Courier Mail Server

Transport Modules

The directory ${COURIER_HOME}/lib/modules contains shared libraries and programs which provide input and output modules to Courier. The actual module itself can actually be installed and invoked anywhere else, the SMTP module, in fact, is installed in bin. The shared library in lib, however, may include functions which rewrite addresses for messages submitted from the input module. All files for module named MODULE are stored in the directory ${COURIER_HOME}/lib/modules/MODULE.

The modules.ctl file

The file ${COURIER_HOME}/lib/modules/modules.ctl contains a list of all the modules that Courier loads. modules.ctl is only used if Courier is compiled with shared library support. If Courier is compiled with static linkage only (by choice, or by necessity), all the module libraries are statically linked into Courier, and modules.ctl is not used. Each line in modules.ctl is in the following form: priority<SP>name<SP>filename, where <SP> designates the space character. The lines are sorted in increasing priority order! "priority" is taken from the module's config file, and filename is the filename of the shared library.

Contents of ${COURIER_HOME}/lib/modules/name directory

The following files will be found in a module subdirectory under lib. The name of the subdirectory is the name field from the modules.ctl file.


This is the module configuration file. Blank lines in the config file are ignored, also lines that start with the # character, which can be used for comments. The module configuration is specified using the NAME=VALUE notation, where NAME is the name of the configuration parameter, and VALUE is its value.


This is the shared library that's loaded by submit and courierd. The shared library provides code to rewrite addresses to and from the canonical format for messages to or from this module. If Courier is compiled with static linkage, this file does not exist - the library is statically linked.

The actual name of the library may vary, and is specified in the config file.

If this shared library does not exist, an attempt is made to load librewrite-old. This allows an updated version of the shared library to be installed in a live, running, system by renaming the current one to librewrite-old, then renaming the new one to librewrite.

Although submit will pick up new rewriting rules immediately, courierd must be SIGHUPed in order to reload the new shared library.

Parameters in config

NAME=name - specifies the name of this module. Should be the same as the directory name.

LIBRARY=filename - specifies the name of the shared library to load. Not used if Courier was compiled with static libraries.

VERSION=version - version of the module interface. Not the actual version of the module, but version of the interface between the module, and Courier. The current version is version 0.

PRIORITY=n - priority of the output module. Courier calls all the modules' rewritedel functions in the increasing priority order until it finds one which accepts messages addressed to the recipient's address.

PROG=pathname - pathname to the output module program. Must be a full path, unless the module itself is in the lib/MODULE directory.  If the PROG parameter is missing, this module is an input only module. If the attempt to execute PROG fails, Courier will attempt to execute PROG-old, which allows an updated output module to be inserted into a live system by renaming the current one PROG-old, then renaming the new output module as PROG.

MAXDELS=n - maximum concurrent deliveries for this module. No more than these many instances of PROG will be started at any given time.

MAXHOST=n - maximum instances of PROG that will be started with the same HOST parameter.

MAXRCPT=n - maximum number of addresses that will be given to any single instance of PROG.

Please note that although these parameters are reread by courierd when it restarts, the individual output module may impose its own restrictions on the valid limits for these parameters.

Module library functions

The module library contains the following functions. In each case, XXXX is used to represent the name of the function in the library for module XXXX. For example, in the "local" module, the rw_install function is called local_rw_install.

struct rw_list *XXXX_rw_install(const struct rw_install_info *info);

The rw_install() function is called after the shared library is open. It receives a pointer to the following structure:

struct rw_install_info {
        int rw_verlo;
        int rw_verhi;
        const char *courier_home;
        } ;
rw_verlo/rw_verhi - reserved for future expansion. Currently set to 0. rw_install function of modules compatible with this Courier interface must check that rw_verlo is set to 0, and ignore rw_verhi.

courier_home - Courier's home directory, the shared library can use this to find its files.

