## SQL and `mod_sql`

Compiling with `mod_sql`
To compile `proftpd` with the `mod_sql` SQL module, you will need to have the libraries and header files of a SQL database installed; the `mysql.h` and `libmysqlclient.a` files for MySQL, the `libpq-fe.h` and `libpq.a` files for Postgres.

`mod_sql` is the module that provides a generic interface between the `proftpd` daemon and the underlying SQL database(s); `mod_sql` relies on backend modules to handle the database-specific operations. For MySQL database, the backend module is `mod_sql_mysql`; `mod_sql_postgres` is to be used when dealing with Postgres databases. Then, to build a `proftpd` daemon to use `mod_sql`, use the `--with-modules` option of the `configure` script, specifying both `mod_sql` and the backed module, e.g.:

      ./configure --with-modules=mod_sql:mod_sql_mysql

Sometimes the necessary header and library files for building in SQL support are in non-standard locations; the `configure` script needs to be told about these non-standard locations so that it can find the necessary files, and build the daemon properly. The `--with-includes` option (for header file locations) and `--with-libraries` option (for library file locations) are used for informing `configure` of such things. For example, if you had installed MySQL using a prefix of `/usr/local/mysql`, so that the path the header file was:

      /usr/local/mysql/include/mysql/mysql.h

and the path the library was:

      /usr/local/mysql/lib/mysql/libmysqlclient.a

then, the above `configure` line would be changed to look like:

      ./configure \
        --with-modules=mod_sql:mod_sql_mysql \
        --with-includes=/usr/local/mysql/include/mysql \
        --with-libraries=/usr/local/mysql/lib/mysql

The same options can be used similarly for specifying Postgres file locations. (NOTE: Postgres 7.2 or higher should be used; earlier versions of Postgres lacked a string-escaping function that allowed for an SQL injection vulnerability. Use the `mod_sql_postgres` from proftpd 1.2.9rc1 or later in conjunction with a newer Postgres library to fix the bug.)

`mod_sql` is capable of using OpenSSL for different ways of encrypting passwords stored in database tables. The `configure` options for building an OpenSSL-capable `mod_sql` might look something like this:

      CFLAGS=-DHAVE_OPENSSL LIBS=-lcrypto ./configure \
        --with-modules=mod_sql:mod_sql_mysql \
        --with-includes=/usr/local/mysql/include/mysql:/usr/local/openssl/include \
        --with-libraries=/usr/local/mysql/lib/mysql:/usr/local/openssl/lib

Note that this example assumes that you have installed OpenSSL using a prefix of `/usr/local/openssl`.

Configuring `mod_sql`
Now that a `proftpd` daemon has been compiled for SQL support, you can begin the task of configuring it so that `mod_sql` can access your SQL database tables. At a very minimum, `mod_sql` assumes that it has access to a table with two columns, one for user names, the other for passwords. For more complete functionality, tables providing full user and group information are needed. The full information that can be provided is described below.

User Information Table1

Column | Type | Required? | Duplicates? | Null? | Purpose
userid | text | yes | no | no | user's login
passwd | text | yes | yes | no | user's password
uid | number | yes | no2 | yes | user's UID
gid | number | no | yes | yes | user's GID
home3 | text | no | yes | yes | user's home
shell4 | text | no | yes | yes | user's shell

Notes:

1. The user table MUST exist.

2. Nothing in `proftpd` or `mod_sql` prevents you from using duplicate UIDs, from given multiple users the same UID. Hower, this is should be done only if you are certain you know what you are doing.

3. See the `SQLDefaultHomedir` and `SQLUserInfo` configuration directives.

4. See the `RequireValidShell` configuration directive.

Group Information Table1

Column | Type | Required? | Null? | Purpose
groupname | text | yes | no | group's name
gid | number | yes | no | group's GID
members2 | text | yes | yes | group's members

Notes:

1. `mod_sql` will normally concatenate all matching group rows; you can have multiple rows for each group with only one member per group, or have a single row with multiple groups, or a mixing of the two. However, if you use the fast option for groupset of the `SQLAuthenticate` directive, you may not have multiple rows per group.

2. Multiple members per group are formatted as comma-separated names (no contained whitespace) in the text for this column.

