% Getting started with InterIMAP
% [Guilhem Moulin](mailto:guilhem@fripost.org);
  [Gustav Eek](mailto:gustav.eek@fripost.org)

This document describes the setup of InterIMAP for a rather usual
user-case, where messages on a remote IMAP server `imap.example.net`
need to be synchronized locally in a bidirectional fashion (changes on
either server are replicated on the other one).


Local IMAP server
=================

Background and rationale
------------------------

On a workstation, one's mail storage is typically found under `~/Maildir`
(in [*Maildir* format][Maildir]) or in `/var/mail/$USER` (in [*mbox*
format][mbox]).  Local mail clients usually access it directly.  They
also often maintain their own cache in order to speed up message header
listing and searches.

While most bidirectional synchronization software (such as [OfflineIMAP])
are able to handle a mail storage in Maildir format, *InterIMAP is
not*.  Instead, InterIMAP needs an [IMAP4rev1] server on *both* peers
to synchronize.  This may sound like a severe limitation at first, but by
seeing both local and remote mail storage through the same “IMAP lens”,
InterIMAP is able to take advantage of the abstraction layer and
perform significant optimizations, yielding [much faster](benchmark.html)
synchronization.
*Note*: InterIMAP uses the [Quick Mailbox Resynchronization][RFC 7162]
extension for stateful synchronization, hence won't work on IMAP servers
that don't advertise [support](https://www.imapwiki.org/Specs#line-68)
for that extension.

Installing an [IMAP4rev1] server on a single-user workstation may sound
overkill, but we argue that most systems, not only servers, come with a
[Message Transfer Agent][MTA] preinstalled.  Just like one may use
`/usr/sbin/sendmail` (or a compatible interface) in order to send mail
out, we propose to use an `imap` binary to access them.

In order to take full advantage of the abstraction layer and of
InterIMAP's optimizations, one should *always* access the mail storage
through the local [IMAP4rev1] server and *never directly*.  Otherwise
the IMAP server will invalidate its cache each time it notices
inconsistencies, potentially causing a severe performance hit.  (*Or
worse*: very likely many [IMAP4rev1] servers are not able to gracefully
reconcile cache inconsistencies.)  As far as the mail client is
concerned, the cost of abstraction seems to be negligible.  (*TODO* link
to benchmark.)  Furthermore, we think that approach is in line with the
[Unix philosophy]: the mail client only takes care of the rendering
part, leaving the rest to the IMAP server (searches, sorting/threading,
as well as storage and caching logic).

Installation
------------

While this document focuses on [Dovecot](https://dovecot.org), a popular
[IMAP4rev1] server, any other [`QRESYNC`][RFC 7162]-capable server
should work equally well.  Run the following command to install the
Dovecot IMAP server on a Debian GNU/Linux system.

    $ sudo apt install dovecot-imapd

(The leading `$ ` in this document are command-line prompt strings,
which are not part of the command themselves.)

Configuration
-------------

Our [`interimap`(1)] instance will use the `imap` binary from Dovecot's
`libexec_dir` in order to access the local mail storage.  We assume that
the mail client can access it in the same fashion.  In other words, that
it can spawn a command and use its standard input (resp. output) for
[IMAP4rev1] commands (resp. responses).  [Mutt] is an example of such a
mail client, for which we propose a configuration snippet
[below](#mutt-config).

Since we don't need the Dovecot services nor master process in this
example, we disable them and create a local configuration file under
`$XDG_CONFIG_HOME/dovecot`.  If you need to keep the system-wise
services (for instance because your [MTA] uses the [LMTP server] for
mailbox delivery) then don't disable them, and modify Dovecot's system
wide configuration instead.  Same thing if your mail client isn't able
to spawn a command for IMAP communication, and instead insists on
connecting to a network socket (in that case you'll even need to
configure [user authentication](https://doc.dovecot.org/latest/core/config/auth/overview.html)
for the IMAP service, which is out of scope for the present document).

