# Courier Authentication Library

This library is used for two purposes:

1. Read the name of a mail account. Determine the local account's home directory, and system userid and groupid.

2. Read an account name, and a password. If valid, determine the account's home directory, system userid, and groupid.

The term "authentication" is used in the following documentation to refer to either one of these two functions. The library contains several alternative authentication modules to choose from, described below.

The configuration file `/etc/authlib/authdaemonrc` contains several settings. The most important of them are:

- A list of authentication modules to activate. By default, this list includes all available authentication modules, even if some are not actually installed at the moment. When the authentication library is set up, only those authentication modules that can be supported by the operating system will be installed. Some of the listed modules may not actually be there, however that's not a problem. Any unavailable authentication modules will be ignored. Also, on some platforms certain authentication modules are installed by optional sub-packages. Installing the sub-package is the only action needed to make use of it.

  The only time the list of authentication modules need to be adjusted is when an available authentication module must be disabled for some reason. This should only be needed in the most unusual circumstances.

- Number of authentication processes. The default setting is to start five authentication processes, which should be sufficient for normal usage. Try increasing this setting if its taking too long to log into an account, and you have determined that this is not due to a bottleneck in the whatever authentication database you're using (LDAP, MySQL, SQLite, or PostgreSQL).

  An authentication request must be completed within thirty seconds, otherwise it gets rejected. When authentication requests come in faster than all five authentication processes can get to them, delays build up, and the timer is ticking. If all the activity maxes out the CPU or I/O bandwidth, nothing can be done about it, short of getting another server. However if there's plenty of available CPU and I/O, increasing the number of processes will do the trick.

## The `authpwd` authentication module

This modules obtains account information and passwords from the `/etc/passwd` file.

### Note

This module doesn't actually read the `/etc/passwd` file, it uses the C library's getpw() functions. The C library implementation could use any mechanism to obtain the equivalent information.

## The `authshadow` authentication module

This module is a version of the `authpwd` module that reads passwords from `/etc/shadow` (the C library's getsp() functions).

## The `authpam` authentication module

This modules uses the system's PAM library (pluggable authentication modules) for authentication. This is, essentially, a way to use existing PAM modules for authentication. Note, however, that the authenticated account's home directory, userid and groupid are still read from the `/etc/passwd` file, since PAM functionality is limited to validating account passwords.

### Note

Not all PAM modules are compatible with Courier's authentication library. PAM modules that make use of PAM's session functions, or authentication token functions, like `pam_krb5` will not work with Courier.

### Note

Additional configuration steps will be required to set up the PAM library to authenticate Courier's services. Courier's IMAP and POP3 servers, for example, require that the “imap” and “pop3” PAM service to be configured.

The specific configuration steps differ from system to system. Consult the system documentation for more information. It might be tempting to throw in a towel and use `authshadow` or `authpwd` if you cannot figure out how to install PAM support, however that is not advisable. It is highly recommended to use `authpam` wherever the PAM library is available.

The exact configuration procedure depends on the PAM implementation. Most PAM libraries use configuration files in the `/etc/pam.d` directory. Therefore, it will be necessary to install the configuration files `/etc/pam.d/imap` and `/etc/pam.d/pop3`. Similarly, Courier's webmail server, SqWebMail, uses `/etc/pam.d/webmail`, and its optional calendar component uses `/etc/pam.d/webmail`. Courier-MTA's authenticated SMTP component uses the `/etc/pam.d/smtp` service.

In nearly all cases all these configuration files will specify an identical PAM library configuration for all services. The exact configuration details are site-specific. Here's an example of a PAM configuration file for a recent version of the most common PAM library:

    auth       required     pam_nologin.so
    auth       required     pam_stack.so service=system-auth
    account    required     pam_stack.so service=system-auth
    session    required     pam_stack.so service=system-auth

Again, the actual configuration is site specific. Examine the contents of existing configuration files in `/etc/pam.d` for similar services (if there's `/etc/pam.d/ppp` it's often a good example to follow) in order to derive the correct setup for Courier.

Older PAM libraries use a single configuration file, usually `/etc/pam.conf`. Append Courier-specific PAM settings to this configuration file, again using settings for existing services as a guide. For example:

    imap  auth    required        pam_unix.so      try_first_pass
    imap  account required        pam_unix.so
    imap  session required        pam_permit.so
    pop3  auth    required        pam_unix.so      try_first_pass
    pop3  account required        pam_unix.so

Some PAM libraries use `pam_pwdb.so` instead of `pam_unix.so`; consult the PAM library's documentation for more information.

