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Proxy passdb ​

Dovecot supports proxying IMAP, POP3, submission server, LMTP Server, and ManageSieve 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. The proxy may or may not perform authentication itself. This requires that the client uses only cleartext authentication, or alternatively the proxy has access to users' passwords in cleartext.

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

The proxy is configured pretty much the same way as login referrals, with the addition of proxy field.

Fields ​

proxy ​

Enables the proxying.

Either this or proxy_maybe is required.

proxy_maybe ​

Enables the proxying.

Either this or proxy 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.

  • proxy_maybe with LMTP require.
  • proxy_maybe with host=<dns name> requires.
  • auth_proxy_self setting in dovecot.conf can be used to specify extra IPs that are also considered to be the proxy's own IPs.

proxy_always ​

Can be used with proxy_maybe to conditionally do proxying to specified remote host (host isn't self) or to let cluster assign a backend host (host is self).

Basically, this setting just always sends the proxy extra field to login process, but not necessarily the host.

Useful when dividing users across multiple clusters.

host=<s> ​

The destination server's IP address.

This field is required.

source_ip=<s> ​

The source IP address to use for outgoing connections.

port=<s> ​

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

protocol=<s> ​

The protocol to use for the connection to the destination server. This field is currently only relevant for LMTP: it can be used to select either lmtp or smtp.

destuser=s ​

Tell client to use a different username when logging in.

proxy_mech=<s> ​

Tell client to use this SASL authentication mechanism when logging in.

proxy_timeout=<time_msecs> ​

Abort connection after this much time has passed.

This overrides the default login_proxy_timeout.

This setting applies only to proxying via login processes, not to lmtp or doveadm processes.

proxy_nopipelining ​

Don't pipeline IMAP commands. This is a workaround for broken IMAP servers that hang otherwise.

proxy_not_trusted ​

IMAP/POP3 proxying never sends the ID/XCLIENT command to remote.

SSL ​

You can use SSL/TLS connection to destination server by returning:

  • ssl = yes

    Use SSL and require a valid verified remote certificate.

    WARNING

    Unless used carefully, this is an insecure setting! 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.

    Note

    ssl_client_ca_dir or ssl_client_ca_file aren't currently used for verifying the remote certificate, although ideally they will be in a future Dovecot version.

    For now you need to add the trusted remote certificates to ssl_ca.

    WARNING

    doveadm proxying doesn't support SSL/TLS currently - any ssl/starttls extra field is ignored.

  • ssl=any-cert: Use SSL, but don't require a valid remote certificate.

  • starttls=yes: Use STARTTLS command instead of doing SSL handshake immediately after connected.

  • starttls=any-cert: Combine starttls and ssl=any-cert.

Additionally you can also tell Dovecot to send SSL client certificate to the remote server using ssl_client_cert and ssl_client_key settings in dovecot.conf.

Set login_trusted_networks to point to the proxies in the backends. This way you'll get the clients' actual IP addresses logged instead of the proxy's.

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.

For POP3 you'll have to modify Dovecot's sources for now (src/pop3/capability.h).

Dovecot also automatically sends updated untagged CAPABILITY reply if it detects that the remote server has different capabilities than what it already advertised to the client, but some clients simply ignore the updated CAPABILITY reply.

Source IPs ​

If your proxy handles a lot of connections (~64k) to the same destination IP, you may run out of TCP ports. The only way to work around this is to use either multiple destination IPs or ports, or multiple source IPs.

Multiple source IPs can be easily used by adding them to login_source_ips. You can also use hostnames which expand to multiple IPs.

By prefixing the setting with ? (e.g. login_source_ips = ?proxy-sources.example.com) Dovecot will use only those IPs that actually exist in the server, allowing you to share the same config file with multiple servers.

It's probably better not to include the server's default outgoing IP address in the setting, as explained here: https://idea.popcount.org/2014-04-03-bind-before-connect/.

