$Cambridge: exim/exim-src/src/auths/README,v 1.5 2006/02/23 12:41:22 ph10 Exp $

AUTHS

The modules in this directory are in support of various authentication
functions. Some of them, such as the base64 encoding/decoding and MD5
computation, are just functions that might be used by several authentication
mechanisms. Others are the SMTP AUTH mechanisms themselves, included in the
final binary if the relevant AUTH_XXX value is set in Local/Makefile. The
general functions are in separate modules so that they get included in the
final binary only if they are actually called from somewhere.

GENERAL FUNCTIONS

The API for each of these functions is documented with the function's code.

  auth_b64encode       encode in base 64
  auth_b64decode       decode from base 64
  auth_call_pam        do PAM authentication (if build with SUPPORT_PAM)
  auth_get_data        issue SMTP AUTH challenge and read response
  auth_xtextencode     encode as xtext
  auth_xtextdecode     decode from xtext

INTERFACE TO SMTP AUTHENTICATION MECHANISMS

These are general SSL mechanisms, adapted for use with SMTP. Each
authentication mechanism has three functions, for initialization, server
authentication, and client authentication.

INITIALIZATION

The initialization function is called when the configuration is read, and can
check for incomplete or illegal settings. It has one argument, a pointer to the
instance block for this configured mechanism. It must set the flags called
"server" and "client" in the generic auth_instance block to indicate whether
the server and/or client functions are available for this authenticator.
Typically this depends on whether server or client configuration options have
been set, but it is also possible to have an authenticator that has only one of
the server or client functions.

SERVER AUTHENTICATION

The second function performs authentication as a server. It receives a pointer
to the instance block, and its second argument is the remainder of the data
from the AUTH command. The numeric variable maximum setting (expand_nmax) is
set to zero, with $0 initialized as unset. The authenticator may set up numeric
variables according to its (old) specification and $auth<n> variables the
preferred ones nowadays; it should leave them set at the end so that they can
be used for the expansion of the generic server_set_id option, which happens
centrally.

This function has access to the SMTP input and output so that it can write
intermediate responses and read more data if necessary. There is a packaged
function in auth_get_data() which outputs a challenge and reads a response.

The yield of a server authentication check must be one of:

  OK          success
  DEFER       couldn't complete the check
  FAIL        authentication failed
  CANCELLED   authentication forced to fail by "*" response to challenge,
                or by a forced string expansion failure
  BAD64       bad base64 data received
  UNEXPECTED  unexpected data received

In the case of DEFER, auth_defer_msg should point to an error message.

CLIENT AUTHENTICATION

The third function performs authentication as a client. It receives a pointer
to the instance block, and four further arguments:

  The smtp_inblock item for the connection to the remote host.

  The normal command-reading timeout value.

  A pointer to a buffer, to be used for receiving responses. It is done this
    way so that the buffer is available for logging etc. in the calling
    function in cases of error.

  The size of the buffer.

The yield of a client authentication check must be one of:

  OK          success
  FAIL_SEND   error after writing a command; errno is set
  FAIL        failed after reading a response;
              either errno is set (for timeouts, I/O failures) or
              the buffer contains the SMTP response line
  CANCELLED   the client cancelled authentication (often "fail" in expansion)
              the buffer may contain a message; if not, *buffer = 0
  ERROR       local problem (typically expansion error); message in buffer

To communicate with the remote host the client should call
smtp_write_command(). If this yields FALSE, the authenticator should return
FAIL. After a successful write, the response is received by a call to
smtp_read_response(), which should use the buffer handed to the client function
as an argument.

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