## The `authpipe` authentication module

This is a generic plug-in module that runs an external script, or a program, in response to authentication requests.

The external program reads from stdin and writes to stdout. It can be persistent and handle many authentication requests. Only one request will be sent to it at a time; each authdaemon process starts its own copy of the external script.

The location of the external program is set by the `--with-pipeprog` configure option, which defaults to `/etc/authlib/authProg`. A sample program is included in the courier-authlib source.

## The `authpipe` protocol

authpipe uses the same protocol as authdaemon clients use to communicate with authdaemond.

There are four possible requests: `PRE`, `AUTH`, `PASSWD` and `ENUMERATE`. Apart from `AUTH`, each request is a single line terminated by newline.

PRE . `authservice` `username` <newline>

  Look up data for an account. `authservice` identifies the service the user is trying to use - e.g. pop3, imap, webmail etc.

  If the account exists, return the account data as a series of ATTR=value newline-terminated lines, followed by a period on a line of its own. Valid attributes are:

            USERNAME=username         -- system account which owns mailbox (name)
            UID=uid                   -- system account which owns mailbox (numeric uid)
            GID=gid                   -- numeric groupid
            HOME=homedir              -- home directory
            ADDRESS=addr              -- e-mail address
            NAME=name                 -- full name
            MAILDIR=maildir           -- Maildir relative to home directory
            QUOTA=quota               -- quota string: maxbytesS,maxfilesC
            PASSWD=cryptpasswd        -- encrypted password
            PASSWD2=plainpasswd       -- plain text password
            OPTIONS=acctoptions       -- option1=val1,option2=val2,...
            .

  Of these, it is mandatory to return ADDRESS, HOME, GID, and either UID or USERNAME; the others are optional.

  If the account is not known, return `FAIL``<newline>`. If there is a temporary failure, such as a database being down, authProg should terminate (thereby closing stdin/stdout) without sending any response. authdaemon will restart the pipe module for the next request, thus ensuring it is properly reinitialized.

AUTH `len`<newline>`len-bytes`

  Validate a login attempt. The AUTH line is followed by len-bytes of authentication data, which does not necessarily end with a newline. The currently defined authentication requests are:

          service \n login \n username \n password [\n]         -- plaintext login
          service \n cram-md5 \n challenge \n response [\n]     -- base-64 encoded challenge and response
          service \n cram-sha1 \n challenge \n response [\n]    -- ditto
          service \n cram-sha256 \n challenge \n response [\n]  -- ditto

  In the case of success, return the complete set of account parameters in the same format as PRE, ending with a period on a line of its own. In the case of failure (e.g. username does not exist, password wrong, unsupported authentication type), return `FAIL``<newline>`. If there is a temporary failure, such as a database being down, authProg should terminate without sending any response.

  Note: if the user provides a plaintext password and authenticates successfully, then you can return it as PASSWD2 (plain text password) even if the database contains an encrypted password. This is useful when using the POP3/IMAP proxy functions of courier-imap.

PASSWD `service`<tab> `username`<tab> `oldpasswd`<tab> `newpasswd`<tab> <newline>

  Request a password change for the given account: validate that the oldpassword is correct, and if so, change it to the newpassword.

  Reply: the string `OK``<newline>` for success, or `FAIL``<newline>` for a data error (e.g. no such account, old password wrong, new password not acceptable). In the case of a temporary failure, such as a database being down, authProg should terminate without sending any response.

ENUMERATE <newline>

  Return a list of all accounts, one per line in the following format, ending with a period on a line of its own:

          username \t uid \t gid \t homedir \t maildir \t options \n
          .

  If your module does not support the ENUMERATE command then return just a period on a line of its own (which will still allow enumeration data from other modules to be returned). In the case of a temporary failure, such as a database being down or an error occuring mid-way through returning account data, authProg should terminate before sending the terminating period.

## The `authuserdb` authentication module

This module uses a GDBM or a DB-based userdb(8) database. This module also incorporates userdb-based challenge-response authentication implementation that was done by a separate `authcram` module in previous versions of the Courier authentication library.

