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Manual Reference Pages  -  MDPOP3D (8)


mdpop3d, in.mdpop3d - Post Office Protocol version 3 daemon for Maildirs.




mdpop3d [options]


This implementation of POP3 daemon works with qmail-stype Maildirs, typically in user’s home directories. It is very small and simple but have all required for pop3 daemon features.

mdpop3d can be used in two modes: it can work with mail storage only, providing only transaction stage of POP3 protocol, and it optionally can provide simple authentication. In this way it differ from e.g. qmail-pop3d, as by default this mdpop3d will work for most situatuons without additional support programs. For many sites with regular users this is sufficient.

mdpop3d can authenticate users using PAM or using plain unix getpwnam(3) and getspnam(3) methods, depending on compilation options. For PAM, service name used is pop3 and may be changed by -p option.

If PAM support is enabled, mdpop3d can be compiled with support of APOP command (note that this command is available only with PAM, and only if specially compiled in and if activated in command line). APOP implementation requires special PAM module capable of checking client-provided md5 hash against user’s password. mdpop3d simply passes md5hash from client and server-generated timestamp to PAM library in place of a password (see USAGE section below), and some PAM module should be able to use this information to check client-supplied credentials.

When mdpop3d authenticates user itself (the default), it will by default look to Maildir subdirectory in user’s home directory as returned by getpwent(3) call (usually this info stored in /etc/passwd file). This default directory name may be changed by -m option or by MAILDIR environment variable.


mdpop3d accepts the following command-line options:
-t timeout set timeout value. If there will be no activity for this number of secounds, mdpop3d will terminate session without any changes in mail directory. This timeout applies also to output: if mdpop3d will not be able to write a line to client in this time, it will also terminate session. Default value is 60 (1 minute), that should be more than enouth for almost all situations. POP protocol is not a long-running protocol like e.g. IMAP, so timeout seemed to be reasonable here.
-c commit maildir on error/timeout. If this option given, mdpop3d will execute implicit QUIT command on any client communication error (i.e. timeout, closed connection etc). This way, messages marked as deleted will be removed from disk even without QUIT command. This violates protocol, but can help with broken mail clients on slow unstable dialup links, where client can loop forewer trying to retrieve all the messages at one session (this is known to be the case for some versions of M$ OutGluck).
-m maildir look to this maildir subdirectory inside user’s home directory instead of default Maildir for user’s mail. This option is not ignored in pre-authenticated mode (-a or -u options), the only difference is that mdpop3d will chdir to this directory from current invocation directory, not from user’s home. To specify current directory (or home dir itself), use ‘-m.’ (i.e. use ‘.’ for maildir). See also MAILDIR environment variable (this option takes precedence).
-a request pre-authenticated mode. mdpop3d goes to transaction state upon invocation, and tries to open Maildir (subject to -m option and MAILDIR environment variable) in current working directory. Authentication procedure in this case should be performed by the calling program, together with all required setuid(2) etc calls. This is the only mode of operation supported by qmail-pop3d alone. mdpop3d will run under whatether user- and group-id as calling process. Unless -u option is also given, mdpop3d will use value of LOGNAME environment variable as a user name for logging purposes, or, if this variable is not set, it will derermine logged in user by a call to getpwuid().
-u username similar to -a option (mdpop3d will go to transaction state), but provides also username for logging purposes. With this option, -a is useless.
-r host specifies host name or ip address of client, used for logging purposes and as PAM_RHOST if compiled with PAM support. The same effect have TCPREMOTEIP or REMOTE_HOST environment variables. If not given, mdpop3d will try to determine client ip address itself using getpeername(2).
-R hostvar this options names environment variable that holds remote host name or ip address, instead of TCPREMOTEIP and REMOTE_HOST. If this option given and no hostvar variable exists, mdpop3d will refuse to start.
-q be somewhat quiet. Normally mdpop3d logs all connections and disconnections via syslog at LOG_INFO priority. With this option it omits this exactly logging (but it still logs error conditions).
-n this is mostly debugging option. With this, mdpop3d will not move messages from new/ directory in maildir to cur/ directory. If this is only one program used for mail retrieval, then this saves some number of system work that may be noticable on heavily loaded machine. mdpop3d still looks to cur/ directory if it exists, and will remove deleted messages from both directories.
-d requests debugging. With this, all data received from client will be logged via syslog with LOG_DEBUG priority. Note that this data will not contain user’s passwords (all text for the PASS command will be replaced by "<hidden>").
-p service use PAM service name service instead of default pop3. May be useful for e.g. supporting ip-based virtual pop service (setup separate PAM configs for each local ip address). Not valid if mdpop3d was compiled without PAM support.
-A enable APOP command. This option valid only if mdpop3d was compiled with both PAM and APOP support.
-T htag enable APOP command and use htag for generating server-side APOP tag (timestamp), placing this htag after at (@) sign. RFC1939 recommends that server-side timestamp constructed in the following form <PID.TIME@HTAG>, where HTAG is FQDN of a server. By default (if only -A option is given), mdpop3d will use gethostname() call to determine this htag. This option valid only if mdpop3d was compiled with both PAM and APOP support.
-V if this option given, mdpop3d will print version information and exit. If compiled with PAM or APOP support, this will be indicated.


