Dovecot processes

Dovecot is split into multiple processes where each process does only one thing. This is partially because it makes the code cleaner, but also because it allows setting up different privileges for each process. The most important processes are:

• Master process (dovecot)
• Log process (log)
• Config process (config)
• Authentication process (auth)
• Login processes (imap-login, pop3-login)
• Mail processes (imap, pop3, lmtp)

Master Process

This process keeps all the other processes running. If a child process dies, another one is restarted automatically if necessary. The master process runs as root, so its functionality is attempted to be kept minimal.

The master process is the only process that opens all inet, unix socket and fifo listeners. These listener fds are then passed to child processes that it forks.

Each service has a pipe that is used by the service's processes to send status updates to master about how many client connections it can still accept. If the master process sees that the service is completely full and can't accept any more client connections, it logs a warning and eventually starts rejecting the client connections for the service.

The login processes are special however: When master process notices that all login processes are full, it instead notifies them about it. The login processes then start closing their oldest connections in order to make space for more client connections. This prevent DoSing the login services by simply opening many idling connections.

Log Process

Most of the logging is done via the log process. Only the master process and processes that start up standalone (e.g. dovecot-lda) bypass the log process.

The Dovecot master process sets up a separate pipe for each service, which is shared by all the processes of that service. The write side of the pipe becomes the processes' stderr fd, while the read side is read by the log process.

Log process has a few benefits over direct logging:

• Log process can start throttling a service that logs too rapidly.
• All stderr output logged by processes will be caught by the log process, so any errors printed by libraries will not be lost. These errors will also always have a log prefix showing which service caused the error.
• Avoids duplicate log lines about crashes. If a panic is logged before a process crashes, there's no unnecessary line logged about process dying with signal 6.

Config Process

The config processes parse the configuration file and feed the parsed output in a simplified format to all the other processes via UNIX socket connections.

The master process reads its configuration in a different way: It first executes the doveconf binary, which reads the configuration into environment variables and then executes back the dovecot master binary.

Standalone tools like dovecot-lda and doveadm first try to read the configuration via UNIX socket connection, but if that fails they do it by executing doveconf.

Authentication Process

The auth process handles everything related to the actual authentication: SASL authentication mechanisms, looking up and verifying the passwords and looking up user information.

There is only a single auth master process, which accepts all incoming connections. This means that the auth process needs to be very efficient in what it does and must not block for long or it will cause all the authentications to hang.

To handle potentially long-running blocking operations there are auth worker processes. These are often used for passdb and userdb lookups. The auth worker processes can also be used for verifying password hashes, which may be necessary if strong hashing algorithms are used.

Login Processes

The login processes implement the required minimum of the IMAP, POP3, ManageSieve or Submission protocols before a user logs in successfully. Each protocol is handled by a separate process (and binary).

These processes are run with least possible privileges. Unfortunately the default UNIX security model still allows them to do much more than they would have to: Accept new connections on a socket, connect to new UNIX sockets and read and write to existing file descriptors. Still, the login process is by default run under a user account that has no special access to anything, and runs inside a non-writable chroot where only a couple of UNIX sockets exist. Doing any damage inside there to the server itself should be difficult. It could of course still create connections to other services that would normally be unavailable from external IP addresses.

When a new connection comes, one of the login processes accept()s it. After that the client typically does nothing more than ask the server's capability list and then log in. The client may also start TLS session before logging in.

Authentication is done by talking to the authentication process. The login process is untrusted by the authentication process, so even if an attacker is able to execute arbitrary code inside a login process, they won't be able to log in without a valid username and password.

After receiving a successful authentication reply from the authentication process, the login process connects to the mail process via UNIX socket and sends the file descriptor it. The mail process verifies from the auth process that the authentication really was successful.

By default each login process will handle only a single connection and afterwards kill itself (but see SSL proxying below). This way attacker can't see other people's connections. This can however be disabled, in which case the security of the design suffers greatly.

The login processes handle SSL/TLS connections themselves completely. They keep proxying the connection to mail processes for the entire lifetime of the connection. This way if a security hole is found from the SSL library, an authenticated user still can't execute code outside the login process.

See login processes for more information about different settings related to login processes.

Mail Processes

These processes handle the actual post-login mail handling using the privileges of the logged in user.

Other Processes

There are also various other processes commonly used:

• anvil: Keep track of which mail processes handle which users
• dict: Proxy process for dict lookups
• dns-client: Asynchronous DNS lookups
• imap-hibernate: IDLEing imap connections can be moved to hibernation processes
• indexer and indexer-worker: For full text search indexing
• stats: Tracking statistics