`/etc/authlib/userdb` is a plain file that can be edited with any text editor. The file contains a list of account names, and their pertinent information. `/etc/authlib/userdb` may alternatively be a directory containing plain text files, which are effectively concatenated together to form the actual list of accounts. The makeuserdb script compiles the account information into a GDBM or DB database file, which can be quickly looked up.

`/etc/authlib/userdb` is loosely equivalent in function to `/etc/passwd` and `/etc/shadow`, and contain analous information: account name, its numeric userid and groupid, home directory, and passwords. `/etc/authlib/userdb` also contains additional Courier-specific metadata, such as account quotas and other account-specific settings. `/etc/authlib/userdb` files can also be maintained by custom-written Perl scripts, instead of being edited by hand.

`/etc/authlib/userdb` allows creation of virtual mail accounts that do not have a corresponding login account -- virtual mail accounts that can share the same, reserved, system userid. `/etc/authlib/userdb` can also be used to completely supersede `/etc/passwd`. With many accounts it can be quite a drain to have to continuously linearly scan `/etc/passwd` in order to look up an account. Instead, a fast database lookup can retrieve the same information from the database file. Review the included manual pages, starting with userdb(8), for more information.

### A brief `userdb` primer

`userdb` is a way to implement many virtual mailboxes - many mailboxes that do not have to have a separate system userid allocated for each one, and there is no system login associated with each mailbox. `userdb` uses a database for mapping virtual addresses to physical maildirs. It should be scalable to thousands of mailboxes. It can also be used to replace linear searches of `/etc/passwd` with a database lookup, see userdb(8).

Note - you still MUST use some valid system userid and groupid that is shared by all virtual mailboxes. Instead of allocating a single userid and groupid per each mailbox, the same userid and groupid is used for all of them.

This is a rough overview of using userdb. For additional information, read userdb(8) and makeuserdb(8). All the scripts will be installed in `/usr/sbin`, so look for them there.

The best way to describe how `userdb` works is to try to create one virtual mail account. As mentioned before, virtual mailboxes still need one system account to be used for uid/gid purposes. Let's call this system account "vmail".

### A simple userdb setup

This approach should be used if you do not have many virtual mailboxes. It's very simple, but quickly becomes cumbersome if you administer many virtual mailboxes.

Create an empty `/etc/authlib/userdb` file:

    # cp /dev/null /etc/authlib/userdb
    # chmod 700 /etc/authlib/userdb

`/etc/authlib/userdb` must have 700 permissions, since it will contain passwords.

Now, run the script pw2userdb, as root. This script converts the contents of `/etc/passwd` to the `/etc/authlib/userdb` format (including the contents of `/etc/shadow`, this is why permissions on `/etc/authlib/userdb` must be 700). This script is usually used where you want to convert a very large `/etc/passwd` to `/etc/authlib/userdb`. `userdb` applications can now use a fast `userdb` database instead of a linear scan of `/etc/passwd` in order to look up system accounts. However, you probably don't want to use this feature right now, so what you want to do is take the output of pw2userdb, and find the entry for the vmail account that you created earlier. Look for a line that starts with 'vmail' followed by tab, followed by familiar fields from `/etc/passwd`. Save the output of pw2userdb in a temporary file, edit it, and remove everything except the line containing vmail, and the very next line, which is a special entry that maps vmail's userid back to the vmail record.

Here's what you might find in the output of pw2userdb:

    vmail   uid=1012|gid=1012|home=/home/vmail|systempw=*
    1012=   vmail

The actual numerical values and the home directory location may vary. Save these two lines as `/etc/authlib/userdb`, and set the permissions on `/etc/authlib/userdb` to 700:

    $ chmod 700 /etc/authlib/userdb

Now, with that out of the way, let's really create a virtual account. In this example we'll create a virtual mailbox for 'john@example.com'.

    # su vmail
    $ cd ~vmail
    $ mkdir john-example
    $ maildirmake john-example/Maildir
    $ exit
    #

You may need to specify a full path to your maildirmake program. The end result is that you created `$HOME/john-example` in vmail's account, which can be thought of as a “virtual home directory” for “john@example.com”, that contains the account's maildir mailbox.

