Dovecot Authentication Protocol v1.1
====================================

General
-------

This is a line based protocol. Each line is a command which ends with an LF
character. The maximum line length isn't defined, but it's currently expected
to fit into 8192 bytes. Authentication mechanism specific data transfers are
the largest single parameters.

Each command is in format:

---%<-------------------------------------------------------------------------
<command name> TAB <parameters separated with TAB>
---%<-------------------------------------------------------------------------

Parameters are split into required and optional parameters. Required parameters
aren't in any specific format, but optional parameters are either booleans
without a value, or a name=value pair. If optional parameter name is unknown,
the parameter should just be ignored.

Typical command looks like (without spaces):

---%<-------------------------------------------------------------------------
command TAB param1 TAB param2 TAB optname=value TAB optboolean
---%<-------------------------------------------------------------------------

There is no way to have TABs or LFs in parameters.

Client <-> Server
-----------------

Client is an untrusted authentication client process. It can serve one or more
users, so from user's point of view it's usually eg. IMAP or SMTP server
process.

Server is an authentication server process.

The connection starts by both client and server sending handshakes:

---%<-------------------------------------------------------------------------
C: "VERSION" TAB <major> TAB <minor>
C: "CPID" TAB <pid>
S: "VERSION" TAB <major> TAB <minor>
S: "SPID" TAB <pid>
S: "CUID" TAB <pid>
S: "COOKIE" TAB <cookie>
S: "MECH" TAB <name> [TAB <parameters>] (multiple times)
S: "DONE"
---%<-------------------------------------------------------------------------

Both client and server should check that they support the same major version
number. If they don't, the other side isn't expected to be talking the same
protocol and should be disconnected. Minor version can be ignored. This
document is version number 1.1.

 * CPID and SPID specify client and server Process Identifiers (PIDs). They
   should be unique identifiers for the specific process. UNIX process IDs are
   good choices.
 * CUID is a server process-specific unique connection identifier. It's
   different each time a connection is established for the server.
 * CPID is used by master's REQUEST command.
 * SPID can be used by authentication client to tell master which server
   process handled the authentication.
 * CUID is currently useful only for APOP authentication.
 * COOKIE returns connection-specific 128 bit cookie in hex. It must be given
   to REQUEST command. (Protocol v1.1+ / Dovecot v2.0+)
 * DONE finishes the handshake from server. CPID finishes the handshake from
   client.

Authentication Mechanisms
-------------------------

MECH command announces an available authentication SASL mechanism. Mechanisms
may have parameters giving some details about them:

anonymous:
  Anonymous authentication

plaintext:
  Transfers plaintext passwords

dictionary:
  Subject to passive (dictionary) attack

active:
  Subject to active (non-dictionary) attack

forward-secrecy:
  Provides forward secrecy between sessions

mutual-auth:
  Provides mutual authentication

private:
  Don't advertise this as available SASL mechanism (eg. APOP)

Authentication Request
----------------------

---%<-------------------------------------------------------------------------
C: "AUTH" TAB <id> TAB <mechanism> TAB service=<service> [TAB <parameters>]
S1: "FAIL" TAB <id> [TAB <parameters>]
S2: "CONT" TAB <id> TAB <base64 data>
S3: "OK" TAB <id> [TAB <parameters>]
---%<-------------------------------------------------------------------------

ID is a connection-specific unique request identifier. It must be a 32bit
number, so typically you'd just increment it by one.

Service is the service requesting authentication, eg. POP3, IMAP, SMTP.

AUTH and USER (see below) common parameters are:

lip=<ip>:
  Local IP  - in standard string format,

rip=<ip>:
  Remote IP - ie. for IPv4 127.0.0.1 and for IPv6 ::1

lport=<port>:
  Local port

rport=<port>:
  Remote port

AUTH-only parameters are:

secured:
  Remote user has secured transport to auth client] (e.g. localhost, SSL, TLS)

valid-client-cert:
  Remote user has presented a valid SSL certificate.

no-penalty:
  Ignore auth penalty tracking for this request

cert_username:
  Username taken from client's SSL certificate.

resp=<base64>:
  Initial response for authentication mechanism.  NOTE: This must be the last
  parameter. Everything after it is ignored. This is to avoid accidental
  security holes if user-given data is directly put to base64 string without
  filtering out tabs.

FAIL parameters may contain:

reason=<str>:
  <str> should be sent to remote user instead of the standard "Authentication
  failed" messages. For example "invalid base64 data". It must NOT be used to
  give exact reason for authentication failure (i.e. "user not found" vs.
  "password mismatch").

temp:
  This is a temporary internal failure, e.g. connection was lost to SQL
  database.

authz:
  Authentication succeeded, but authorization failed (master user's password
  was ok, but destination user was not ok). Added in Dovecot v1.2.

A CONT response means that the authentication continues, and more data is
expected from client to finish the authentication. Given base64 data should be
sent to client. The client may continue the process issuing

---%<-------------------------------------------------------------------------
C: "CONT" TAB <id> TAB <base64 data>
---%<-------------------------------------------------------------------------

The <id> must match the <id> of the AUTH command.

FAIL and OK may contain multiple unspecified parameters which authentication
client may handle specially. The only one specified here is "user=<userid>"
parameter, which should always be sent if the userid is known.

Server <-> Master
-----------------

Master is a trusted process which may query results of previous client
authentication or information about a specific user. Master is optional and in
SMTP AUTH case it's not needed.

The connection starts by both server and master sending handshakes:

---%<-------------------------------------------------------------------------
S: "VERSION" TAB <major> TAB <minor>
S: "SPID" TAB <pid>
M: "VERSION" TAB <major> TAB <minor>
---%<-------------------------------------------------------------------------

Auth with client <-> server, both should check that the version numbers are
valid.

SPID can be used to let master identify the server process.

Master Requests
---------------

---%<-------------------------------------------------------------------------
M: "REQUEST" TAB <id> TAB <client-pid> TAB <client-id> TAB <cookie>
M: "USER" TAB <id> TAB <userid> TAB service=<service> [TAB <parameters>]
S: "NOTFOUND" TAB <id>
S: "FAIL" TAB <id> TAB <error message>
S: "USER" TAB <id> TAB <userid> [TAB <parameters>]
---%<-------------------------------------------------------------------------

Master commands can request information about existing authentication request,
or about a specified user.

USER command's service and parameters are the same as with AUTH client request.

ID is a connection-specific unique request identifier. It must be a 32bit
number, so typically you'd just increment it by one.

NOTFOUND reply means that the user wasn't found. (v1.x also reported unknown
request IDs with NOTFOUND.)

FAIL reply means an internal error occurred. Usually either a configuration
mistake or temporary error caused by lost resource (eg. database down). Also
unknown request IDs are reported as FAILs (since v2.0).

USER reply is sent if request succeeded. It can return parameters:

uid=<uid>:
  System user ID.

gid=<gid>:
  System group ID.

home=<dir>:
  Home directory.

chroot=<dir>:
  Chroot directory.

mail=<data>:
  Mail location.

system_user=<user>:
  System user name which can be used to get extra groups.  This will probably
  be replaced later by giving just multiple gid fields.

(This file was created from the wiki on 2017-02-24 04:44)