Run the following command to terminate and disable the system-wide
Dovecot processes.

    $ sudo systemctl mask --now dovecot.socket dovecot.service


Create a new directory `$XDG_CONFIG_HOME/dovecot` holding the local
Dovecot 2.4 configuration:

    $ install -m0700 -vd ${XDG_CONFIG_HOME:-~/.config}/dovecot
<!-- -->
    $ cat >${XDG_CONFIG_HOME:-~/.config}/dovecot/dovecot.conf <<-EOF
		dovecot_config_version = 2.4.0
		dovecot_storage_version = 2.4.0

		ssl = no
		mail_home = /home/%{user | username | lower}
		mail_driver = maildir
		mail_path = ~/Mail
		namespace inbox {
		    inbox     = yes
		    separator = /
		}
	EOF

Some remarks on the above:

  * SSL/TLS is explicitly turned off in order to avoid warnings when
    running `` `doveconf -nc ${XDG_CONFIG_HOME:-~/.config}/dovecot/dovecot.conf` ``.
  * Messages will be stored in Maildir format under `~/Mail`.  Ensure
    the directory is either *empty* or *doesn't exist* before
    continuing!  You may want to choose a different
    [format](https://doc.dovecot.org/latest/core/config/mailbox_formats/overview.html)
    here, or simply set `mailbox_list_layout = fs` in order to use a
    nicer (File System like) Maildir layout.
  * The `separator` setting defines the IMAP hierarchy delimiter.  This
    is orthogonal to the Maildir layout delimiter, and you can safely
    change it later (even on an existing mail store).  Popular hierarchy
    delimiters include `/` (slash) and `.` (period).

Now test the configuration by starting a pre-authenticated [IMAP4rev1]
session and issuing two commands, first `` `LIST "" "*"` `` to
recursively list all mailboxes (along with their hierarchy delimiter),
then `` `LOGOUT` `` to… log out and exit.  (The "`C: `" and "`S: `"
prefixes respectively denote client commands and server responses.)

    $ doveadm -c ${XDG_CONFIG_HOME:-~/.config}/dovecot/dovecot.conf exec imap
    S: * PREAUTH [CAPABILITY IMAP4rev1 …] Logged in as myuser
    C: a LIST "" "*"
    S: * LIST (\HasNoChildren) "/" INBOX
    S: a OK List completed (0.001 + 0.000 secs).
    C: q LOGOUT
    S: * BYE Logging out
    S: q OK Logout completed (0.001 + 0.000 secs).

Create a wrapper under `~/.local/bin` in order to avoid hard-coding the
local Dovecot configuration path:

    $ install -Dm 0755 /dev/stdin ~/.local/bin/dovecot-imap <<-EOF
		#!/bin/sh
		set -ue
		export PATH="/usr/bin:/bin"
		exec env -i PATH="\$PATH" HOME="\$HOME" USER="\$USER" \\
		    doveadm -c "\${XDG_CONFIG_HOME:-\$HOME/.config}/dovecot/dovecot.conf" \\
		        exec imap
	EOF

You can now start a pre-authenticated [IMAP4rev1] session like the one
above by simply running `` `~/.local/bin/dovecot-imap` ``.


InterIMAP
========

On Debian 10 (codename *Buster*) and later, installing the package is
one command away.  Simply run the following:

    $ sudo apt install interimap

Create directories for the InterIMAP configuration and data files:

    $ install -m0700 -vd ${XDG_CONFIG_HOME:-~/.config}/interimap ${XDG_DATA_HOME:-~/.local/share}/interimap

Create the configuration file.  The included sample file
`/usr/share/doc/interimap/interimap.sample` can be used as baseline, but
for the sake of clarity we start from an empty file here.