Now, let's connect the dots here, and create an entry in `/etc/authlib/userdb` for `john@example.com`:

    # userdb "john@example.com" set home=/home/vmail/john-example \
                                    uid=UUU gid=GGG

This command runs the script named `userdb` , which is installed, by default in `/usr/sbin`. Replace UUU and GGG with the userid and groupid of the vmail account. If you now look in `/etc/authlib/userdb`, you will see that a new record for “john@example.com” has been appended to the end of the file.

One more detail: we need to set the IMAP password for this mailbox:

    # userdbpw | userdb "john@example.com" set imappw

On most modern Linux and BSD distributions, you can specify the -md5 option to userdbpw, in order to use MD5 password hashes, instead of crypt. The traditional password function allows passwords only up to 8 characters long; everything in excess is ignored. The newer MD5 passwords, now supported by most modern systems, allow longer passwords.

Use "`systempw`" instead of "`imappw`" if you would like to use the same password for the POP3 server, and for all other services. The "`imappw`" field is only checked by the IMAP server. If not defined, "`systempw`" is used instead. The field `pop3pw` is checked only by Courier's POP3 server. If it is not defined the POP3 server will check `systempw` too.

Finally, compile the database:

    # makeuserdb

This command creates the actual database, `/etc/authlib/userdb.dat` and `/etc/authlib/userdbshadow.dat` from the plain text file `/etc/authlib/userdb`. Courier will now start accepting logins to this mailbox. Adding and removing mailboxes can be done while Courier is running.

Courier reads `/etc/authlib/userdb.dat` and `/etc/authlib/userdbshadow.dat` only. The plain text source, `/etc/authlib/userdb` is not read by Courier itself. Changes take effect only when makeuserdb runs.

### Large virtual domain farm

The previous approach used a single flat file, `/etc/authlib/userdb`. This will work for up to a couple of hundred accounts. An slightly different approach can scale to thousands of domains and mailboxes.

Instead of creating a `/etc/authlib/userdb` file, create a subdirectory:

    # mkdir /etc/authlib/userdb
    # chmod 700 /etc/authlib/userdb

Now, create `/etc/authlib/userdb/default`, containing pw2userdb's output for the vmail account, as previously described.

This time, you probably want to create all mailboxes for the same domain in a separate subdirectory:

    # su - vmail
    $ cd ~vmail
    $ mkdir -p domains/example-com
    $ mkdir domains/example-com/john
    $ maildirmake domains/example-com/john
    $ exit

The idea is that all the maildirs for `@example.com` will now be found in `~vmail/domains/example-com`. All maildirs for `domain.org` will be in `~vmail/domains/domain.org`. The actual layout and naming conventions are entirely up to you to define.

Here's how configure `/etc/authlib/userdb`:

    $ userdb "example-com/john@example.com" set \
                 home=/home/vmail/domains/example-com/john \
                 uid=UUU gid=GGG

This creates the file `/etc/authlib/userdb/example-com` (the first parameter to the userdb command), and appends a record named "john@example.com". You will store all `userdb` entries for `@example.com` in the file `/etc/authlib/userdb/example-com`. All entries for `@domain.org` will be maintained in `/etc/authlib/userdb/domain-org`, and so on.

    $ userdbpw | userdb "example-com/john@example.com" set imappw

This sets the IMAP access password for this account. Finally:

    $ makeuserdb

### Beyond `userdb`

`userdb` is a simple, straightforward solution that scales to a couple of thousand of mail accounts, depending on the hardware. Beyond that, one of database-based modules will need to be used, such as `authldap`, `authmysql`, `authsqlite`, `authpgsql`. Since `userdb` is maintained as plain text files that are easily parsed by a script, migrating data from userdb will not be difficult.

## The `authmysql` authentication module

This module reads the list of mail accounts and passwords from a table in a MySQL database. The `/etc/authlib/authmysqlrc` configuration file defines the particular details regarding the MySQL database and the schema of the mail account table.

## The `authpgsql` authentication module

This module reads the list of mail accounts and passwords from a table in a PostgreSQL database. The `/etc/authlib/authpgsqlrc` configuration file defines the particular details regarding the PostgreSQL database and the schema of the mail account table.

## The `authsqlite` authentication module

This module reads the list of mail accounts and passwords from a table in a SQLite database file. The `/etc/authlib/authsqliterc` configuration file defines the particular details regarding the SQLite database file and the schema of the mail account table.

## The `authldap` authentication module

This module reads the list of mail accounts and passwords from an LDAP directory. The `/etc/authlib/authldaprc` configuration file defines the particular details regarding the LDAP directory layout.

