Dovecot Authentication Protocol

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 16384 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

The parameters use Dovecot’s generic “tab-escaping”, where \001 is used as the escape character. After it follows:

  • 0: NUL character

  • 1: \001 character

  • t: TAB

  • r: CR

  • l: LF

  • anything else writes the character itself (skipping the \001)

Client <-> Server

Client is an untrusted authentication client process. For example in Dovecot the imap-login process is an auth client. The same auth client can perform multiple authentications against different users.

Server is the auth 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 specifies the version number 1.2.

  • 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:

session=<id>

Unique session ID. Mainly used for logging.

lip=<ip>

Local IP connected to by the client. In standard string format, e.g. 127.0.0.1 or ::1.

rip=<ip>

Remote client IP

lport=<port>

Local port connected to by the client.

rport=<port>

Remote client port

real_rip, real_lip, real_lport, real_rport

When Dovecot proxy is used, the real_rip/real_port are the proxy’s IP/port and real_lip/real_lport are the backend’s IP/port where the proxy was connected to.

local_name=<name>

TLS SNI name

debug

Enable debugging for this lookup.

forward_fields

List of fields that will become available via %{forward_*} variables. The list is double-tab-escaped, like: tab_escaped[tab_escaped(key=value)[<TAB>...]

AUTH-only parameters are:

secured[=tls]

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

transport=<value>

The value can be “insecure”, “trusted” or “TLS”.

tls_cipher=<cipher>

TLS cipher being used.

tls_cipher_bits=<bits>

The number of bits in the TLS cipher.

tls_pfs=<string>

TLS perfect forward secrecy algorithm (e.g. DH, ECDH)

tls_protocol=<name>

TLS protocol name (e.g. SSLv3, TLSv1.2)

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.

client_id

IMAP ID string

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”).

code=temp_fail (v2.3+), temp (<=v2.2)

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

code=authz_fail (v2.3+), authz (v1.2..v2.2)

Authentication succeeded, but authorization failed (master user’s password was ok, but destination user was not ok).

code=user_disabled (v2.3+), user_disabled (v2.2)

User is disabled (password may or may not have been correct)

code=pass_expired (v2.3+), pass_expired (v2.2)

User’s password has expired.

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 (e.g. 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.

There can be also other extra fields.