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+% interimap(1)
+% [Guilhem Moulin](mailto:guilhem@fripost.org)
+% July 2015
+
+Name
+====
+
+InterIMAP - Fast bidirectional synchronization for QRESYNC-capable IMAP servers
+
+Synopsis
+========
+
+`interimap` [*OPTION* ...] [*COMMAND*] [*MAILBOX* ...]
+
+Description
+===========
+
+`interimap` performs stateful synchronization between two IMAP4rev1
+servers.
+Such synchronization is made possible by the [`QRESYNC` IMAP
+extension][RFC 7162]; for convenience reasons servers must also support
+the [`LIST-EXTENDED`][RFC 5258], [`LIST-STATUS`][RFC 5819] (or
+[`NOTIFY`][RFC 5465]) and [`UIDPLUS`][RFC 4315] IMAP extensions.
+See also the **[supported extensions](#supported-extensions)** section
+below.
+
+Stateful synchronization is only possible for mailboxes supporting
+persistent message Unique Identifiers (UID) and persistent storage of
+mod-sequences (MODSEQ); any non-compliant mailbox will cause `interimap`
+to abort.
+Furthermore, because UIDs are allocated not by the client but by the
+server, `interimap` needs to keep track of associations between local
+and remote UIDs for each mailbox.
+The synchronization state of a mailbox consists of its `UIDNEXT` and
+`HIGHESTMODSEQ` values on each server; it is then assumed that each
+message with UID smaller than `UIDNEXT` have been replicated to the
+other server, and that the metadata (such as flags) of each message with
+MODSEQ at most `HIGHESTMODSEQ` have been synchronized.
+Conceptually, the synchronization algorithm is derived from [RFC 4549]
+with the [RFC 7162] (sec. 6) amendments, and works as follows:
+
+ 1. `SELECT` (on both servers) a mailbox the current `UIDNEXT` or `HIGHESTMODSEQ`
+ values of which differ from the values found in the database (for
+ either server). Use the `QRESYNC` `SELECT` parameter from [RFC
+ 7162] to list changes (vanished messages and flag updates) since
+ `HIGHESTMODSEQ` to messages with UID smaller than `UIDNEXT`.
+
+ 2. Propagate these changes onto the other server: get the corresponding
+ UIDs from the database, then:
+ a. issue a `UID STORE` command, followed by `UID EXPUNGE`, to
+ remove messages that have not already been deleted on both
+ servers; and
+ b. issue some `UID STORE` commands to propagate flag updates (send
+ a single command for each flag list in order the reduce the
+ number of round trips).
+
+ (Conflicts may occur if the metadata of a message has been updated
+ on both servers with different flag lists; in that case, `interimap`
+ issues a warning and updates the message on each server with the
+ union of both flag lists.)
+ Repeat this step if the server sent some updates in the meantime.
+ Otherwise, update the `HIGHESTMODSEQ` value in the database.
+
+ 3. Process new messages (if the current `UIDNEXT` value of the mailbox
+ differs from the one found in the database) by issuing a `UID FETCH`
+ command; process each received message on-the-fly by issuing an
+ `APPEND` command with the message's `RFC822` body, `FLAGS` and
+ `INTERNALDATE`.
+ Repeat this step if the server received new messages in the
+ meantime. Otherwise, update the `UIDNEXT` value in the database.
+ Go back to step 2 if the server sent some metadata (such as flag)
+ updates in the meantime.
+
+ 4. Go back to step 1 to proceed with the next unsynchronized mailbox.
+
+Commands
+========
+
+By default, `interimap` synchronizes each mailbox listed by the `LIST ""
+"*"` IMAP command;
+the *list-mailbox*, *list-select-opts* and *ignore-mailbox* options from
+the [configuration file](#configuration-file) can be used to shrink that
+list and save bandwidth.