mdpop3d may use some environment variables. Since this program should be invoked in some "friendly" environment (i.e. inetd(8) or some other daemon), and because environment used for non-critical tasks (mostly logging), usage of environment variables is safe.

TCPREMOTEIP, REMOTE_HOST if set and not empty, will be used as an ip address of client accessing service (mdpop3d uses first of those that is available). Some inetd implementations sets one of this variables when invoking programs (one such implementation is courier tcpd), and some other programs may set them as well (e.g. stunnel(8)). Command-line option -h has precedence. The only usage of this address is for logging purposes, and, if mdpop3d compiled with PAM support, address will become PAM_RHOST PAM item.
LOGNAME when mdpop3d invoked with -a option, this variable, if set, used as a username of logged in (and authenticated by calling program) user. The only usage of this username is for logging purposes. Command line option -u have precedence here. Note that this variable will be used only in pre-authenticated mode.
MAILDIR look to this directory for user’s mail (Maildir by default). May be overwritten by -m option.


Typically mdpop3d will be invoked directly by inetd(8) daemon, like this:

pop3 stream tcp nowait root /usr/sbin/in.mdpop3d mdpop3d

This is all that needed for usual functionality to serve regular unix users’s mails. Also, mdpop3d may be invoked by some authenticator program, with correct userid, correct current directory and providing -a option. If you’re familiar with qmail, than the qmail-popup(8) program is the best reference for this.

mdpop3d will always serve only one request, it is not a long-running process. Then client issues QUIT command, or when there is timeout or client disconnect condition, or if mdpop3d failed to authenticate user, it will terminate session and exit.

If APOP support was compiled in (requires also PAM support) and enabled in command line (with -A option), mdpop3d will accept and handle APOP command. This is alternative of using USER and PASS command that avoids transmission of passwords in cleartext over network, but also requires that cleartext password to be known by server. In order to use/enable APOP command, one should provide some PAM module that will have access to original client’s password and can compute md5 hash from it and a timestamp string supplied by mdpop3d and compare computed value with client-supplied md5 hash. After receiving APOP command, mdpop3d will check if it is syntactically correct (by ensuring that supplied md5 hash value consists of exactly 32 lowercase hexadecimal digits), and will call pam library providing the string "APOP", space, this md5 hash as received from client, space and server-generated timestamp string, all in place of a password. For example, "password" provided to PAM may look like this:

APOP 0123456789abcdef0123456789abcdef <12.3456@host>

PAM module can check if password have this form ("APOP " prefix should be sufficient) and handle it accordingly. Example PAM config entry that support both APOP and USER/PASS:

auth sufficient
auth required ...

