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 it's 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)
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)
auth_policy_request_attributes: Request attributes specification (see attributes section below)
Default: auth_policy_request_attributes = login=%{orig_username} pwhash=%{hashed_password} remote=%{real_rip}
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
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"}}
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.