A suggested LDAP schema can be found in the files `authldap.schema` and `authldap.ldif`, which is included in Courier authentication library's source code, and may be installed on your system.

## `authcustom`

This is a do-nothing module where custom authentication code can be added. This authentication module is just a stub that doesn't really do anything. It's purpose is to serve as a placeholder where custom authentication code can be easily implement.

## Account options

The authentication library has a facility for keep arbitrary “name=value”-type settings, called “options”, for individual accounts. This feature is only available with `userdb`, `LDAP`, `MySQL`, `SQLite` and `PostgresSQL` modules. Individual account options are not supported with system-based authentication modules (password/shadow files, or PAM).

See auth_generic(3) for a description of option names used by various Courier packages. Other applications can make up names for their own settings, and use them in the same way.

Account options are specified via the authentication modules in the following manner:

`userdb`

  Use the userdb command to set a field called "`options`". Example:

      userdb user1@example.com set options=disableimap=1,sharedgroup=44
      makeuserdb

  The option text string here is "`disableimap=1,sharedgroup=44`". It specifies two options.

`LDAP`

  Account options are defined by the `LDAP_AUXOPTIONS` setting in the `authldaprc` configuration file. `LDAP_AUXOPTIONS` consists of a comma-separated list of "`attribute=setting`". "attribute" is the name of an LDAP attribute, and "setting" is the corresponding account setting name. The value of the attribute becomes the value of the setting. Unless you value your sanity, the names of LDAP attributes should be the same as the actual setting names (in which case "=setting" may be dropped and `LDAP_AUXOPTIONS` becomes a simple comma-separated list of supported settings), but they don't have to be.

  `LDAP_AUXOPTIONS` is nothing more than a simple mapping of LDAP attributes to account settings. A `LDAP_AUXOPTIONS` of "shared=sharedgroup,disableimap" means that the LDAP attribute called "shared" contains the "sharedgroup" setting, as described previously; and an LDAP attribute of disableimap contains the setting of the same name.

MySQL, SQLite, and PostgreSQL

  Account options are defined by `MYSQL_AUXOPTIONS_FIELD`, `SQLITE_AUXOPTIONS_FIELD`, or `POSTGRESQL_AUXOPTIONS_FIELD`, in its corresponding configuration file. In the most simplest case, add a character field to the database, and put the field name into the `MYSQL_AUXOPTIONS_FIELD`, `SQLITE_AUXOPTIONS_FIELD`, or `POSTGRESQL_AUXOPTIONS_FIELD` configuration file setting. For each account, the character field should contain the literal option string. Yes, you'll just put "shared=sharedgroup,disableimap" literally, in that field.

  Fortunately, there is a cleaner way to do this, which avoid driving a database designer batty. Keep in mind that the contents of `MYSQL_AUXOPTIONS_FIELD`/`SQLITE_AUXOPTIONS_FIELD`/`POSTGRESQL_AUXOPTIONS_FIELD` are simply inserted directly into the SQL query that fetches the account information. MySQL, SQLite, and PostgreSQL have a rich SQL that can be used to manufacture a suitable option string from plain, garden-variety, database fields. That is, you may define individual table fields like "disableimap", and "disablepop3", then provide a suitable (albeit ugly) SQL fragment that combines them together into the expected option string. An example of such an SQL string is provided in the comments portion of the configuration file.

  ### Note

  When using the alternative custom query option, the option string is the last field that the custom SQL query should return.

### Note

The following list of account options is a combined list of implemented options supported by Courier, Courier-IMAP, and SqWebMail packages. Some of the following information is obviously not applicable for a particular package. The inapplicable bits should be obvious.

The following options are recognized by the various Courier packages:

`disableimap=``n`

  If "n" is 1, IMAP access to this account should be disabled.

`disablepop3=``n`

  If "n" is 1, POP3 access to this account should be disabled.

`disableinsecureimap=``n`

  If "n" is 1, unencrypted IMAP access to this account should be disabled.

`disableinsecurepop3=``n`

  If "n" is 1, unencrypted POP3 access to this account should be disabled.

`disablewebmail=``n`

  If "n" is 1, webmail access to this account should be disabled.

`disableshared=``n`

  If "n" is 1, this account should not have access to shared folders or be able to share its own folders with other people.

