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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 Trying 192.168.34.25... Connected to server.example. Escape character is '^]'. 220 server.example ESMTP Exim 4.20 ... ehlo client.example 250-server.example Hello client.example [10.8.4.5] 250-SIZE 52428800 250-PIPELINING 250-AUTH PLAIN 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
AUTH_CRAM_MD5=yes AUTH_CYRUS_SASL=yes AUTH_PLAINTEXT=yes AUTH_SPA=yes |
in ‘Local/Makefile’, respectively. The first of these supports the CRAM-MD5 authentication mechanism (RFC 2195), and the second provides an interface to the Cyrus SASL authentication library. The third 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 fourth authenticator supports Microsoft's Secure Password Authentication mechanism.
The authenticators are configured using the same syntax as other drivers (see section Format of driver configurations). 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:
cram: driver = cram_md5 public_name = CRAM-MD5 server_secret = ${if eq{$auth1}{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 | ||
33.2 The AUTH parameter on MAIL commands | ||
33.3 Authentication on an Exim server | ||
33.4 Testing server authentication | ||
33.5 Authentication by an Exim client |
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| Use: authenticators | Type: string* | Default: unset |
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_cipher}{}} |
(Older documentation incorrectly states that $tls_cipher
contains the cipher
used for incoming messages. In fact, during SMTP delivery, it contains the
cipher used for the delivery.)
| Use: authenticators | Type: string | Default: unset |
This option must always be set. It specifies which of the available authenticators is to be used.
| Use: authenticators | 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.
| Use: authenticators | Type: string* | 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 Authentication on an Exim server below for further discussion.
| Use: authenticators | Type: string* | Default: unset |
This option must be set for a plaintext
server authenticator, where it
is used directly to control authentication. See section Using plaintext in a server
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.
| Use: authenticators | Type: string* | 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.
| Use: authenticators | Type: string* | 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.
| Use: authenticators | Type: string* | 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.
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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:
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.
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.
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.
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When Exim receives an EHLO command, it advertises the public names of those authenticators that are configured as servers, subject to the following conditions:
auth_advertise_hosts
(default *).
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_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
auth_advertise_hosts
; or
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 $sender_host_authenticated
contains the name (not the
public name) of the authenticator driver that successfully authenticated the
client from which the message was received. This variable is empty if there was
no successful authentication.
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Exim's -bh
option can be useful for testing server authentication
configurations. The data for the AUTH command has to be sent using base64
encoding. A quick way to produce such data for testing is the following Perl
script:
use MIME::Base64; printf ("%s", encode_base64(eval "\"$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.
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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:
$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.
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.
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