, but some (such as SunOS 4) use built-in rules that cannot be changed without the source code.

Beginning with V8.7, sendmail uses a system-service switch on Solaris, DEC OSF/1, and Ultrix.^[61] Otherwise, sendmail uses the service switch defined by this ServiceSwitchFile option.

^[61] Other operating systems might have service-switch files, but sendmail has not yet been ported to those systems.

The form for redefining the switched-services file is as follows:

O ServiceSwitchFile=path              configuration file (V8.7 and later) 
-OServiceSwitchFile=path              command line (V8.7 and later) 
define(`confSERVICE_SWITCH_FILE',path)    mc configuration (V8.7 and later) 

If this option is defined on Solaris, DEC OSF/1, or Ultrix, it is ignored. Otherwise, path is used as the full pathname of the file that is to be used as the service switch. If path is omitted, the default is NULL. If the entire option is omitted, the default is /etc/mail/service.switch. The default for the mc technique is to omit this option.

The service-switch file must live in a safe directory and must itself have safe permissions, or sendmail will refuse to use the information in it. If your site unavoidably must make the service-switch file unsafe, you might be able to overcome that problem with the DontBlameSendmail option (DontBlameSendmail).

The form of each line in the file defined by path is:

service  how how

Here, service is either hosts (which states how hostnames are looked up), aliases (which states how aliases are looked up), or passwd (which states how passwd(5) information is looked up). For each service, there might be one or more how methods (not all of which make sense with all services). The service and the hows must be separated from each other by whitespace. The possible methods (values for each how) are files (the information is in a file or database, such as /etc/hosts), netinfo (for information on NeXT machines), nis (the information is in an nis map), nisplus (the information is in an nisplus map), dns (the host information is looked up with DNS), or hesiod (the information is listed with a Hesiod service).^[62]

^[62] Currently, the list is limited to those shown. Future versions of sendmail might offer others.

For example, consider the contents of the following /etc/service.switch file:

aliases nis
passwd nis files
hosts dns

Here, sendmail will look up aliases in the nis map mail.aliases. Password information, such as local user login names and full name information from the GECOS field, will first be looked up in the nis map passwd.byname. If not found there, they will then be looked up in the file /etc/passwd. The last line tells sendmail to look up A, AAAA, CNAME, PTR, and MX records using the DNS services.

The hosts line can also determine how MX records are treated (Section 9.2.5). If "dns" does not appear in that line, sendmail disables lookups of MX records. If sendmail is configured to look up hosts with nis first, then DNS, it will do the MX lookup in DNS before the nis lookup.

For Solaris, hosts is looked up with the nsswitch.conf(4) service. For DEC OSF/1 and Ultrix, hosts is looked up with the svc.conf(5) service. For all others the file defined by the ServiceSwitchFile is examined for a line that begins with the word hosts. If that line is missing or if the file doesn't exist, dns is returned by default. But if NAMED_BIND was not defined (NAMED_BIND) when sendmail was compiled, the default returned is nis for Solaris and SunOS, and on other systems it is files.

Note that on systems such as SunOS, a version of gethostbyname(3) is still called that ignores the sendmail program's service-switch file. On such systems you might need to download the source, recompile, and install a version that works correctly.

The ServiceSwitchFile option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.