+However if some extra argument are provided on the command line,
+`interimap` ignores these options and synchronizes the given
+*MAILBOX*es instead. Note that each *MAILBOX* is taken “as is”; in
+particular, it must be [UTF-7 encoded][RFC 2152], unquoted, and the list
+wildcards ‘\*’ and ‘%’ are passed verbatim to the IMAP server.
+
+If the synchronization was interrupted during a previous run while some
+messages were being replicated (but before the `UIDNEXT` or
+`HIGHESTMODSEQ` values have been updated), `interimap` performs a “full
+synchronization” on theses messages: downloading the whole UID and flag
+lists on each servers allows `interimap` to detect messages that have
+been removed or for which their flags have changed in the meantime.
+Finally, after propagating the offline changes for these messages,
+`interimap` resumes the synchronization for the rest of the mailbox.
+
+Specifying one of the commands below makes `interimap` perform an action
+other than the default [`QRESYNC`][RFC 7162]-based synchronization.
+
+`--repair` [*MAILBOX* ...]
+
+: List the database anomalies and try to repair them. (Consider only
+ the given *MAILBOX*es if non-optional arguments are provided.)
+ This is done by performing a so-called “full synchronization”,
+ namely:
+ 1/ download all UIDs along with their flag list both from the
+ local and remote servers;
+ 2/ ensure that each entry in the database corresponds to an
+ existing UID; and
+ 3/ ensure that both flag lists match.
+ Any message found on a server but not in the database is replicated
+ on the other server (which in the worst case, might yield a message
+ duplicate).
+ Flag conflicts are solved by updating each message to the union of
+ both lists.
+
+`--delete` *MAILBOX* [*MAILBOX* ...]
+
+: Delete the given *MAILBOX*es on each target (by default each server
+ plus the database, unless `--target` specifies otherwise) where it
+ exists.
+ Note that per the [IMAP4rev1 standard][RFC 3501] deletion is not
+ recursive. Thus *MAILBOX*'s children are not deleted.
+
+`--rename` *SOURCE* *DEST*
+
+: Rename the mailbox *SOURCE* to *DEST* on each target (by default
+ each server plus the database, unless `--target` specifies
+ otherwise) where it exists.
+ `interimap` aborts if *DEST* already exists on either target.
+ Note that per the [IMAP4rev1 standard][RFC 3501] renaming is
+ recursive. Thus *SOURCE*'s children are moved to become *DEST*'s
+ children instead.
+
+Options
+=======
+
+`--config=`*FILE*
+
+: Specify an alternate [configuration file](#configuration-file).
+ Relative paths start from *$XDG_CONFIG_HOME/interimap*, or *~/.config/interimap*
+ if the `XDG_CONFIG_HOME` environment variable is unset.
+
+`--target={local,remote,database}`
+
+: Limit the scope of a `--delete` or `--rename` command to the given
+ target. Can be repeated to act on multiple targets. By default all
+ three targets are considered.
+
+`--watch`[`=`*seconds*]
+
+: Don't exit after a successful synchronization. Instead, keep
+ synchronizing forever. Sleep for the given number of *seconds* (by
+ default 1 minute if `--notify` is unset, and 15 minutes if
+ `--notify` is set) between two synchronizations. Setting this
+ options enables `SO_KEEPALIVE` on the socket for *type*s other than
+ `tunnel`.
+
+`--notify`
+
+: Whether to use the [IMAP `NOTIFY` extension][RFC 5465] to instruct
+ the server to automatically send updates to the client. (Both local
+ and remote servers must support [RFC 5465] for this to work.)
+ This greatly reduces IMAP traffic since `interimap` can rely on
+ server notifications instead of manually polling for updates.
+ If the connection remains idle for 15 minutes (configurable with
+ `--watch`), then `interimap` sends a `NOOP` command to avoid being
+ logged out for inactivity.
+
+`-q`, `--quiet`
+
+: Try to be quiet.
+
+`--debug`
+
+: Turn on debug mode. Debug messages are written to the given *logfile*.
+ Note that this include all IMAP traffic (except literals).
