This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.

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.


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.

Required Minimum Configuration

auth_policy_server_url =
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=%{orig_username} pwhash=%{hashed_password} remote=%{real_rip}
#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}




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


{"login":"john.doe","pwhash":"1234","remote":"", "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


for this to work.


{"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:

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:

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:

status result value is ignored.

External Auth Policy Servers