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

Login processes

The main purpose of login processes is to handle the IMAP and POP3 connections before the user has logged in. The login processes don't need to be able to do anything else than let the user log in, so they can run in highly restricted environment. By default they are run as a non-privileged "dovenull" user chrooted into a non-writable directory containing only authentication UNIX sockets.

Login processes also handle proxying the SSL and TLS connections even after the user has logged in. This way all the SSL code runs in the same restricted environment, which means that a security hole in the SSL library gives the attacker access only to the restricted chroot, rather than possibly all the users' mails.

The default login settings should be good enough for small sites. There are two ways to run the login processes: the high-security mode and the high-performance mode. Both are discussed separately below.

Login process creation

login_processes_count and login_max_processes_count setting control how new login processes are created. login_processes_count specifies the number of login processes that are tried to be kept listening for new connections. However when a lot of connections arrive at the same time this number will increase automatically as described below.

High-security mode

You can enable high-security mode with:

service imap-login {
  service_count = 1
  #process_min_avail = 0
  #process_limit = $default_process_limit
  #vsz_limit = 64M
}
service pop3-login {
  service_count = 1
}

This is the default. It works by using a new imap-login or pop3-login process for each incoming connection. Since the processes run in a highly restricted chroot, running each connection in a separate process means that in case there is a security hole in Dovecot's pre-authentication code or in the SSL library, the attacker can't see other users' connections and can't really do anything destructive.

Since one login process can handle only one connection, the service's process_limit setting limits the number of users that can be logging in at the same time (defaults to default_process_limit=100). SSL/TLS proxying processes are also counted here, so if you're using SSL/TLS you'll need to make sure this count is higher than the maximum number of users that can be logged in simultaneously.

High-performance mode

You can enable high-performance mode with:

service imap-login {
  service_count = 0
  #client_limit = $default_client_limit
  #process_min_avail = 0
  #vsz_limit = 64M
}
service pop3-login {
  service_count = 0
}

It works by using a number of long running login processes, each handling a number of connections. This loses much of the security benefits of the login process design, because in case of a security hole the attacker is now able to see other users logging in and steal their passwords.