+ Depending on the chosen authentication mechanism, this might include
+ authentication credentials.
+
+`-h`, `--help`
+
+: Output a brief help and exit.
+
+`--version`
+
+: Show the version number and exit.
+
+Configuration file
+==================
+
+Unless told otherwise by the `--config=FILE` command-line option,
+`interimap` reads its configuration from *$XDG_CONFIG_HOME/interimap/config*
+(or *~/.config/interimap/config* if the `XDG_CONFIG_HOME` environment
+variable is unset) as an [INI file].
+The syntax of the configuration file is a series of `OPTION=VALUE`
+lines organized under some `[SECTION]`; lines starting with a ‘#’ or
+‘;’ character are ignored as comments.
+The `[local]` and `[remote]` sections define the two IMAP servers to
+synchronize.
+Valid options are:
+
+*database*
+
+: SQLite version 3 database file to use to keep track of associations
+ between local and remote UIDs, as well as the `UIDVALIDITY`,
+ `UIDNEXT` and `HIGHESTMODSEQ` of each known mailbox on both servers.
+ Relative paths start from *$XDG_DATA_HOME/interimap*, or
+ *~/.local/share/interimap* if the `XDG_DATA_HOME` environment
+ variable is unset. This option is only available in the default
+ section.
+ (Default: `HOST.db`, where *HOST* is taken from the `[remote]` or
+ `[local]` sections, in that order.)
+
+*list-reference*
+
+: An optional “reference name” to use for the initial `LIST` command,
+ indicating the context in which the *MAILBOX*es are interpreted.
+ For instance, by specifying `list-reference=perso/` in the `[local]`
+ section, *MAILBOX* names are interpreted relative to `perso/` on the
+ local server; in other words the remote mailbox hierarchy is mapped
+ to the `perso/` sub-hierarchy on the local server. This is useful
+ for synchronizing multiple remote servers against different
+ namespaces belonging to the same local IMAP server (using a
+ different `interimap` instance for each local namespace ↔ remote
+ synchronization).
+
+ (Note that if the reference name is not a level of mailbox hierarchy
+ and/or does not end with the hierarchy delimiter, by [RFC 3501] its
+ interpretation by the IMAP server is implementation-dependent.)
+
+*list-mailbox*
+
+: A space separated list of mailbox patterns to use when issuing the
+ initial `LIST` command (overridden by the *MAILBOX*es given as
+ command-line arguments).
+ Names containing special characters such as spaces or brackets need
+ to be enclosed in double quotes. Within double quotes C-style
+ backslash escape sequences can be used (‘\\t’ for an horizontal tab,
+ ‘\\n’ for a new line, ‘\\\\’ for a backslash, etc.), as well as
+ hexadecimal escape sequences ‘\\xHH’.
+ Furthermore, non-ASCII names must be [UTF-7 encoded][RFC 2152].
+ Two wildcards are available, and passed verbatim to the IMAP server:
+ a ‘\*’ character matches zero or more characters, while a ‘%’
+ character matches zero or more characters up to the hierarchy
+ delimiter.
+ This option is only available in the default section.
+ (The default pattern, `*`, matches all visible mailboxes on the
+ server.)
+
+*list-select-opts*
+
+: An optional space separated list of selectors for the initial `LIST`
+ command. (Requires a server supporting the [`LIST-EXTENDED` IMAP
+ extension][RFC 5258].) Useful values are `SUBSCRIBED` (to list only
+ subscribed mailboxes), `REMOTE` (to also list remote mailboxes on a
+ server supporting mailbox referrals), and `RECURSIVEMATCH` (to
+ list parent mailboxes with children matching one of the above
+ *list-mailbox* patterns). This option is only available in the
+ default section.
+
+*ignore-mailbox*
+
+: An optional Perl Compatible Regular Expressions ([PCRE]) covering
+ mailboxes to exclude: any ([UTF-7 encoded][RFC 2152] and unquoted)
+ mailbox listed in the initial `LIST` responses is ignored if it
+ matches the given expression.
