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33. SMTP authentication

The “authenticators” section of Exim's run time configuration is concerned with SMTP authentication. This facility is an extension to the SMTP protocol, described in RFC 2554, which allows a client SMTP host to authenticate itself to a server. This is a common way for a server to recognize clients that are permitted to use it as a relay. SMTP authentication is not of relevance to the transfer of mail between servers that have no managerial connection with each other.

Very briefly, the way SMTP authentication works is as follows:

If you are setting up a client, and want to know which authentication mechanisms the server supports, you can use Telnet to connect to port 25 (the SMTP port) on the server, and issue an EHLO command. The response to this includes the list of supported mechanisms. For example:

  $ telnet server.example 25
  Connected to server.example.
  Escape character is '^]'.
  220 server.example ESMTP Exim 4.20 ...
  ehlo client.example
  250-server.example Hello client.example []
  250-SIZE 52428800
  250 HELP

The second-last line of this example output shows that the server supports authentication using the PLAIN mechanism. In Exim, the different authentication mechanisms are configured by specifying authenticator drivers. Like the routers and transports, which authenticators are included in the binary is controlled by build-time definitions. The following are currently available, included by setting


in Local/Makefile, respectively. The first of these supports the CRAM-MD5 authentication mechanism (RFC 2195), and the second can be configured to support the PLAIN authentication mechanism (RFC 2595) or the LOGIN mechanism, which is not formally documented, but used by several MUAs. The third authenticator supports Microsoft's Secure Password Authentication mechanism.

The authenticators are configured using the same syntax as other drivers (see section 6.16). If no authenticators are required, no authentication section need be present in the configuration file. Each authenticator can in principle have both server and client functions. When Exim is receiving SMTP mail, it is acting as a server; when it is sending out messages over SMTP, it is acting as a client. Authenticator configuration options are provided for use in both these circumstances.

To make it clear which options apply to which situation, the prefixes server_ and client_ are used on option names that are specific to either the server or the client function, respectively. Server and client functions are disabled if none of their options are set. If an authenticator is to be used for both server and client functions, a single definition, using both sets of options, is required. For example:

    driver = cram_md5
    public_name = CRAM-MD5
    server_secret = ${if eq{$1}{ph10}{secret1}fail}
    client_name = ph10
    client_secret = secret2

The server_ option is used when Exim is acting as a server, and the client_ options when it is acting as a client.

Descriptions of the individual authenticators are given in subsequent chapters. The remainder of this chapter covers the generic options for the authenticators, followed by general discussion of the way authentication works in Exim.

33.1. Generic options for authenticators


Type:  string
Default:  unset

This option must always be set. It specifies which of the available authenticators is to be used.


Type:  string
Default:  unset

This option specifies the name of the authentication mechanism that the driver implements, and by which it is known to the outside world. These names should contain only upper case letters, digits, underscores, and hyphens (RFC 2222), but Exim in fact matches them caselessly. If public_name is not set, it defaults to the driver's instance name.


Type:  string, expanded
Default:  unset

When a server is about to advertise an authentication mechanism, the condition is expanded. If it yields the empty string, “0”, “no”, or “false”, the mechanism is not advertised. If the expansion fails, the mechanism is not advertised. If the failure was not forced, and was not caused by a lookup defer, the incident is logged. See section 33.3 below for further discussion.


Type:  string, expanded
Default:  unset

If this option is set and authentication debugging is enabled (see the -d command line option), the string is expanded and included in the debugging output when the authenticator is run as a server. This can help with checking out the values of variables. If expansion of the string fails, the error message is written to the debugging output, and Exim carries on processing.


Type:  string, expanded
Default:  unset

When an Exim server successfully authenticates a client, this string is expanded using data from the authentication, and preserved for any incoming messages in the variable $authenticated_id. It is also included in the log lines for incoming messages. For example, a user/password authenticator configuration might preserve the user name that was used to authenticate, and refer to it subsequently during delivery of the message. If expansion fails, the option is ignored.


Type:  string, expanded
Default:  unset

This option allows a server to discard authenticated sender addresses supplied as part of MAIL commands in SMTP connections that are authenticated by the driver on which server_mail_auth_condition is set. The option is not used as part of the authentication process; instead its (unexpanded) value is remembered for later use. How it is used is described in the following section.

33.2. The AUTH parameter on MAIL commands

When a client supplied an AUTH= item on a MAIL command, Exim applies the following checks before accepting it as the authenticated sender of the message:

When $authenticated_sender is set for a message, it is passed on to other hosts to which Exim authenticates as a client. Do not confuse this value with $authenticated_id, which is a string obtained from the authentication process, and which is not usually a complete email address.

