Login processes

The main purpose of login processes is to handle the IMAP, POP3, Submission (v2.3), and ManageSieve 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.

For explanation on the various settings for services, see Services

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. The only way out of it is to find and exploit a kernel security hole.

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. With TLS/SSL connections, the login process will not terminate, and remains to perform proxying between imap backend process and the client.

If the maximum login process count is reached, the oldest process in logging-in state (ie. non-proxying) is destroyed.
To avoid startup latency for new client connections, set process_min_avail to higher than zero. That many idling processes are always kept around waiting for new connections.
vsz_limit should be fine at its default 64MB value.

High-performance mode

You can enable high-performance mode with:

service imap-login {
  service_count = 0
  #client_limit = $default_client_limit
  process_min_avail = 4 # number of CPU cores
  vsz_limit = 1G
}
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 (in Dovecot or SSL library) the attacker is now able to see other users logging in and steal their passwords, read their mails, etc.

process_min_avail should be set to be at least the number of CPU cores in the system, so that all of them will be used.
Otherwise new processes are created only once an existing one's connection count reaches client_limit
Default client_limit * process_limit = 1000*100 = 100k connections
vsz_limit should be increased to avoid out of memory errors, especially if you're using SSL/TLS.

Login access check sockets

Dovecot login processes can check via UNIX socket if the incoming connection should be allowed to log in. This is most importantly implemented to enable TCP wrappers support for Dovecot.

TCP wrappers support

You must have built Dovecot with support for TCP wrappers. You can do this by giving --with-libwrap parameter to configure.

Add to dovecot.conf:

login_access_sockets = tcpwrap

service tcpwrap {
  unix_listener login/tcpwrap {
    group = $default_login_user
    mode = 0600
    user = $default_login_user
  }
}

Remember to configure your rules! The format is described in hosts.allow(5) and hosts.deny(5). Files used are usually /etc/hosts.allow and /etc/hosts.deny.