Proxying

Dovecot supports proxying IMAP and POP3 connections to other hosts. The proxying can be done for all users, or only for some specific users. There are two ways to do the authentication:

1. Forward the password to the remote server and let it perform the actual authentication. This requires that the client uses only plaintext authentication.

2. Let Dovecot proxy perform the authentication and login to remote server using the proxy's master password. This allows client to use also non-plaintext authentication.

The proxy is configured pretty much the same way as login referrals, with the addition of proxy field. The common fields to use for both proxying ways are:

• proxy and proxy_maybe: Enables the proxying. Either one of these fields is required.

  • proxy_maybe can be used to implement "automatic proxying". If the proxy destination matches the current connection, the user gets logged in normally instead of being proxied. If the same happens with proxy, the login fails with "Proxying loops" error. This feature exists only in v1.1+.

• host=s: The destination server's IP address. This field is required. Note that currently it's required to use an IP address since no DNS resolving is done. (DNS resolution proposal: look here)

• port=s: The destination server's port. The default is 143 with IMAP and 110 with POP3.

• destuser=s: Tell client to use a different username when logging in.

• proxy_timeout: Abort connection after this many seconds. (v1.2.4+)

In v1.2.rc4+ the connections to destination server can be TLS/SSL encrypted by returning:

• ssl=yes: Use SSL and require a valid verified remote certificate. WARNING: Unless used carefully, this is an insecure setting! Currently host must be an IP address, so this setting accepts any certificate signed by a trusted CA. The host name isn't checked in any way against the certificate's CN. The only way to use this securely is to only use and allow your own private CA's certs, anything else is exploitable by a man-in-the-middle attack.

• ssl=any-cert: Use SSL, but don't require a valid remote certificate.
• starttls: Use STARTTLS command instead of doing SSL handshake immediately after connected.
  • With v2.0+ you can use "starttls=any-cert". With older versions you need to specify both "ssl=any-cert" and "starttls".

The destination servers don't need to be running Dovecot, but you should make sure that the Dovecot proxy doesn't advertise more capabilities than the destination server can handle. For IMAP you can do this by changing imap_capability setting. For POP3 you'll have to modify Dovecot's sources for now (src/pop3/capability.h). v1.2.rc4+ automatically sends updated untagged CAPABILITY reply if it detects that the remote server has different capabilities than what it already advertised to the client. Note that some clients simply ignore the updated CAPABILITY reply.

Password forwarding

Make sure that the authentication succeeds with any given password. You can do this by using empty passwords. v1.1+ requires also that you return nopassword field.

Master password

This way of forwarding requires the destination server to support master user feature. The users will be normally authenticated in the proxy and the common proxy fields are returned, but you'll need to return two fields specially:

• destuser=s: Both the logging username and the master username need to be included in this.

• pass=s: This field contains the master user's password.

For the master user logins it'd be cleaner to use a SASL mechanism with authorization ID, but for now this isn't supported.

If the destination server is Dovecot, you can return these fields like:

• destuser=%u*proxy

• pass=secret

Then in the destination Dovecot's config file set auth_master_user_separator=* and create a master user named proxy with password secret. See MasterPassword for more information how to configure this.

Example password forwarding SQL configuration

Create the SQL table:

CREATE TABLE proxy (
  user varchar(255) NOT NULL,
  host varchar(16) default NULL,
  destuser varchar(255) NOT NULL default '',
  PRIMARY KEY  (user)
);

Insert data to SQL corresponding your users.

Working data could look like this:

The important parts of dovecot.conf:

# If you want to trade a bit of security for higher performance, change these settings:
login_process_per_connection = no
login_processes_count = 20

# If you are not moving mailboxes from host to one on daily basis you can
# use authentication cache pretty safely.
auth_cache_size = 4096

auth default {
  mechanisms = plain

  # dovecot-auth only needs to be able to connect to SQL
  user = nobody

  # Userdb settings are not used with proxy but there need to be something.
  userdb static {
    args = uid=0 gid=0
  }
  passdb sql {
    args = /usr/local/etc/dovecot-sql.conf
  }
}

The important parts of dovecot-sql.conf:

# Database driver: mysql, pgsql
driver = mysql

# Database connect string.
# Only MySQL driver support multiple hosts for now.
connect = host=sqlhost1 host=sqlhost2 dbname=mail user=dovecot password=secret

# Query
password_query = SELECT NULL AS password, 'Y' as nopassword, host, destuser, 'Y' AS proxy FROM proxy WHERE user = '%u'

Example proxy_maybe SQL configuration

Create the SQL table:

CREATE TABLE users (
  user varchar(255) NOT NULL,
  domain varchar(255) NOT NULL,
  password varchar(100) NOT NULL,
  host varchar(16) NOT NULL,
  home varchar(100) NOT NULL,
  PRIMARY KEY (user)
);

The important parts of dovecot.conf:

# user/group who owns the message files:
mail_uid = vmail
mail_gid = vmail

auth default {
  mechanisms = plain

  # dovecot-auth only needs to be able to connect to SQL
  user = nobody

  passdb sql {
    args = /usr/local/etc/dovecot-sql.conf
  }
  userdb sql {
    args = /usr/local/etc/dovecot-sql.conf
  }
}

The important parts of dovecot-sql.conf:

driver = mysql

password_query = \
  SELECT concat(user, '@', domain) AS user, password, host, 'Y' AS proxy_maybe \
  FROM users WHERE user = '%n' AND domain = '%d'

user_query = SELECT home FROM users WHERE user = '%n' AND domain = '%d'

Example proxy LDAP configuration

see: PasswordDatabase/ExtraFields for more information, and a worked out example