imapd(8)
NAME
imapd - The Courier IMAP server
SYNOPSIS
/usr/lib/courier/couriertcpd {couriertcpd options}
{/usr/sbin/imaplogin} [modules...]
{/usr/bin/imapd} {./Maildir}
/usr/bin/imapd {./Maildir}
DESCRIPTION
imapd is the Courier IMAP server that provides IMAP access to Maildir mailboxes. Normally you don't have to worry about it, as imapd runs automatically after receiving a network connection, accompanied by the appropriate userid and password. couriertcpd opens network ports that receive incoming IMAP connections. After an incoming network connections is established, couriertcpd runs the command specified by its first argument, which is imaplogin passing the remaining arguments to imaplogin. imaplogin reads the IMAP login userid and password, then runs the modules specified by its remaining options, which are Courier server authentication modules described in the authlib(7)[1] manual page. The last daisy-chained command is imapd, which is the actual IMAP server, which is started from the logged-in account's home directory. The sole argument to imapd is the pathname to the default IMAP mailbox, which is usually ./Maildir. Some authentication modules are capable of specifying a different filename, by setting the MAILDIR environment variable. imapd may also be invoked from the shell prompt, in which case it issues a PREAUTH response, then changes the current directory to either its argument, or the contents of the MAILDIR environment variable, then attempts to talk IMAP on standard input and output. imapd implements IMAP4REV1, as defined by RFC 2060[2].
FILES AND ENVIRONMENT VARIABLES
AUTH*
imapd examines several environment variables whose names start with
AUTH - these environment variables are set by imaplogin and the
authentication modules. Their absence tells imapd that it's running
from the command line.
MAILDIR
MAILDIR - if defined, imapd changes its directory to the one
specified by this environment variable. Otherwise imapd changes its
directory to the one specified on the command line.
`pwd`/.
The current directory is assumed to be the main INBOX Maildir.
`pwd`/.folder
Maildir folders, each one containing their own tmp, new, cur,
etc...
Other environment variables are initialized from the /etc/courier/imapd
and /etc/courier/imapd-ssl configuration files. These files are loaded
into the environment by the system startup script that runs
couriertcpd.
Realtime concurrent folder status updates
Setting the IMAP_ENHANCEDIDLE to 1 in /etc/courier/imapd enables
realtime concurrent folder status updates. When relatime folder status
updates are enabled all IMAP mail clients that have the same folder
open will be immediately notified of any changes to the folder's
contents.
The Courier IMAP server always allows more than one mail client to have
the same folder opened. However, when two or more clients have the same
folder opened, the mail clients may not necessarily know when another
client added or removed messages from the folder. The base IMAP
protocol specification requires IMAP mail clients to explicitly check
for any changes to the folder's contents. No provisions exists to
notify the mail client immediately when the folder's contents are
modified by another mail client.
The IDLE extension to the base IMAP protocol provides a delivery
mechanism for notifying mail clients of changes to the mail folder's
contents. Although at this time it's not known to which extent the IDLE
extension is supported by IMAP mail clients, the Courier IMAP server
fully implements the IDLE extension provided that the following
requirements are met:
Gamin or FAM
Either Gamin[3] or FAM[4] must be properly installed and configured
prior to installing the Courier IMAP server.
Gamin/FAM is an application library that provides an interface to
the operating system's kernel that applications can use to be
notified when specific files or directories are changed, and the
Courier IMAP server leverages this API to implement realtime
concurrent folder status updates. According to the most recently
available documentation, Gamin is a Linux-specific library, and FAM
builds and runs on Linux and IRIX. FAM should also build on other
platforms, but without a supported kernel monitor FAM will fall
back to a polling mode. At press time, FAM's web site reports that
FAM succesfully builds (in polling mode) on FreeBSD and Solaris.
FAM (but not Gamin) also works with NFS filesystems. On NFS clients
fam transparently forwards file monitoring requests to a peer fam
process on the NFS server.
Installation and configuration of Gamin or FAM is beyond the scope
of this document. This documentation presumes that Gamin or FAM is
succesfully installed. Use the resources and tools on Gamin's or
FAM's web site for assistance with setting them up. Systems that
use GNOME or KDE desktops already have FAM or Gamin installed, as
FAM or Gamin is used by the current versions of both desktops.
IDLE IMAP capability
IDLE must be listed in the IMAP_CAPABILITY setting in the
/etc/courier/imapd configuration file.
IMAP_USELOCKS
This setting in /etc/courier/imapd must be enabled. This setting
uses dot-lock files to synchronize updates to folder indexes
between multiple IMAP clients that have the same folder opened.
This setting is safe to use with NFS, as it does not use actual
file locking calls, and does not require the services of the
problematic NFS lock daemon.
An IMAP mail client that fully supports the IDLE protocol extension.
Of course, an IMAP client that supports the IDLE protocol extension
is required. At press time the status and extent of IDLE support in
most IMAP mail clients is not known.
IMAP_ENHANCEDIDLE
This setting in /etc/courier/imapd actually enables concurrent
realtime folder status updates using the IDLE extension. Note that
it is possible to enable the IDLE extension even if FAM or Gamin is
not available, or without enabling either the IMAP_USELOCKS and/or
IMAP_ENHANCEDIDLE settings. The resulting consequences are
described are as follows:
1. Without IMAP_USERLOCKS there exists a small possibility that
multiple mail clients will receive a slightly inconsistent
folder index if both clients try to update the contents of the
folder at the same time. Usually, the worst case result is that
some clients will eventually end up downloading the same
message twice from the server, and caching it incorrectly in
the local cache (if the IMAP client caches message contents).