+ Note that the *MAILBOX*es given as command-line arguments bypass the
+ check and are always considered for synchronization. This option is
+ only available in the default section.
+
+*logfile*
+
+: A file name to use to log debug and informational messages. (By
+ default these messages are written to the error output.) This
+ option is only available in the default section.
+
+*type*
+
+: One of `imap`, `imaps` or `tunnel`.
+ `type=imap` and `type=imaps` are respectively used for IMAP and IMAP
+ over SSL/TLS connections over an INET socket.
+ `type=tunnel` causes `interimap` to create an unnamed pair of
+ connected sockets for interprocess communication with a *command*
+ instead of opening a network socket.
+ Note that specifying `type=tunnel` in the `[remote]` section makes
+ the default *database* to be `localhost.db`.
+ (Default: `imaps`.)
+
+*host*
+
+: Server hostname, for `type=imap` and `type=imaps`.
+ (Default: `localhost`.)
+
+*port*
+
+: Server port.
+ (Default: `143` for `type=imap`, `993` for `type=imaps`.)
+
+*proxy*
+
+: An optional SOCKS proxy to use for TCP connections to the IMAP
+ server (`type=imap` and `type=imaps` only), formatted as
+ `PROTOCOL://[USER:PASSWORD@]PROXYHOST[:PROXYPORT]`.
+ If `PROXYPORT` is omitted, it is assumed at port 1080.
+ Only [SOCKSv5][RFC 1928] is supported (with optional
+ [username/password authentication][RFC 1929]), in two flavors:
+ `socks5://` to resolve *hostname* locally, and `socks5h://` to let
+ the proxy resolve *hostname*.
+
+*command*
+
+: Command to use for `type=tunnel`. Must speak the [IMAP4rev1
+ protocol][RFC 3501] on its standard output, and understand it on its
+ standard input. The value is passed to `` `/bin/sh -c` `` if it
+ contains shell metacharacters; otherwise it is split into words and
+ the resulting list is passed to `execvp`(3).
+
+*STARTTLS*
+
+: Whether to use the [`STARTTLS`][RFC 2595] directive to upgrade to a
+ secure connection. Setting this to `YES` for a server not
+ advertising the `STARTTLS` capability causes `interimap` to
+ immediately abort the connection.
+ (Ignored for *type*s other than `imap`. Default: `YES`.)
+
+*auth*
+
+: Space-separated list of preferred authentication mechanisms.
+ `interimap` uses the first mechanism in that list that is also
+ advertised (prefixed with `AUTH=`) in the server's capability list.
+ Supported authentication mechanisms are `PLAIN` and `LOGIN`.
+ (Default: `PLAIN LOGIN`.)
+
+*username*, *password*
+
+: Username and password to authenticate with. Can be required for non
+ pre-authenticated connections, depending on the chosen
+ authentication mechanism.
+
+*compress*
+
+: Whether to use the [`IMAP COMPRESS` extension][RFC 4978] for servers
+ advertising it.
+ (Default: `NO` for the `[local]` section, `YES` for the `[remote]`
+ section.)
+
+*null-stderr*
+
+: Whether to redirect *command*'s standard error to `/dev/null` for
+ `type=tunnel`. (Default: `NO`.)
+
+*SSL_protocols*
+
+: A space-separated list of SSL protocols to enable or disable (if
+ prefixed with an exclamation mark `!`. Known protocols are `SSLv2`,
+ `SSLv3`, `TLSv1`, `TLSv1.1`, `TLSv1.2`, and `TLSv1.3`. Enabling a
+ protocol is a short-hand for disabling all other protocols.
+ (Default: `!SSLv2 !SSLv3 !TLSv1 !TLSv1.1`, i.e., only enable TLSv1.2
+ and above.)
+
+*SSL_cipher_list*
+
+: The cipher list to send to the server. Although the server
+ determines which cipher suite is used, it should take the first
+ supported cipher in the list sent by the client. See
+ [`ciphers`(1ssl)] for more information.