In this case, pam_apop module should check if password starts with "APOP ", then obtain original password, compute md5 hash from timestamp string and this password and compare with supplied hash, and then return either success or failure. If it doesn’t starts with "APOP ", then this module should return PAM_IGNORE, so that request will be processed by pam_unix module.

Note that such hypotetic pam_apop module is very site-dependant and not provided with mdpop3d. I would be glad to hear if anyone ever use the APOP feature at all...

If mdpop3d compiled with PAM support, it is trivial to support "virtual" maildirs using only one system account. For this, PAM module should be written that will check client-supplied credentials (username and password), and sets PAM_USER to the owner of virtual mail storage and $MAILDIR environment (either using pam_putenv() or setenv()) to point to maildir for a user relative to mail owner home. With this, mdpop3d can be used for both virtual mail storage and for regular user’s maildirs simultaneously, having properly configured PAM module stacking. Please refer to PAM documentation for further details. To support many virtual domains, one can form POP3 username using both name of a user and domain name. Again, such a module does not provided with mdpop3d (yet), while it can be of general use.


mdpop3d supports the following POP3 commands:
NOOP no operation
USER user specifies username (not valid in transaction state)
PASS password specifies user’s password (valid only after USER)
APOP user md5digest alternative way to specify user credentials. This command recognized only if APOP support compiled in and activated in command line (-A option).
QUIT terminates session. In transaction state mdpop3d will update maildir.
LIST [msgno] get size of message msgno or for all messages
UIDL [msgno] return unique identifier(s) for message(s)
TOP msgno lines return headers of message msgno and at most lines lines of body
RETR msgno return (retrieve) message msgno
DELE msgno marks message msgno as deleted
RSET reset message deleted flags from all previously deleted messages
CAPA list of server’s capabilities. Currently, mdpop3d lists UIDL, TOP and USER as a responce.
When responding to client’s commands, mdpop3d is somewhat quiet. For example almost all positive responces consists of just three characters +OK followed by CR, LF pair, no additional information used. POP3 used almost by software clients, not humans, and that additional info will always be discarded.


When using own implementation of password checking (via getpwnam(3) et al), password aging is not checked, and userid also (thus, mdpop3d will allow user with uid=0 and/or expired password to log in) -- only minimal checking done. This can be easily "cured" by using PAM that is far more preferable method anyway.

Any possible message from PAM discarded completely. This really isn’t a bug in this daemon itself, but in difficulties communicating of non-interactive application uses predefined protocol with pam framework. On any error in pam mdpop3d responds with generic "login incorrect" message.

mdpop3d will not allow user to log in with empty password, and there is no way to tell it to do so. This may be considered a bug, but I mostly disagree.

POP protocol transmits plaintext passwords over network. For unsecure networks this may be not acceptable. As a workaround there may be some "autheticator" that sets up a secure (encrypted) connection and calls this mdpop3d program. One example of this is ssl wrapper such as stunnel(8).

mdpop3d reports incorrect message size(s). It uses message file size as returned by stat(2) system call as a message size, but this does not includes extra carriage return (CR) character at the end of each line, as required by POP3 protocol. It is a protocol violation, but I think that it isn’t a (serious) issue. Many other pop3 daemons do the same.

mdpop3d will refuse to work with files in Maildir that have very long names and thus may overflow static mdpop3d buffer. Such a files will be silently ignored. Currently limit of filename is about 1018 characters, that should be sufficient for all environments where mdpop3d will run.


mail(1), qmail-pop3d(8), sendmail(8), inetd(8), stunnel(8), rfc1939 (Post Office Protocol - Version 3), rfc2449 (POP3 Extension Mechanism)


This software can be distributed under the terms of the GNU General Public License (GPL) version 2 or any further version.


This software and manual page has been written by Michael Tokarev,

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System Daemons IN.MDPOP3D (8) 12 Dec 2000

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