    $ install -m0600 /dev/null ${XDG_CONFIG_HOME:-~/.config}/interimap/config

 1. The file is in [INI format][INI file].  First, set general options
    in the default section:

        $ cat >${XDG_CONFIG_HOME:-~/.config}/interimap/config <<-EOF
			# only consider subscribed mailboxes
			list-select-opts = SUBSCRIBED
			#list-mailbox = "*"

			# ignore the mailbox named 'virtual' and its descendants
			# WARN: for version 0.4 and earlier it should be ^virtual(?:/|$)
			ignore-mailbox = ^virtual(?:\x00|$)
		EOF

 2. Next, append a `[local]` section pointing to the wrapper defined
    above:

        $ cat >>${XDG_CONFIG_HOME:-~/.config}/interimap/config <<-EOF

			[local]
			type = tunnel
			command = exec ~/.local/bin/dovecot-imap
		EOF

    (The command will be passed to `` `/bin/sh -c` `` as it contains
    the metacharacter `~`.  We use the `exec` built-in utility so the
    shell process doesn't linger around during the IMAP session.)

 3. And finally append a `[remote]` section with your account
    information at `imap.example.net` (adapt the values accordingly):

        $ cat >>${XDG_CONFIG_HOME:-~/.config}/interimap/config <<-EOF

			[remote]
			type = imaps
			host = imap.example.net
			username = myname
			password = xxxxxxxx
		EOF

At this point running `` `interimap` `` should create the database and
copy the entire remote mail store locally.  (If `~/Mail` was not empty,
it will also copy its content remotely, possibly *yielding duplicates*.)
This might take a while depending on the volume of messages to
synchronize.

    $ interimap
    Creating new schema in database file …/imap.example.net.db
    database: Created mailbox INBOX
    […]

A user unit for systemd is provided.  Run the following command to
enable and start the service:

    $ systemctl --user enable --now interimap.service

By default the connection to the IMAP servers remains open, and a status
update is requested every minute.  Thanks to the [`QRESYNC`][RFC 7162]
IMAP extension a status update scales linearly with the number of
mailboxes (unlike [OfflineIMAP] *not* with the number of messages).  And
thanks to the `COMPRESS` extension, the typical volume of data exchanged
[is rather small](benchmark.html#live-sync).  You may even want to
override the default settings and reduce the interval between status
updates to 20s:

    $ mkdir -p ${XDG_CONFIG_HOME:-~/.config}/systemd/user/interimap.service.d
<!-- -->
    $ cat >${XDG_CONFIG_HOME:-~/.config}/systemd/user/interimap.service.d/override.conf <<-EOF
		[Service]
		ExecStart=
		ExecStart=/usr/bin/interimap --watch=20
	EOF
<!-- -->
    $ systemctl --user daemon-reload
<!-- -->
    $ systemctl --user restart interimap.service


Email client configuration
==========================

[Mutt]  {#mutt-config}
------

Add the following snippet to the configuration file:

    $ cat >>~/.muttrc <<-EOF
		set tunnel    = "exec ~/.local/bin/dovecot-imap"
		set folder    = "imap://foo"
		set spoolfile = "imap://foo"
	EOF


Further Reading and Resources
=============================

Other use-cases:

:   [Multi-remote setup](multi-account.html)

Benchmarks:

:   [Benchmark metrics and comparison](benchmark.html)

Manual

:   [`interimap`(1)]


[IMAP4rev1]: https://tools.ietf.org/html/rfc3501
[INI file]: https://en.wikipedia.org/wiki/INI_file
[`interimap`(1)]: interimap.1.html
[LMTP server]: https://doc.dovecot.org/latest/core/config/delivery/lmtp.html
[Maildir]: https://en.wikipedia.org/wiki/Maildir
[mbox]: https://en.wikipedia.org/wiki/Mbox
[MTA]: https://en.wikipedia.org/wiki/Message_transfer_agent
[Mutt]: http://mutt.org/
[OfflineIMAP]: https://www.offlineimap.org/
[RFC 7162]: https://tools.ietf.org/html/rfc7162
[Unix philosophy]: https://en.wikipedia.org/wiki/Unix_philosophy