Disconnection Delay ​

To avoid reconnection load spikes when a backend server dies, you can tell proxy to spread the client disconnections over a longer time period (after the server side of the connection is already disconnected).

login_proxy_max_disconnect_delay controls this (disabled by default).

Forwarding Fields ​

You can forward arbitrary variables by returning them prefixed with forward_.

Dovecot will use a protocol-dependent extension to forward these variables to the next hop. The next hop imports these to the auth request as passdb extra fields, so they are visible in, e.g., %{passdb:forward_variable}.

If the proxying continues, all these fields are further forwarded to the next hop again.

This feature requires that the sending host is in login_trusted_networks.

See forwarding parameters in proxying for more details on how this is implemented for different protocols, which includes limits to the key and value lengths and counts.

Note

Most importantly the IMAP ID command restricts the forward key length to just 20 bytes (excluding forward_ prefix). Larger keys are silently dropped.

Moving Users Between Backends/Clusters ​

A safe way to move users from one cluster to another is to do it like:

  • Set delay_until=<timestamp> passdb extra fields where <timestamp> is the current timestamp plus some seconds into future (e.g. 31s). You may also want to append, e.g., +5 for some load balancing if a lot of users are moved at once.

  • Set host=<new host> passdb extra fields. This update should be atomic together with the delay_until field.

  • Use doveadm kick to kick the user's existing connections.

    • The processes may still continue running in the backend for a longer time. If you want to be absolutely sure, you could also run a script to kill -9 all processes for the user in the backend. This of course has its own problems.

The idea here is that while the user's connections are being kicked and the backend processes are finishing up and shutting down, new connections are being delayed in the proxy. This delay should be long enough that the user's existing processes are expected to die, but not so large that clients get connection timeouts. A bit over 30 seconds is likely a good value. Once the delay_until timestamp is reached, the connections continue to the new host.

If you have a lot of users, it helps to group some of them together and do the host/delay_until updates on a per-group basis rather than per-user basis.

ID Command Forwarding ​

If you want to forward, for some reason, the IMAP ID command provided by the client, set imap_id_retain = yes.

This will also enable client_id variable in variable expansions for auth requests, which will contain the ID command as IMAP arglist.

Password Forwarding ​

If you don't want proxy itself to do authentication, you can configure it to succeed with any given password. You can do this by returning an empty password and 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:

  • master=<s>: This contains the master username (e.g. proxy). It's used as SASL authentication ID.

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

See master users for more information how to configure this.

OAuth2 Forwarding ​

If you want to forward OAuth2 authentication database tokens, return field proxy_mech=%m as extra field.

Examples ​

Password Forwarding with Static DB ​

See static authentication database.

Password Forwarding with SQL ​

Create the SQL table:

sql
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:

| user | host | destuser | | john |192.168.0.1 | | | joe | 192.168.0.2 | joe@example.com |

# If you want to trade a bit of security for higher performance, change
# these settings:
service imap-login {
  service_count = 0
}
service pop3-login {
  service_count = 0
}

# If you are not moving mailboxes between hosts on a daily basis you can
# use authentication cache pretty safely.
auth_cache_size = 4096

auth_mechanisms = plain
passdb {
  driver = sql
  args = /usr/local/etc/dovecot/dovecot-sql.conf.ext
}
driver = mysql
connect = host=sqlhost1 host=sqlhost2 dbname=mail user=dovecot password=secret
password_query = SELECT NULL AS password, 'Y' as nopassword, \
    host, destuser, 'Y' AS proxy \
    FROM proxy WHERE user = '%u'

proxy_maybe with SQL ​

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

auth_mechanisms = plain

passdb {
  driver = sql
  args = /usr/local/etc/dovecot/dovecot-sql.conf.ext
}
userdb sql {
  driver = sql
  args = /usr/local/etc/dovecot/dovecot-sql.conf.ext
}
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 user AS username, domain, home \
    FROM users WHERE user = '%n' AND domain = '%d'

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