Authentication policy support
=============================

Dovecot supports (v2.2.25+) external authentication policy server. This server
can be used to decide whether the connecting user is permitted, tarpitted or
outright rejected. While dovecot can do tarpitting and refusal on its own, this
adds support for making cluster-wide decisions to make it easier to deter and
defeat bruteforce attacks.

Configuration
-------------

The auth-policy server is a core feature and does not require plugin(s) to
work. To activate this feature, you need to configure it.

 * *auth_policy_server_url*: URL of the policy server, url is appended with
   ?command=allow/report unless it ends with '&', at which just
   command=allow/report is added.
    * /Default/: None (*REQUIRED* configuration)
    * Example: 'auth_policy_server_url = http://example.com:4001/'
 * *auth_policy_server_api_header*: Header and value to add to request (for API
   authentication)
    * /Default/: None (No authentication is done)
    * See https://en.wikipedia.org/wiki/Basic_access_authentication#Client_side

    * Example: 'Authorization: Basic <base64-encoded value>'
 * *auth_policy_server_timeout_msecs*: Request timeout in milliseconds
    * /Default/: 'auth_policy_server_timeout_msecs = 2000'
 * *auth_policy_hash_mech*: Hash mechanism to use for password, you can use any
   hash mechanism supported by Dovecot (md4,md5,sha1,sha256,sha512)
    * /Default/: 'auth_policy_hash_mech = sha256'
 * *auth_policy_hash_nonce*: Cluster-wide nonce to add to hash
    * /Default/: None (*REQUIRED* configuration)
    * Example: 'auth_policy_hash_nonce = localized_random_string'
 * *auth_policy_request_attributes*: Request attributes specification (see
   attributes section below)
    * /Default/: 'auth_policy_request_attributes = login=%{requested_username}
      pwhash=%{hashed_password} remote=%{rip} device_id=%{client_id}
      protocol=%s'
 * *auth_policy_reject_on_fail*: If policy request fails for some reason should
   users be rejected
    * /Default/: 'auth_policy_reject_on_fail = no'
 * *auth_policy_hash_truncate*: How many *bits* to use from password hash.
    * /Default/: 'auth_policy_hash_truncate = 12'
 * *auth_policy_check_before_auth*: Whether to do policy lookup before
   authentication is started
    * /Default/: 'auth_policy_check_before_auth = yes'
 * *auth_policy_check_after_auth*: Whether to do policy lookup after
   authentication is completed
    * /Default/: 'auth_policy_check_after_auth = yes'
 * *auth_policy_report_after_auth*: Whether to report authentication result
    * /Default/: 'auth_policy_report_after_auth = yes'

Required Minimum Configuration
------------------------------

---%<-------------------------------------------------------------------------
auth_policy_server_url = http://example.com:4001/
auth_policy_hash_nonce = localized_random_string
#auth_policy_server_api_header = Authorization: Basic <base64-encoded value>
#auth_policy_server_timeout_msecs = 2000
#auth_policy_hash_mech = sha256
#auth_policy_request_attributes = login=%{requested_username}
pwhash=%{hashed_password} remote=%{rip} device_id=%{client_id} protocol=%s
#auth_policy_reject_on_fail = no
#auth_policy_hash_truncate = 12
#auth_policy_check_before_auth = yes
#auth_policy_check_after_auth = yes
#auth_policy_report_after_auth = yes
---%<-------------------------------------------------------------------------

Password hash algorithm
-----------------------

To generate the hash, you concatenate nonce, login name, nil byte, password and
run it through the hash algorithm once. The hash is truncated when truncation
is set to non-zero. The hash is truncated by first choosing bits from MSB to
byte boundary (rounding up), then right-shifting the remainding bits.

---%<-------------------------------------------------------------------------
hash = H(nonce||user||'\x00'||password)
bytes = round8(bits*8)
hash = HEX(hash[0:bytes] >> (bytes-bits*8))
---%<-------------------------------------------------------------------------

Request attributes
------------------

Auth policy server requests are JSON requests. The JSON format can be specified
with auth_policy_request_attributes. The syntax is key=value pairs, and key can
contain one or more / to designate that a JSON object should be made. For
example:

---%<-------------------------------------------------------------------------
login=%{orig_username} pwhash=%{hashed_password} remote=%{real_rip}
---%<-------------------------------------------------------------------------

produces

---%<-------------------------------------------------------------------------
{"login":"john.doe","pwhash":"1234","remote":"127.0.0.1"}
---%<-------------------------------------------------------------------------

And

---%<-------------------------------------------------------------------------
login=%{orig_username} pwhash=%{hashed_password} remote=%{real_rip}
attrs/cos=%{userdb:cos}
---%<-------------------------------------------------------------------------

produces

---%<-------------------------------------------------------------------------
{"login":"john.doe","pwhash":"1234","remote":"127.0.0.1",
"attrs":{"cos":"premium"}}
---%<-------------------------------------------------------------------------

Since v2.2.29/v2.3 you can include IMAP ID command result in auth policy
requests, this is achieved with using %{client_id}, which will expand to IMAP
ID command arglist. You must set

---%<-------------------------------------------------------------------------
imap_id_retain=yes
---%<-------------------------------------------------------------------------

for this to work.

Since v2.3.2 the request contains 'tls' attribute when TLS has been used. TLS
is also detected if it's offloaded by a load balancer that can provide this
information using HAProxy v2 protocol to dovecot.

Response
--------

---%<-------------------------------------------------------------------------
{"status":-1,"msg":"go away"}
---%<-------------------------------------------------------------------------

'status' values: see below

Mode of operation
-----------------

Auth policy check: Authentication ''before'' userdb/passdb
----------------------------------------------------------

First query is done *before* password and user databases are consulted. This
means that any userdb/passdb attributes are left empty.

The command used here is 'allow' and will appear on the URL as command=allow.

'status' result values:

 * *-1*: Reject
 * *0*: Accept
 * *(Any other positive value)*: Tarpit for this number of seconds.

Auth policy check: Authentication ''after'' successful userdb/passdb lookup
---------------------------------------------------------------------------

Second lookup is done *after* authentication succeeds.

The command used here is 'allow' and will appear on the URL as command=allow.

'status' result values:

 * *-1*: Authentication fail
 * *>= 0*: Authentication succeed

Auth policy check: Reporting after authentication succeeds
----------------------------------------------------------

A report request is sent at end of authentication.

The command used here is 'report' and will appear on the URL as command=report.

The JSON request is sent with two additional attributes:

 * *success*: boolean true/false depending on whether the overall
   authentication succeeded
 * *policy_reject*: boolean true/false whether the failure was due to policy
   server

'status' result value is ignored.

External Auth Policy Servers
----------------------------

 * http://oxpedia.org/wiki/index.php?title=AppSuite:Dovecot_Antiabuse_Shield

(This file was created from the wiki on 2019-03-05 04:42)