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## 36. The cyrus_sasl authenticator

The code for this authenticator was provided by Matthew Byng-Maddick of A L Digital Ltd (http://www.aldigital.co.uk).

The cyrus_sasl authenticator provides server support for the Cyrus SASL library implementation of the RFC 2222 (“Simple Authentication and Security Layer”). This library supports a number of authentication mechanisms, including PLAIN and LOGIN, but also several others that Exim does not support directly. In particular, there is support for Kerberos authentication.

The cyrus_sasl authenticator provides a gatewaying mechanism directly to the Cyrus interface, so if your Cyrus library can do, for example, CRAM-MD5, then so can the cyrus_sasl authenticator. By default it uses the public name of the driver to determine which mechanism to support.

Where access to some kind of secret file is required, for example in GSSAPI or CRAM-MD5, it is worth noting that the authenticator runs as the Exim user, and that the Cyrus SASL library has no way of escalating privileges by default. You may also find you need to set environment variables, depending on the driver you are using.

### 36.1 Using cyrus_sasl as a server

The cyrus_sasl authenticator has four private options. It puts the username (on a successful authentication) into $auth1. For compatibility with previous releases of Exim, the username is also placed in $1. However, the use of this variable for this purpose is now deprecated, as it can lead to confusion in string expansions that also use numeric variables for other things.

server_hostname | Use: cyrus_sasl | Type: string† | Default: `$primary_hostname`

This option selects the hostname that is used when communicating with the library. It is up to the underlying SASL plug-in what it does with this data.

server_mech | Use: cyrus_sasl | Type: string | Default: `public_name`

This option selects the authentication mechanism this driver should use. It allows you to use a different underlying mechanism from the advertised name. For example:

    sasl:
      driver = cyrus_sasl
      public_name = X-ANYTHING
      server_mech = CRAM-MD5
      server_set_id = $auth1

server_realm | Use: cyrus_sasl | Type: string | Default: unset

This specifies the SASL realm that the server claims to be in.

server_service | Use: cyrus_sasl | Type: string | Default: `smtp`

This is the SASL service that the server claims to implement.

For straightforward cases, you do not need to set any of the authenticator’s private options. All you need to do is to specify an appropriate mechanism as the public name. Thus, if you have a SASL library that supports CRAM-MD5 and PLAIN, you could have two authenticators as follows:

    sasl_cram_md5:
      driver = cyrus_sasl
      public_name = CRAM-MD5
      server_set_id = $auth1

    sasl_plain:
      driver = cyrus_sasl
      public_name = PLAIN
      server_set_id = $auth1

Cyrus SASL does implement the LOGIN authentication method, even though it is not a standard method. It is disabled by default in the source distribution, but it is present in many binary distributions.

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