The rw_search function

struct rw_list *rw_search(const char *);
The rw_search function can be called by a library function in order to return the rw_list (see below) structure of a function in another library. rw_search may NOT be called form XXXX_rw_install, because the library containing this function may not've been installed yet. rw_search may be called from the XXXX_rw_init function.

const char *XXXX_rw_init()

After all modules are installed, each module's rw_init() function is called, which can complete any additional setup. rw_init should return a null pointer. Otherwise, rw_init should return the error message text. Courier will refuse to start if rw_init does not return a null pointer.

The library's rw_install function must return a pointer to the following structure. If rw_install returns a NULL pointer, Courier will refuse to start.

struct rw_list {
        int rw_version;
        void (*rewrite)(struct rw_info *info, void (*func)(struct rw_info *));
        void (*rewrite_del)(struct rw_info *info, void (*func)(struct rw_info *),
                    void (*delfunc)(struct rw_info *info, const struct rfc822token *host,
                                const struct rfc822 *addr));
        int  (*filter_msg)(const char *,
                        const char *,
                        const char *,
                        const char *,

                        char *,
} ;
rw_version - shared libraries compatible with this module interface must set rw_version to zero.

rewrite - this function is called to rewrite a single address. The first argument points to the following structure:

struct rw_info {
    int mode;
    struct rfc822token *ptr;
    void (*err_func)(int, const char *, struct rw_info *);
    const struct rfc822token *sender;
    const char *smodule;
    void *udata;
} ;
mode contains the following values, ORed into a bitmask: RW_ENVSENDER - rewrite envelope sender, RW_ENVRECIPIENT - rewrite envelope recipient, RW_HEADER - rewrite header. When calling rewrite(), one of these three bits will be set. Additional bits that may be set: RW_OUTPUT - rewrite() should convert canonical format to the transport format. If this bit is not set, rewrite should convert from the transport format to the canonical format. In fact, the main courierd does not call rewrite with the RW_OUTPUT bit set, because that function is performed by the dedicated output module, which may handle rewriting on its own. Also, the RW_SUBMIT can be set together with RW_ENVSENDER or RW_ENVRECIPIENT, indicating that this call is as a result of a message being submitted for delivery (as opposed to address verification for EXPN/VRFY functionality).

It is possible that none of those bits are set, when invoked by another rewrite function.

ptr is the address to rewrite, as rfc822 tokens.

udata contains an arbitrary pointer, for usage by rewrite's caller.

When mode has RW_ENVRECIPIENT set, sender points to the envelope sender format in the canonical format (previous result of RW_ENVSENDER), otherwise this field is unused. If sender is NULL, this should be interpreted as an empty envelope sender (or if rewrite is being called in test mode.

smodule is initialized when mode has the RW_SUBMIT bit set. It will point to the name of the module argument to submit - the module sending the message.

err_func is the function to call in case of an error.

rewrite is expected to call either func, (it's second argument), or err_func, before returning. If rewriting succeeds, func is called. If rewriting failed, rewrite must call the err_func is function. errcode will be the RFC822-style error number, errmsg is the error message, which may be multiline (includes newlines). The text in errmsg is NOT prefixed by the error number.

After calling func, or err_func, rewrite is expected to immediately terminate. rewrite may alter the 'ptr' link list in any form or fashion it wants, except that it may NOT malloc or free any node, or a part thereof. However, it can relink portions of the link list, or modify the link pointers to include tokens created by rewrite internally.

After func or err_func terminates, rewrite may deallocate everything it allocated, then terminate itself.

This interface allows rewrite to execute very quickly, without allocating or deallocating any memory. If new RFC822 tokens are required, they can be allocated on the stack, and linked into the original list.

The rewrite_del function is called to determine if the module can accept delivery of a message to this address. The rewrite_del of all installed libraries are called until one of them calls the delfunc function. If rewrite_del cannot accept delivery of a message, it should call func. The rewrite_del function should call the delfunc function to indicate that this module can accept mail addressed to the address specified by info->ptr. rewrite_del receives the pointer to the rw_info structure, then the host and the address information for the output module, as rfc822token lists. If the mode field has the RW_SUBMIT bit set, rewrite_del can find the message's envelope sender address in canonical format) in info->sender.