+
+*SSL_fingerprint*
+
+: Fingerprint of the server certificate's Subject Public Key Info, in
+ the form `[ALGO$]DIGEST_HEX` where `ALGO` is the used algorithm (by
+ default `sha256`).
+ Attempting to connect to a server with a non-matching certificate
+ SPKI fingerprint causes `interimap` to abort the connection during
+ the SSL/TLS handshake.
+ The following command can be used to compute the SHA-256 digest of a
+ certificate's Subject Public Key Info:
+
+ openssl x509 -in /path/to/server/certificate.pem -pubkey \
+ | openssl pkey -pubin -outform DER \
+ | openssl dgst -sha256
+
+*SSL_verify*
+
+: Whether to verify the server certificate chain.
+ Note that using *SSL_fingerprint* to specify the fingerprint of the
+ server certificate is an orthogonal authentication measure as it
+ ignores the CA chain.
+ (Default: `YES`.)
+
+*SSL_CApath*
+
+: Directory to use for server certificate verification if
+ `SSL_verify=YES`.
+ This directory must be in “hash format”, see [`verify`(1ssl)] for
+ more information.
+
+*SSL_CAfile*
+
+: File containing trusted certificates to use during server
+ certificate authentication if `SSL_verify=YES`.
+
+Supported extensions
+====================
+
+`interimap` takes advantage of servers supporting the following
+extensions to the [IMAP4rev1 protocol][RFC 3501] (those marked as
+“recommended” give the most significant performance gain):
+
+ * `LITERAL+` ([RFC 2088], recommended);
+ * `MULTIAPPEND` ([RFC 3502], recommended);
+ * `COMPRESS=DEFLATE` ([RFC 4978], recommended);
+ * `NOTIFY` ([RFC 5465], recommended);
+ * `SASL-IR` ([RFC 4959]); and
+ * `UNSELECT` ([RFC 3691]).
+
+Known bugs and limitations
+==========================
+
+ * Using `interimap` on two identical servers with a non-existent or
+ empty *database* will duplicate each message due to the absence of
+ local ↔ remote UID association. Hence one needs to manually empty
+ the mail store on one end when migrating to `interimap` from another
+ synchronisation solution.
+
+ * `interimap` is single threaded and doesn't use IMAP command
+ pipelining. Synchronization could be boosted up by sending
+ independent commands (such as the initial `LIST` and `STATUS`
+ commands) to both servers in parallel, and for a given server, by
+ sending independent commands (such as flag updates) in a pipeline.
+
+ * Because the [IMAP protocol][RFC 3501] doesn't have a specific
+ response code for when a message is moved to another mailbox (either
+ using the `MOVE` command from [RFC 6851], or via `COPY` + `STORE` +
+ `EXPUNGE`), moving a message causes `interimap` to believe that it
+ was deleted while another one (which is replicated again) was added
+ to the other mailbox in the meantime.
+
+ * `PLAIN` and `LOGIN` are the only authentication mechanisms currently
+ supported.
+
+ * `interimap` will probably not work with non [RFC][RFC 3501]-compliant
+ servers. In particular, no work-around is currently implemented
+ beside the tunables in the [configuration file](#configuration-file).
+ Moreover, few IMAP servers have been tested so far.
+
+Standards
+=========
+
+ * M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas and L. Jones,
+ _SOCKS Protocol Version 5_,
+ [RFC 1928], March 1996.
+ * M. Leech, _Username/Password Authentication for SOCKS V5_,
+ [RFC 1929], March 1996.
+ * J. Myers, _IMAP4 non-synchronizing literals_,
+ [RFC 2088], January 1997.
+ * D. Goldsmith and M. Davis,
+ _A Mail-Safe Transformation Format of Unicode_,
+ [RFC 2152], May 1997.
