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+% pullimap(1)
+% [Guilhem Moulin](mailto:guilhem@fripost.org)
+% March 2016
+
+Name
+====
+
+PullIMAP - Pull mails from an IMAP mailbox and deliver them to a SMTP
+session
+
+Synopsis
+========
+
+`pullimap` [**\-\-config=***FILE*] [**\-\-idle**[**=***SECONDS*]]
+[**\-\-no-delivery**] [**\-\-quiet**] *SECTION*
+
+Description
+===========
+
+`pullimap` retrieves messages from an IMAP mailbox and deliver them to a
+SMTP or LMTP transmission channel.  It can also remove old messages
+after a configurable retention period.
+
+A *statefile* is used to keep track of the mailbox's `UIDVALIDITY` and
+`UIDNEXT` values.  While `pullimap` is running, the *statefile* is also
+used to keep track of UIDs being delivered, which avoids duplicate
+deliveries in case the process is interrupted.
+See the [**Control flow**](#control-flow) section below for details.
+
+Options
+=======
+
+`--config=`*FILE*
+
+: Specify an alternate configuration file.  Relative paths start from
+  *$XDG_CONFIG_HOME*, or *~/.config* if the `XDG_CONFIG_HOME`
+  environment variable is unset.
+
+`--idle`[`=`*seconds*]
+
+: Don't exit after a successful poll.  Instead, keep the connection open
+  and issue `IDLE` commands (require an IMAP server supporting [RFC
+  2177]) to watch for updates in the mailbox.  This also enables
+  `SO_KEEPALIVE` on the socket.  Each `IDLE` command is terminated after
+  at most *seconds* (29 minutes by default) to avoid being logged out
+  for inactivity.
+
+`--no-delivery`
+
+: Update the *statefile*, but skip SMTP/LMTP delivery.  This is mostly
+  useful for initializing the *statefile* when migrating to
+  `pullimap` from another similar program such as [`fetchmail`(1)] or
+  [`getmail`(1)].
+
+`-q`, `--quiet`
+
+: Try to be quiet.
+
+`--debug`
+
+: Turn on debug mode.  Debug messages are written to the error output.
+  Note that this include all IMAP traffic (except literals).  Depending
+  on the chosen authentication mechanism, this might include
+  authentication credentials.
+
+`-h`, `--help`
+
+: Output a brief help and exit.
+
+`--version`
+
+: Show the version number and exit.
+
+
+Configuration file
+==================
+
+Unless told otherwise by the `--config=FILE` option, `pullimap` reads
+its configuration from *$XDG_CONFIG_HOME/pullimap* (or
+*~/.config/pullimap* 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.
+Valid options are:
+
+*statefile*
+
+: State file to use to keep track of the *mailbox*'s `UIDVALIDITY` and
+  `UIDNEXT` values.  Relative paths start from
+  *$XDG_DATA_HOME/pullimap*, or *~/.local/share/pullimap* if the
+  `XDG_DATA_HOME` environment variable is unset.
+  (Default: the parent section name of the option.)
+
+*mailbox*
+
+: The IMAP mailbox to pull messages from.
+  Support for persistent message Unique Identifiers (UID) is required.
+  (Default: `INBOX`.)
+
+*deliver-method*
+
+: `PROTOCOL:[ADDRESS]:PORT` where to deliver messages.  Both [SMTP][RFC
+  5321] and [LMTP][RFC 2033] servers are supported, and [SMTP
+  pipelining][RFC 2920] is used when possible.
+  (Default: `smtp:[127.0.0.1]:25`.)
+
+*deliver-ehlo*
+
+: Hostname to use in `EHLO` or `LHLO` commands.
+  (Default: `localhost.localdomain`.)
+
+*deliver-rcpt*
+
+: Message recipient.
+  (Default: the username associated with the effective uid of the
+  `pullimap` process.)
+
+*purge-after*
+
+: Retention period (in days), after which messages are removed from the
+  IMAP server.  (The value is at best 24h accurate due to the IMAP
+  `SEARCH` criterion ignoring time and timezone.)
+  If *purge-after* is set to `0` then messages are deleted immediately
+  after delivery.  Otherwise `pullimap` issues an IMAP `SEARCH` command
+  to list old messages; if `--idle` is set then the `SEARCH` command is
+  issued again every 12 hours.
+
+*type*
+
+: One of `imap`, `imaps` or `tunnel`.
+  `type=imap` and `type=imaps` are respectively used for IMAP and IMAP
+  over SSL/TLS connections over a INET socket.
+  `type=tunnel` causes `pullimap` to open a pipe to a *command* instead
+  of a raw socket.
+  (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.
+
+*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 `pullimap` to immediately abort the
+  connection.
+  (Ignored for *type*s other than `imap`.  Default: `YES`.)
+
+*auth*
+
+: Space-separated list of preferred authentication mechanisms.
+  `pullimap` 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: `YES`.)
+
+*null-stderr*
+
+: Whether to redirect *command*'s standard error to `/dev/null` for type
+  `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`, and `TLSv1.2`.  Enabling a
+  protocol is a short-hand for disabling all other protocols.
+  (Default: `!SSLv2 !SSLv3`, i.e., only enable TLSv1 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 (or its public key) in the form
+  `ALGO$DIGEST_HEX`, where `ALGO` is the used algorithm (default
+  `sha256`).
+  Attempting to connect to a server with a non-matching certificate
+  fingerprint causes `pullimap` to abort the connection during the
+  SSL/TLS handshake.
