Exim Internet Mailer

1. Generic options for authenticators

When Exim is authenticating as a client, it skips any authenticator whose client_condition expansion yields “0”, “no”, or “false”. This can be used, for example, to skip plain text authenticators when the connection is not encrypted by a setting such as:

client_condition = ${if !eq{$tls_out_cipher}{}}

When client authentication succeeds, this condition is expanded; the result is used in the log lines for outbound messages. Typically it will be the user name used for authentication.

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

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.

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.

This option must be set for a plaintext server authenticator, where it is used directly to control authentication. See section 34.2 for details.

For the gsasl authenticator, this option is required for various mechanisms; see chapter 38 for details.

For the other authenticators, server_condition can be used as an additional authentication or authorization mechanism that is applied after the other authenticator conditions succeed. If it is set, it is expanded when the authenticator would otherwise return a success code. If the expansion is forced to fail, authentication fails. Any other expansion failure causes a temporary error code to be returned. If the result of a successful expansion is an empty string, “0”, “no”, or “false”, authentication fails. If the result of the expansion is “1”, “yes”, or “true”, authentication succeeds. For any other result, a temporary error code is returned, with the expanded string as the error text.

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.

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.

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.

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:

• If the connection is not using extended SMTP (that is, HELO was used rather than EHLO), the use of AUTH= is a syntax error.

• If the value of the AUTH= parameter is “<>”, it is ignored.

• If acl_smtp_mailauth is defined, the ACL it specifies is run. While it is running, the value of $authenticated_sender is set to the value obtained from the AUTH= parameter. If the ACL does not yield “accept”, the value of $authenticated_sender is deleted. The acl_smtp_mailauth ACL may not return “drop” or “discard”. If it defers, a temporary error code (451) is given for the MAIL command.

• If acl_smtp_mailauth is not defined, the value of the AUTH= parameter is accepted and placed in $authenticated_sender only if the client has authenticated.

• If the AUTH= value was accepted by either of the two previous rules, and the client has authenticated, and the authenticator has a setting for the server_mail_auth_condition, the condition is checked at this point. The valued that was saved from the authenticator is expanded. If the expansion fails, or yields an empty string, “0”, “no”, or “false”, the value of $authenticated_sender is deleted. If the expansion yields any other value, the value of $authenticated_sender is retained and passed on with 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.

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 client host must match auth_advertise_hosts (default *).

• It the server_advertise_condition option is set, its expansion must not yield the empty string, “0”, “no”, or “false”.

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 10.9.8.0/24 network are permitted (by the ACL that runs for RCPT) to relay without authentication, you should set

auth_advertise_hosts = ! 10.9.8.0/24

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 particular mechanism to encrypted connections, by a setting such as:

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

If the session is encrypted, $tls_in_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

• The client host does not match auth_advertise_hosts; or

• No authenticators are configured with server options; or

• Expansion of server_advertise_condition blocked the advertising of all the server authenticators.

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 “esmtpa” or “esmtpsa” instead of “esmtp” or “esmtps”, 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.

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 '\0user@domain.com\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.

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:

• For each authenticator that is configured as a client, in the order in which they are defined in the configuration, it searches the authentication mechanisms announced by the server for one whose name matches the public name of the authenticator.

• When it finds one that matches, it runs the authenticator’s client code. The variables $host and $host_address are available for any string expansions that the client might do. They are set to the server’s name and IP address. If any expansion is forced to fail, the authentication attempt is abandoned, and Exim moves on to the next authenticator. Otherwise an expansion failure causes delivery to be deferred.

• If the result of the authentication attempt is a temporary error or a timeout, Exim abandons trying to send the message to the host for the moment. It will try again later. If there are any backup hosts available, they are tried in the usual way.

• If the response to authentication is a permanent error (5xx code), Exim carries on searching the list of authenticators and tries another one if possible. If all authentication attempts give permanent errors, or if there are no attempts because no mechanisms match (or option expansions force failure), what happens depends on whether the host matches hosts_require_auth or hosts_try_auth. In the first case, a temporary error is generated, and delivery is deferred. The error can be detected in the retry rules, and thereby turned into a permanent error if you wish. In the second case, Exim tries to deliver the message unauthenticated.

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.