+ * C. Newman, _Using TLS with IMAP, POP3 and ACAP_,
+ [RFC 2595], June 1999.
+ * M. Crispin, _Internet Message Access Protocol - Version 4rev1_,
+ [RFC 3501], March 2003.
+ * M. Crispin,
+ _Internet Message Access Protocol (IMAP) - `MULTIAPPEND` Extension_,
+ [RFC 3502], March 2003.
+ * A. Melnikov,
+ _Internet Message Access Protocol (IMAP) `UNSELECT` command_,
+ [RFC 3691], February 2004.
+ * M. Crispin,
+ _Internet Message Access Protocol (IMAP) - `UIDPLUS` extension_,
+ [RFC 4315], December 2005.
+ * A. Melnikov,
+ _Synchronization Operations for Disconnected IMAP4 Clients_,
+ [RFC 4549], June 2006.
+ * A. Gulbrandsen, _The IMAP `COMPRESS` Extension_,
+ [RFC 4978], August 2007.
+ * R. Siemborski and A. Gulbrandsen, _IMAP Extension for Simple
+ Authentication and Security Layer (SASL) Initial Client Response_,
+ [RFC 4959], September 2007.
+ * A. Gulbrandsen and A. Melnikov,
+ _The IMAP `ENABLE` Extension_,
+ [RFC 5161], March 2008.
+ * B. Leiba and A. Melnikov,
+ _Internet Message Access Protocol version 4 - `LIST` Command Extensions_,
+ [RFC 5258], June 2008.
+ * A. Gulbrandsen, C. King and A. Melnikov,
+ _The IMAP `NOTIFY` Extension_,
+ [RFC 5465], February 2009.
+ * A. Melnikov and T. Sirainen,
+ _IMAP4 Extension for Returning `STATUS` Information in Extended LIST_,
+ [RFC 5819], March 2010.
+ * A. Gulbrandsen and N. Freed,
+ _Internet Message Access Protocol (IMAP) - `MOVE` Extension_,
+ [RFC 6851], January 2013.
+ * A. Melnikov and D. Cridland,
+ _IMAP Extensions: Quick Flag Changes Resynchronization (`CONDSTORE`)
+ and Quick Mailbox Resynchronization (`QRESYNC`)_,
+ [RFC 7162], May 2014.
+
+[RFC 7162]: https://tools.ietf.org/html/rfc7162
+[RFC 5258]: https://tools.ietf.org/html/rfc5258
+[RFC 5819]: https://tools.ietf.org/html/rfc5819
+[RFC 4315]: https://tools.ietf.org/html/rfc4315
+[RFC 4549]: https://tools.ietf.org/html/rfc4549
+[RFC 2152]: https://tools.ietf.org/html/rfc2152
+[RFC 3501]: https://tools.ietf.org/html/rfc3501
+[RFC 1928]: https://tools.ietf.org/html/rfc1928
+[RFC 1929]: https://tools.ietf.org/html/rfc1929
+[RFC 2595]: https://tools.ietf.org/html/rfc2595
+[RFC 4978]: https://tools.ietf.org/html/rfc4978
+[RFC 2088]: https://tools.ietf.org/html/rfc2088
+[RFC 3502]: https://tools.ietf.org/html/rfc3502
+[RFC 4959]: https://tools.ietf.org/html/rfc4959
+[RFC 3691]: https://tools.ietf.org/html/rfc3691
+[RFC 6851]: https://tools.ietf.org/html/rfc6851
+[RFC 5161]: https://tools.ietf.org/html/rfc5161
+[RFC 5465]: https://tools.ietf.org/html/rfc5465
+
+[INI file]: https://en.wikipedia.org/wiki/INI_file
+[PCRE]: https://en.wikipedia.org/wiki/Perl_Compatible_Regular_Expressions
+[`ciphers`(1ssl)]: https://www.openssl.org/docs/manmaster/apps/ciphers.html
+[`verify`(1ssl)]: https://www.openssl.org/docs/manmaster/apps/verify.html