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diff --git a/doc/getting-started.md b/doc/getting-started.md new file mode 100644 index 0000000..371449d --- /dev/null +++ b/doc/getting-started.md @@ -0,0 +1,286 @@ +% 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 synchronisation 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 though the same “IMAP lens”, +InterIMAP is able to take advantage of the abstraction layer and +perform significant optimizations, yielding much faster synchronization. +(*TODO* link to benchmark.) +*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 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` command 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://wiki.dovecot.org/Authentication) +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 configuration: + + $ install -m0700 -vd ${XDG_CONFIG_HOME:-~/.config}/dovecot +<!-- --> + $ cat >${XDG_CONFIG_HOME:-~/.config}/dovecot/dovecot.conf <<-EOF + ssl = no + mail_location = maildir:~/Mail + namespace { + inbox = yes + list = 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://wiki.dovecot.org/MailboxFormat) + here, or simply append `:LAYOUT=fs` to the `mail_location` value 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 + + 3. And finally append a `[remote]` section with your account + information at `imap.example.org` (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 + +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 (*TODO* metrics). 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 + + +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: + +: *TODO* + +Benchmarks: + +: *TODO* + +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/configuration_manual/protocols/lmtp_server/ +[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 |