+
+*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`.
+
+Control flow
+============
+
+`pullimap` opens the *statefile* corresponding to a given configuration
+*SECTION* with `O_DSYNC` to ensure that written data is flushed to the
+underlying hardware by the time [`write`(2)] returns.  Moreover an
+exclusive lock is placed on the file descriptor immediately after
+opening to prevent multiple `pullimap` processes from accessing the
+*statefile* concurrently.
+
+Each *statefile* consists of a series of 32-bits big-endian integers.
+Usually there are only two integers: the first is the *mailbox*'s
+`UIDVALIDITY` value, and the second is the *mailbox*'s last seen
+`UIDNEXT` value (`pullimap` then assumes that all messages with UID
+smaller than this `UIDNEXT` value have already been retrieved and
+delivered).
+The [IMAP4rev1 specification][RFC 3501] does not guaranty that untagged
+`FETCH` responses are sent ordered by UID in response to an `UID FETCH`
+command.  Thus it would be unsafe for `pullimap` to update the `UIDNEXT`
+value in its *statefile* while the `UID FETCH` command is progress.
+Instead, for each untagged `FETCH` response received while the `UID
+FETCH` command is in progress, `pullimap` delivers the message `BODY` to
+the SMTP or LMTP server (specified with *deliver-method*) then appends
+the message UID to the *statefile*.
+When the `UID FETCH` command eventually terminates, `pullimap` updates
+the `UIDNEXT` value in the *statefile* and truncate the file down to 8
+bytes.  Keeping track of message UIDs as they are received avoids
+duplicate in the event of a crash or connection loss while the `UID
+FETCH` command is in progress.
+
+In more details, `pullimap` works as follows:
+
+ 1. Issue an `UID FETCH` command to retrieve message `ENVELOPE` and
+    `BODY` (and `UID`) with UID bigger or equal than the `UIDNEXT` value
+    found in the *statefile*.
+    While the `UID FETCH` command is in progress, perform the following
+    for each untagged `FETCH` response sent by the server:
+
+    i) if no SMTP/LMTP transmission channel was opened, open one to the
+       server specified with *deliver-method* and send an `EHLO` (or
+       `LHO`) command with the domain specified by *deliver-ehlo*;
+
+    ii) perform a mail transaction (using [SMTP pipelining][RFC 2920] if
+        possible) to send the retrieved message BODY to the SMTP or LMTP
+        session; and
+
+    ii) append the message UID to the *statefile*.
+
+ 2. If a SMTP/LMTP transmission channel was opened, send a `QUIT` command
+    to terminate it gracefully.
+
+ 3. Issue an `UID STORE` command to mark all retrieved messages (and
+    stalled UIDs found in the *statefile* after the eigth byte) as
+    `\Seen`.
+
+ 4. Update the *statefile* with the new UIDNEXT value (bytes 5-8).
+
+ 5.  Truncate the *statefile* down to 8 bytes (so that it contains only
+     two 32-bits integers, respectively the *mailbox*'s current
+     `UIDVALIDITY` and `UIDNEXT` values).
+
+ 6. If `--idle` was set, issue an `IDLE` command; stop idling and go
+    back to step 1. whenever a new message is received.
+
+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, _Local Mail Transfer Protocol_,
+   [RFC 2033], October 1996
+ * J. Myers, _IMAP4 non-synchronizing literals_,
+   [RFC 2088], January 1997
+ * B. Leiba, _IMAP4 IDLE command_,
+   [RFC 2177], June 1997
+ * C. Newman, _Using TLS with IMAP, POP3 and ACAP_,
+   [RFC 2595], June 1999
+ * N. Freed, _SMTP Service Extension for Command Pipelining_,
+   [RFC 2920], September 2000
+ * M. Crispin, _Internet Message Access Protocol - Version 4rev1_,
+   [RFC 3501], March 2003
+ * M. Crispin,
+   _Internet Message Access Protocol (IMAP) - UIDPLUS extension_,
+   [RFC 4315], December 2005
+ * 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
+ * J. Klensin, _Simple Mail Transfer Protocol_,
+   [RFC 5321], October 2008
+
+[RFC 4315]: https://tools.ietf.org/html/rfc4315
+[RFC 2177]: https://tools.ietf.org/html/rfc2177
+[RFC 2595]: https://tools.ietf.org/html/rfc2595
+[RFC 4959]: https://tools.ietf.org/html/rfc4959
+[RFC 2088]: https://tools.ietf.org/html/rfc2088
+[RFC 5321]: https://tools.ietf.org/html/rfc5321
+[RFC 2033]: https://tools.ietf.org/html/rfc2033
+[RFC 2920]: https://tools.ietf.org/html/rfc2920
+[RFC 3501]: https://tools.ietf.org/html/rfc3501
+[RFC 4978]: https://tools.ietf.org/html/rfc4978
+[RFC 1928]: https://tools.ietf.org/html/rfc1928
+[RFC 1929]: https://tools.ietf.org/html/rfc1929
+
+[INI file]: https://en.wikipedia.org/wiki/INI_file
+[`fetchmail`(1)]: http://www.fetchmail.info/
+[`getmail`(1)]: http://pyropus.ca/software/getmail/
+[`write`(2)]: http://man7.org/linux/man-pages/man2/write.2.html
+[`ciphers`(1ssl)]: https://www.openssl.org/docs/manmaster/apps/ciphers.html
+[`verify`(1ssl)]: https://www.openssl.org/docs/manmaster/apps/verify.html