Whenever an AUTH= value is ignored, the incident is logged. The ACL for MAIL, if defined, is run after AUTH= is accepted or ignored. It can therefore make use of $authenticated_sender. The converse is not true: the value of $sender_address is not yet set up when the acl_smtp_mailauth ACL is run.

33.3. Authentication on an Exim server

When Exim receives an EHLO command, it advertises the public names of those authenticators that are configured as servers, subject to the following conditions:

The order in which the authenticators are defined controls the order in which the mechanisms are advertised.

Some mail clients (for example, some versions of Netscape) require the user to provide a name and password for authentication whenever AUTH is advertised, even though authentication may not in fact be needed (for example, Exim may be set up to allow unconditional relaying from the client by an IP address check). You can make such clients more friendly by not advertising AUTH to them. For example, if clients on the network are permitted (by the ACL that runs for RCPT) to relay without authentication, you should set

  auth_advertise_hosts = !

so that no authentication mechanisms are advertised to them.

The server_advertise_condition controls the advertisement of individual authentication mechanisms. For example, it can be used to restrict the advertisement of a patricular mechanism to encrypted connections, by a setting such as:

  server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}}

If the session is encrypted, $tls_cipher is not empty, and so the expansion yields “yes”, which allows the advertisement to happen.

When an Exim server receives an AUTH command from a client, it rejects it immediately if AUTH was not advertised in response to an earlier EHLO command. This is the case if

Otherwise, Exim runs the ACL specified by acl_smtp_auth in order to decide whether to accept the command. If acl_smtp_auth is not set, AUTH is accepted from any client host.

If AUTH is not rejected by the ACL, Exim searches its configuration for a server authentication mechanism that was advertised in response to EHLO and that matches the one named in the AUTH command. If it finds one, it runs the appropriate authentication protocol, and authentication either succeeds or fails. If there is no matching advertised mechanism, the AUTH command is rejected with a 504 error.

When a message is received from an authenticated host, the value of $received_protocol is set to “asmtp” instead of “esmtp”, and $sender_host_authenticated contains the name (not the public name) of the authenticator driver that successfully authenticated the client from which the message was received. This variable is empty if there was no successful authentication.

33.4. Testing server authentication

Exim's -bh option can be useful for testing server authentication configurations. The data for the AUTH command has to be sent using base64 encoding. A quick way to produce such data for testing is the following Perl script:

  use MIME::Base64;
  printf ("%s", encode_base64(eval "\"$ARGV[0]\""));

This interprets its argument as a Perl string, and then encodes it. The interpretation as a Perl string allows binary zeros, which are required for some kinds of authentication, to be included in the data. For example, a command line to run this script on such data might be

  encode '\0user\0password'

Note the use of single quotes to prevent the shell interpreting the backslashes, so that they can be interpreted by Perl to specify characters whose code value is zero.

Warning 1: If either of the user or password strings starts with an octal digit, you must use three zeros instead of one after the leading backslash. If you do not, the octal digit that starts your string will be incorrectly interpreted as part of the code for the first character.

Warning 2: If there are characters in the strings that Perl interprets specially, you must use a Perl escape to prevent them being misinterpreted. For example, a command such as

  encode '\\0pas$$word'

gives an incorrect answer because of the unescaped “@” and “$” characters.

If you have the mimencode command installed, another way to do produce base64-encoded strings is to run the command

  echo -e -n `\0user\0password' | mimencode

The -e option of echo enables the interpretation of backslash escapes in the argument, and the -n option specifies no newline at the end of its output. However, not all versions of echo recognize these options, so you should check your version before relying on this suggestion.

33.5. Authentication by an Exim client

The smtp transport has two options called hosts_require_auth and hosts_try_auth. When the smtp transport connects to a server that announces support for authentication, and the host matches an entry in either of these options, Exim (as a client) tries to authenticate as follows:

When Exim has authenticated itself to a remote server, it adds the AUTH parameter to the MAIL commands it sends, if it has an authenticated sender for the message. If the message came from a remote host, the authenticated sender is the one that was receiving on an incoming MAIL command, provided that the incoming connection was authenticated and the server_mail_auth condition allowed the authenticated sender to be retained. If a local process calls Exim to send a message, the sender address that is built from the login name and qualify_domain is treated as authenticated. However, if the authenticated_sender option is set on the smtp transport, it overrides the authenticated sender that was received with the message.

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