Ubuntu Manpage: InterIMAP - Fast bidirectional synchronization for QRESYNC-capable IMAP servers

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; for convenience reasons
servers must also support the LIST-EXTENDED, LIST-STATUS (or NOTIFY) and UIDPLUS IMAP
extensions. See also the 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
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
MAILBOXes instead. Note that each MAILBOX is taken “as is”; in particular, it must be
UTF-7 encoded, unquoted, and the list wildcards `*' and `%' are passed verbatim to the
IMAP server. If the local and remote hierarchy delimiter differ, then within the MAILBOX
names the local delimiter should be used (it is transparently substituted for remote
commands and responses).

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-based synchronization.

--repair [MAILBOX ...]
List the database anomalies and try to repair them. (Consider only the given
MAILBOXes 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 MAILBOXes on each target (by default each server plus the
database, unless --target specifies otherwise) where it exists. Note that per the
IMAP4rev1 standard 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
renaming is recursive. Thus SOURCE’s children are moved to become DEST’s children
instead.

OPTIONS

--config=FILE
Specify an alternate 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 types other than tunnel.

--notify
Whether to use the IMAP NOTIFY extension 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, which includes all IMAP traffic besides
literals, are written to the given logfile. The LOGIN and AUTHENTICATE commands
are however redacted (in order to avoid disclosing authentication credentials)
unless the --debug flag is set multiple times.

-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 MAILBOXes 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 MAILBOXes 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. 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. Hard-coding the
hierarchy delimiter in this setting is not advised because the server might
silently change it at some point. A null character should be used instead. For
instance, if list-mailbox is set "foo\x00bar" then, assuming the hierarchy
delimiter is `/', only the mailbox named foo/bar is considered for synchronization.

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.) 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 and unquoted) mailbox listed in the initial LIST
responses is ignored if it matches the given expression after trimming the
reference names and substituting the hierarchy delimiter with the null character.
For instance, specifying ^virtual(?:\x00|$) excludes the mailbox named “virtual” as
well as its descendants. Note that the MAILBOXes 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.

log-prefix
A printf(3)-like format string to use as prefix for each log message. Interpreted
sequences are %n and %m, expanding respectively to the component name
(local/remote) and to the name of the mailbox relevant for the log entry.
Conditions on a specifier %X can be obtained with %?X?then? or %?X?then&else?,
which expands to then if the %X specifier expands to a non-empty string, and to
else (or the empty string if there is no else condition) if it doesn’t. Literal %
characters need to be escaped as %%, while &, ? and \ characters need to be
\-escaped. (Default: %?n?%?m?%n(%m)&%n?: ?.)

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 inter-process
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 or IP address, for type=imap and type=imaps. The value can
optionally be enclosed in square brackets to force its interpretation as an IP
literal (hence skip name resolution). (Default: localhost.)

port Server port. (Default: 143 for type=imap, 993 for type=imaps.)

proxy 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 is supported
(with optional username/password authentication), 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 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 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 types 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 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. This
option is ignored when the --debug flag is set. (Default: NO.)

SSL_protocols
Space-separated list of SSL/TLS protocol versions to explicitly enable (or disable
if prefixed with an exclamation mark !). Potentially known protocols are SSLv2,
SSLv3, TLSv1, TLSv1.1, TLSv1.2, and TLSv1.3, depending on the OpenSSL version used.
Enabling a protocol is a short-hand for disabling all other protocols.

DEPRECATED: Use SSL_protocol_min and/or SSL_protocol_max instead.

SSL_protocol_min, SSL_protocol_max
Set minimum resp. maximum SSL/TLS protocol version to use for the connection.
Potentially recognized values are SSLv3, TLSv1, TLSv1.1, TLSv1.2, and TLSv1.3,
depending on the OpenSSL version used.

SSL_cipherlist, SSL_ciphersuites
Sets the TLSv1.2 and below cipher list resp. TLSv1.3 cipher suites. The
combination of these lists is sent to the server, which then determines which
cipher to use (normally the first supported one from the list sent by the client).
The default suites depend on the OpenSSL version and its configuration, see
ciphers(1ssl) for more information.

SSL_fingerprint
Space-separated list of acceptable fingerprints for the server certificate’s
Subject Public Key Info, in the form [ALGO$]DIGEST_HEX where ALGO is the digest
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

Specifying multiple digest values can be useful in key rollover scenarios and/or
when the server supports certificates of different types (for instance a dual-cert
RSA/ECDSA setup). In that case the connection is aborted when none of the
specified digests matches.

SSL_verify
Whether to 1/ verify the server certificate chain; and 2/ match its Subject
Alternative Name (SAN) or Subject CommonName (CN) against the value of the host
option. (Default: YES.)

Note that using SSL_fingerprint to specify the fingerprint of the server
certificate provides an independent server authentication measure as it pins
directly its key material and ignore its chain of trust.

SSL_CAfile
File containing trusted certificates to use during server certificate verification
when SSL_verify=YES.

Trusted CA certificates are loaded from the default system locations unless one (or
both) of SSL_CAfile or SSL_CApath is set.

SSL_CApath
Directory to use for server certificate verification when SSL_verify=YES. This
directory must be in “hash format”, see verify(1ssl) for more information.

Trusted CA certificates are loaded from the default system locations unless one (or
both) of SSL_CAfile or SSL_CApath is set.

SSL_hostname
Name to use for the TLS SNI (Server Name Indication) extension. The default value
is taken from the host option when it is a hostname, and to the empty string when
it is an IP literal. Setting SSL_hostname to the empty string explicitly disables
SNI.

SUPPORTED EXTENSIONS

       interimap  takes advantage of servers supporting the following extensions to the IMAP4rev1
       protocol (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);

       • 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. (Should
they arise, an external tool such as doveadm-deduplicate(1) can be used to weed them
out.) Hence one needs to manually empty the mail store on one end when migrating to
interimap from another synchronization 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 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.

• Because the IMAP protocol doesn’t provide a way for clients to determine whether a
disappeared mailbox was deleted or renamed, interimap aborts when a known mailbox
disappeared from one server but not the other. The --delete (resp. --rename) command
should be used instead to delete (resp. rename) the mailbox on both servers as well as
within interimap’s internal database.

• PLAIN and LOGIN are the only authentication mechanisms currently supported.

• interimap will probably not work with non RFC-compliant servers. In particular, no
work-around is currently implemented beside the tunables in the 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.

BUGS AND FEEDBACK

       Bugs or feature requests for interimap should be  filed  with  the  Debian  project’s  bug
       tracker at <https://www.debian.org/Bugs/>.

AUTHORS

       Guilhem Moulin (mailto:guilhem@fripost.org).

                                            July 2015                                interimap(1)