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documentation  with `make html`.

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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 an SMTP session
+
+Synopsis
+========
+
+`pullimap` [**\-\-config=***FILE*] [**\-\-idle**[**=***SECONDS*]]
+[**\-\-no-delivery**] [**\-\-quiet**] *SECTION*
+
+Description
+===========
+
+`pullimap` retrieves messages from an IMAP mailbox and deliver them to
+an 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](#configuration-file).
+    Relative paths start from *$XDG_CONFIG_HOME/pullimap*, or *~/.config/pullimap*
+    if the `XDG_CONFIG_HOME` environment variable is unset.
+
+`--idle`[`=`*seconds*]
+
+:   Don't exit after a successful poll.  Instead, keep the connection open
+    and issue `IDLE` commands (require an IMAP server supporting [RFC
+    2177]) to watch for updates in the mailbox.  This also enables
+    `SO_KEEPALIVE` on the socket.
+    Each `IDLE` command is terminated after at most *seconds* (29
+    minutes by default) to avoid being logged out for inactivity.
+
+`--no-delivery`
+
+:   Update the *statefile*, but skip SMTP/LMTP delivery.  This is mostly
+    useful for initializing the *statefile* when migrating to `pullimap`
+    from another similar program such as [`fetchmail`(1)] or
+    [`getmail`(1)].
+
+`-q`, `--quiet`
+
+:   Try to be quiet.
+
+`--debug`
+
+:   Turn on debug mode.  Debug messages are written to the error output.
+    Note that this include all IMAP traffic (except literals).
+    Depending on the chosen authentication mechanism, this might include
+    authentication credentials.
+
+`-h`, `--help`
+
+:   Output a brief help and exit.
+
+`--version`
+
+:   Show the version number and exit.
+
+Configuration file
+==================
+
+Unless told otherwise by the `--config=FILE` command-line option,
+`pullimap` reads its configuration from *$XDG_CONFIG_HOME/pullimap/config*
+(or *~/.config/pullimap/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.
+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 ([UTF-7 encoded][RFC 2152] and unquoted) 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.  Note that the local part needs to quoted if it
+    contains special characters; see [RFC 5321] for details.
+    (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` (or
+    extended `SEARCH` on servers advertizing the [`ESEARCH`][RFC 4731]
+    capability) 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 an INET socket.
+    `type=tunnel` causes `pullimap` to create an unnamed pair of
+    connected sockets for interprocess communication with a *command*
+    instead of opening a network 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.  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 `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=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 `pullimap` 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`.
+
+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 a `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 `RFC822`
+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 a `UID FETCH` command to retrieve message `ENVELOPE` and
+    `RFC822` (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* (the
+        channel is kept open and shared for all messages retrieved while
+        the `UID FETCH` IMAP command is in progress);
+
+     i. perform a mail transaction (using [SMTP pipelining][RFC 2920] if
+        possible) to deliver the retrieved message `RFC822` body to the
+        SMTP or LMTP session; and
+
+     i. append the message UID to the *statefile*.
+
+ 2. If an SMTP/LMTP transmission channel was opened, send a `QUIT` command
+    to terminate it gracefully.
+
+ 3. Issue a `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 when a new message is received (or when the `IDLE`
+    timeout expires).
+
+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.
+ * D. Goldsmith and M. Davis,
+   _A Mail-Safe Transformation Format of Unicode_,
+   [RFC 2152], May 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.
+ * A. Melnikov and D. Cridland, _IMAP4 Extension to SEARCH Command for
+   Controlling What Kind of Information Is Returned_,
+   [RFC 4731], November 2006.
+ * 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 2152]: https://tools.ietf.org/html/rfc2152
+[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
+[RFC 4731]: https://tools.ietf.org/html/rfc4731
+
+[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