Like rewrite, rewrite_del may make arbitrary changes to info->ptr, except that it may not deallocate memory used for the existing list. rewrite_del may modify the link list, and allocate memory for new rfc822 tokens. After calling either func or delfunc, rewrite_del should terminate immediately, and deallocate any allocated memory. rewrite_del must keep track of any allocated memory separate, and cannot assume that info->ptr hasn't changed.

When RW_SUBMIT bit is set, rewrite_del can be used to perform additional recipient-specific code, which may be too expensive to run every time courier goes through the queue. The udata field contains a pointer to caller-specific data. The sender field contains a pointer to the envelope sender, in canonical format. Like rewrite, rewrite_del may muck around with the rfc822token list in ptr. rewrite_del functions are called in order according to the configured module priority. By setting a higher priority, it is possible to have rewrite_del rewrite the address so that it would be accepted by another module's rewrite_del, down the chain.

The last function, rw_filter_msg, is called to run an arbitrary spam filter which can be used to selectively reject messages. rw_filter_msg will be called after rewrite_del accepted the message for delivery. The arguments are as follows:

This function is called twice: once when submit receives the recipient address (file descriptor is -1), and the second time after the message is received, but before it is accepted. Therefore, this function should execute quickly. Also note that ESMTP does not have a well-defined way to selectively reject messages to individual recipients once the contents of the message are received. If a message is addressed to multiple recipients, and at at least one recipient's filter rejects the message, Courier replies with a rejection code, and the sending mail server will assume that the message to all recipients has failed.

rw_filter_msg should return 0 if the message is acceptable, a negative value to permanently reject the message to this recipient, and a positive value for a temporary rejection.

Running PROG

PROG is the output module that will be started by Courier when it comes up. PROG can be a shell command, it is executed via "$SHELL -c". It will be started under userid mail, group mail, in ${COURIER_HOME}/lib/modules/NAME. Courier will communicate with PROG on standard input and output.

If PROG succesfully initializes, it should fork, and the parent should exit with status of 0. Courier waits until PROG exits. If it exits with a non-0 exit code, Courier will fail starting up. The child process will then continue to read and write from standard output.

COURIER_HOME, MAXDELS, MAXHOST, and MAXRCPTS will be placed in the environment, prior to executing PROG.

Tip: if the environment variables are not set, PROG can presume that it's being run in test mode, from the command line, and forego the fork.

If PROG terminates, Courier will consider it a fatal error (Courier detects the input channel being closed).

If PROG gets an end-of-file indication, it can assume that Courier is being shut down, so it should promptly cease all activities, without waiting for pending messages to be delivered.

To start delivery for a particular message, PROG will receive a newline-terminated command, specifying the message, and the recipients, and the delivery ID. Once PROG finishes delivering messages, PROG should write the results of the delivery into the message's control file, then print the delivery ID on its standard output, terminated by newline.  If the module's config file specifies that the module can handle multiple deliveries at the same time, PROG may receive additional deliveries before it finishes delivering the first message.

The command that prog receives is a newline-terminated line that looks like this:


<tab> represents the ASCII tab character. This is basically a list of tab-separated fields. The first field is the message id number (the inode number).

sender is the message envelope sender, after it's rewritten to the module format, by the module shared library.

delid is the "delivery ID", which is a small integer, representing this delivery. After PROG finishes delivering the message, it should print the message's delivery ID on standard output after saving the delivery status of each recipient in the control file.

The host field specifies the host where the message should be delivered to, as determined by the module's output rewrite rule. Following the host, there will be one or more num/address pairs. address is the recipient's address as determined by the output rewrite rule, and num is the recipient's number in the message's list of recipients (this is used to save the delivery status in the control file). Note that the address can be an empty string, so there will be two consecutive tabs there.