`group=``name`

  This option is used by Courier-IMAP in calculating access control lists. This option places the account as a member of access group `name`. Instead of granting access rights on individual mail folders to individual accounts, the access rights can be granted to an access group “name”, and all members of this group get the specified access rights.

  The access group name “administrators” is a reserved group. All accounts in the `administrators` group automatically receive all rights to all accessible folders.

  ### Note

  This option may be specified multiple times to specify that the account belongs to multiple account groups.

`sharedgroup=``name`

  Another option used by Courier-IMAP. Append "name" to the name of the top level virtual shared folder index file. This setting restricts which virtual shared folders this account could possibly access (and that's on top of whatever else the access control lists say). See the virtual shared folder documentation for more information.

  For technical reasons, group names may not include comma, tab, "/" or "|" characters.

## Running authtest

The authtest command may be used to verify that the authentication library is working:

    authtest userid
    authtest userid password
    authtest userid password newpassword
    authenumerate

Running authtest with one argument should display the selected account's home directory, userid, groupid, and other related data. The second argument to authtest, if supplied, specifies the account's password. The two argument form of authtest validates the password, and displays an indication whether the given password is valid, or not. The three argument form of the authtest command attemps to change the account's password. The second argument is the old password, the third argument is the new password.

See `README.authdebug.html` for more information.

### Changing account passwords

For the virtual domain modules (`authldap`, `authmysql`, `authsqlite`, `authpgsql` and friends) changing the login is a no-brainer. The tricky situation is when SqWebMail uses system passwords to log in (the `authpwd`, `authshadow`, or `authpam` authentication module). Different systems use different ways to keep login passwords. Many systems use the traditional `/etc/passwd` and `/etc/shadow` files. Other systems use a binary database; other systems use NIS. And on some systems the password file lookup library is a wrapper that goes against an external LDAP directory, or a database. For maximum compatibility, SqWebMail changes login passwords by running the passwd command. This is the traditinal *nix command that changes login passwords. passwd is an interactive command. It's normally run from a terminal. SqWebMail uses an expect script - as mentioned in the introduction - to answer interactive prompts from passwd. The expect script expects to get a plain, garden-variety, passwd command, which acts something like this:

         # passwd
         Changing password for luser
         (current) UNIX password:         (old password typed here)
         New UNIX password:               (new password typed here)
         Retype new UNIX password:        (new password retyped here)
         passwd: all authentication tokens updated successfully
         #

Systems that use a passwd command with very different prompts may find that the default expect script will fail. In which case it will be necessary to tweak the expect script to match the prompts from the system's passwd command.

Modern systems use a passwd command that rejects "bad" passwords - passwords that are based on dictionary words, are too short, or are obvious for other reasons. When testing the ability to change system passwords be sure to use randomly-generated gibberish for the test passwords. Otherwise, the default expect script will actually be working, but you won't be the wiser. For security reasons, the actual messages from passwd will not be shown by.

The expect script is installed as `/usr/local/libexec/courier-authlib/authsystem.passwd` (assuming default options to the configure script).

## Authentication internals

The following structure describes an authentication module:

> Example 1. struct authstaticinfo

>     struct authstaticinfo {
>     	const char *auth_name;
>     	int (*auth_func)(const char *, const char *, char *, int,
>     			 int (*)(struct authinfo *, void *),
>     			 void *);
>     	int (*auth_prefunc)(const char *, const char *,
>     			    int (*)(struct authinfo *, void *),
>     			    void *);
>     	void (*auth_cleanupfunc)();
>     	int (*auth_changepwd)(const char *, /* service */
>     			      const char *, /* userid */
>     			      const char *, /* oldpassword */
>     			      const char *); /* new password */

>     	void (*auth_idle)();
>     	/* Not null - gets called every 5 mins when we're idle */

>     	void (*auth_enumerate)( void(*cb_func)(const char *name,
>     					       uid_t uid,
>     					       gid_t gid,
>     					       const char *homedir,
>     					       const char *maildir,
>     					       void *void_arg),
>     				void *void_arg);
>     	} ;

An authentication module is a shared library that defines a single function called “courier_auth_`NAME`_init”, where “NAME” is the name of the authentication module. The shared library does not need to export any other symbols, this is the only function that needs to be exported. The function returns a pointer to the authstaticinfo structure. For example, the relevant code from the `authmysql` module is:

> Example 2. authmysql

>     static struct authstaticinfo authmysql_info={
>     	"authmysql",
>     	auth_mysql,
>     	auth_mysql_pre,
>     	auth_mysql_cleanup,
>     	auth_mysql_changepw,
>     	auth_mysql_cleanup,
>     	auth_mysql_enumerate};

>     struct authstaticinfo *courier_authmysql_init()
>     {
>     	return &authmysql_info;
>     }

`auth_func` points to a function that handles an authentication request. The function is invoked as follows:

> Example 3. auth_func

>     int result=auth_func(const char *service, const char *authtype,
>     			const char *authdata,
>     			int (*callback_func)(struct authinfo *, void *),
>     			void *callback_arg);

“service” is the name of the service being authenticated, such as “`imap`” or “`pop3`”. “authtype” defines the authentication format, and “authdata” is the actual authentication request.

Two authentication formats are defined at this time. The “authtype” string is set to one of the following strings:

“login”

  Tradition userid/password authentication. `authdata` points to a string that consists of: the userid; a newline character; the password; a final newline character.

“cram-md5”, or “cram-sha1”

  Challenge/response authentication. `authdata` points to a string that consists of: the base64-encoded challenge; a newline character; the base64-encoded response string; and a final newline character. Furthermore, the base64-encoded response string consists of: the login ID, a space character, and the response as a hexadecimal string (yes, base64-encoding of the response string is not strictly necessary).

### Note

Not all authentication modules may implement all authentication formats. An authentication module that does not implement a particular authentication format should handle it the same way as an invalid login ID.

The authentication function should return a negative value if the login ID is invalid. The authentication library will try the next authentication module.

The authentication function should return a positive value if the login ID is valid, but the password is invalid. The authentication library will not try any more authentication modules.

Otherwise, the authentication module should call the `callback_func` function, and return the same value that's returned by this function.

The authentication module should pass through `callback_arg` to the callback function as a second argument. The first argument is a pointer to the authinfo structure, which is described in detail in the auth_generic(3) manual page. The authentication module is responsible for allocating this structure. After the callback function returns this structure can be deallocated. The authentication module initializes the following fields:

`auth_pre_func` points to a function that obtains account information. The function is invoked as follows:

> Example 4. auth_pre_func

>     int auth_pre_func(const char *user, const char *service,
>                       int (*callback)(struct authinfo *, void *), void *arg);

This function does the same thing as “auth_func” except that the password is not actually verified. If the account exists, the callback function is invoked with the same callback arguments.

`auth_cleanup_func` points to a function that will be invoked just before the authentication module is uninstalled, giving it the opportunity for some last-minute cleanup.

`auth_idle` points to a function that will be invoked when no authentication requests are received for a couple of minutes, giving the authentication module an opportunity to close any database connections, so that they do not get shut down by the server, for inactivity, resulting in an error the next time an authentication request is received.

`auth_changepwd` points to a function that will be invoked to change a password on an account, as follows.

> Example 5. auth_changepwd

>     int auth_changepwd(const char *service, const char *user,
>     			const char *oldpw, const char *newpw);

`service` is the name of the service whose password is to be changed (such as “imap” or “pop3”). `auth_changepwd` should return 0 if the password was changed succesfully, a negative value if `user` is invalid (the next authentication module will be tried), or a positive value if the password change request failed (no more modules will be tried).

`auth_enumerate` points to a function that enumerates the list of all login IDs known to the authentication module. The first argument `auth_enumerate` is a callback function. `auth_enumerate` invokes the callback function once for each login ID, supplying the login ID, the userid, groupid, home directory and maildir as arguments. The last argument to the callback function is passed through from the second argument to `auth_enumerate`.

After enumerating all login IDs `auth_enumerate` calls the callback function one last time, with a NULL pointer for the login ID, then returns. If an error is encountered while enumerating the login IDs, `auth_enumerate` terminates without invoking the callback function with a NULL login ID.

## FILES

`/etc/authlib/authdaemonrc` - authdaemond configuration file

`/etc/authlib/authldaprc` - authldap configuration file

`/etc/authlib/authmysqlrc` - authmysql configuration file

`/etc/authlib/authsqliterc` - authsqlite configuration file

`/etc/authlib/authpgsqlrc` - authpgsql configuration file

## SEE ALSO

courier(8), userdb(8)