Clearing the local message cache will quickly eliminate any
residual confusion that results from this situation.
2. Without FAM or Gamin, and IMAP_ENHANCEDIDLE set, the Courier
IMAP server will manually check for changes to the folder's
contents every 60 seconds, in IDLE mode (instead of in real
time).
Verifying realtime concurrent folder status updates
Use the following procedure to verify that realtime concurrent folder
status updates are properly working. It is helpful to be familiar with
the IMAP protocol. If that's not the case, just be extra careful in
entering the IMAP protocol commands. The following instructions
describe the procedure for connecting to the IMAP server, and manually
issuing IMAP protocol commands, as if they originate from an IMAP
client. The following instructions use "C:" to indicate IMAP client
commands that must be entered, and "S:" to indicate the expected
replies from the server.
Note
The actual replies from the server may differ slightly, due to the
actual server configuration, and other minor factors. The following
examples have long lines wrapped for readability. Slight observed
differences from the expected replies are normal, but they should
still be substantively the same.
1. Prepare a test account with a couple of messages. Open two or three
terminal windows. In each window, connect to the IMAP server, and
enter IDLE mode:
S:* OK Courier-IMAP ready. Copyright 1998-2002 Double Precision, Inc.
See COPYING for distribution information.
C:a login userid password
S:a OK LOGIN Ok.
C:a SELECT INBOX
S:* FLAGS (\Draft \Answered \Flagged \Deleted \Seen \Recent)
* OK [PERMANENTFLAGS (\Draft \Answered \Flagged \Deleted \Seen)]
Limited
* 2 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 939609418] Ok
a OK [READ-WRITE] Ok
C:a IDLE
S:+ entering ENHANCED idle mode
Note
The default Courier IMAP server configuration permits a maximum
of four connections from the same IP address. It may be
necessary to adjust this setting in /etc/courier/imapd for the
duration of this test.
2. The last message from the server must be "entering ENHANCED idle
mode". Otherwise, it means that some of the necessary prerequisites
have not been met. Verify that FAM or Gamin was set up prior to
installing The Courier IMAP server (use ldd(1) to verify that the
imapd executable is linked with the libfam library), and verify the
settings in the /etc/courier/imapd.
3. Open another terminal window, connect to the server, and modify the
flags of one of the messages:
S:* OK Courier-IMAP ready. Copyright 1998-2002 Double Precision, Inc.
See COPYING for distribution information.
C:a login userid password
S:a OK LOGIN Ok.
C:a SELECT INBOX
S:* FLAGS (\Draft \Answered \Flagged \Deleted \Seen \Recent)
* OK [PERMANENTFLAGS (\Draft \Answered \Flagged \Deleted \Seen)]
Limited
* 2 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 939609418] Ok
a OK [READ-WRITE] Ok
C:STORE 1 +FLAGS (\Deleted)
* 1 FETCH (FLAGS (\Deleted))
a OK STORE completed.
4. The last command sets the \Deleted flag on the first message in the
folder. Immediately after entering the last command, "* 1 FETCH
(FLAGS (\Deleted))" should also appear in all other terminal
windows. On systems where FAM uses the fall-back polling mode this
response may appear after a brief delay of a few seconds. The delay
should never exceed 15-20 seconds.
5. Verify that all terminal windows reliably receive folder status
updates in real time by alternatively entering the commands "a
STORE 1 -FLAGS (\Deleted)" and "a STORE 1 +FLAGS (\Deleted)", to
toggle the deleted flag on the first message. Observe that the
message is received by all terminal windows quickly, and reliably.
6. With the \Deleted flag set on the first message, enter the EXPUNGE
command, which removes the deleted message from the folder:
C:a EXPUNGE
S:* 1 EXPUNGE
* 2 EXISTS
* 0 RECENT
S:a OK EXPUNGE completed
The lines that begin with the "*" character should also appear in
all other terminal windows (depending on the initial folder state
one of the terminal windows may have a different RECENT message,
which is fine).
7. Use a mail client to create and send a test message to the test
account. As soon as the mail server delivers the message, the
following messages should appear in every terminal window:
* 3 EXISTS
* 0 RECENT
* 3 FETCH (FLAGS ())
The numbers in these messages may be different, depending upon the
initial contents of the test mail folder. One of the terminal
windows should have a different RECENT count, and one of the
terminal windows should include a \Recent flag in the untagged
FLAGS message. These difference are acceptable; the important thing
is to make sure that all terminal windows have the same EXISTS
message.
SEE ALSO
authlib(7)[1], userdb(8)[5]
AUTHOR
Sam Varshavchik
Author
NOTES
1. authlib(7)
[set $man.base.url.for.relative.links]/authlib.html
2. RFC 2060
http://www.rfc-editor.org/rfc/rfc2060.txt
3. Gamin
http://www.gnome.org/~veillard/gamin/
4. FAM
http://oss.sgi.com/projects/fam/
5. userdb(8)
[set $man.base.url.for.relative.links]/userdb.html
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