The two SQL statements below should work for any ANSI SQL compliant databases, and are known to work for MySQL and PostgreSQL. They both fully specify the tables as described above, with reasonable defaults for field length and data type. More stringent definitions are suggested: if you plan on keeping home directory or shell information in the database, those fields could be defined as `NOT NULL`, or even `UNIQUE` for home directory. Similarly, if you plan on being able to use the groupsetfast argument to the `SQLAuthenticate` directive, you should create both the groupname and gid fields as `UNIQUE`.

To create a user table:

      CREATE TABLE users (
        userid VARCHAR(30) NOT NULL UNIQUE,
        passwd VARCHAR(80) NOT NULL,
        uid INTEGER UNIQUE,
        gid INTEGER,
        homedir VARCHAR(255),
        shell VARCHAR(255)
      )

(Note: if you plan to reuse the same UID for multiple users, then you will need to remove the `UNIQUE` from the `uid` column description). To create a group table:

      CREATE TABLE groups (
        groupname VARCHAR(30) NOT NULL,
        gid INTEGER NOT NULL,
        members VARCHAR(255)
      )

The key configuration directives for `mod_sql` are:

- `SQLConnectInfo`, for setting the information for connecting to the database server
- `SQLAuthenticate`, for controlling how the module will perform its authentication lookups
- `SQLAuthTypes`, for defining which authentication methods to use
- `SQLUserInfo`, for modifying the names of the columns using which the module will lookup values from the user table
- `SQLGroupInfo`, for modifying the names of the columns using which the module will lookup values from the group table

A list of `mod_sql` configuration directives can be found here.

Frequently Asked Questions
Whenever questions arise about problems using `mod_sql`, the first place to look will be the server debugging output and in a `SQLLogFile`. `mod_sql` is very verbose with its debugging information, and you can see everything it is doing. However, there are times when there is too much information in the debugging output, and you are unsure of how to remedy the problem. These are some of the frequent questions.

Question: Why is `proftpd` only looking in my SQL tables when a user logs in?
Answer: You probably configured `mod_sql` to be "authoritative" in your `SQLAuthenticate` setting by using the * option. Conversely, if you actually want `proftpd` to only authenticate SQL-defined users, the * "authoritative" suffix is what you would want to use.

Question: Why does my SQL user not use the UID/GID I configured for her in my SQL table?
Answer: More than likely, you gave this user a UID that is below the default `SQLMinUserUID` (999), or a GID that is below the default `SQLMinUserGID` (999). Use the `SQLMinUserUID`, `SQLMinUserGID`, and/or `SQLMinID` configuration directives to set these limits lower as needed.

Question: Do I have to configure a real shell for my SQL-defined users?
Answer: No. The `proftpd` daemon only checks the shell for a user in order to provide compatibilty with other FTP daemons, which do the same check; `proftpd` itself does not spawn the shell. See the `RequireValidShell` configuration directive for turning this check off.

Question: How come my `SQLLog QUIT` is not run if the session aborts or dies?
Answer: Not all FTP clients are polite and issue the `QUIT` before the session ends. Perhaps their session is timed out, or dropped due to network problems. Use `EXIT` as the FTP command in your `SQLLog` directive, rather than `QUIT`, as mentioned in the `SQLLog` documentation.

Question: How can I make `mod_sql` go faster?
Answer: There are a couple of things you might try. First, if using a version of `mod_sql` from ProFTPD-1.2.7rc1 or later, make use of the `SQLNegativeCache` configuration directive.

Other forms of this question are "Why does `mod_sql` iterate through every user in the database?", or "Why is `mod_sql` so slow during logins?" Here's the reason: `mod_sql` is designed to handle all authentication functions that the daemon throws at it. This includes the functions that iterate through all users (`setpwent()`, `getpwent()`, `endpwent()`) and the functions that iterate through all groups (`setgrent()`, `getgrent()`, `endgrent()`).

When you see `mod_sql` iterating through all groups or users, it is doing so because it has been asked to do so by the daemon. Since there is no good way to keep an open query around without adding more to the various backend modules than we already have, `mod_sql` pre-caches all users when `setpwent()` is called, and pre-caches all groups when `setgrent()` is called. This allows the `getpwent()` and `getgrent()` calls to be simple, at the cost of more time during login.

In simple situations, these functions are never called. When you start limiting access to directories, files, or various FTP commands based on user or group, that is when the daemon needs to iterate through the users and groups to check permissions. A basic FTP server, including virtual and anonymous servers, will never make the (potentially, very) expensive user iteration calls, but may iterate through all groups.

The `SQLAuthenticate` directive provides a method to tune `mod_sql`; by default, `mod_sql` will handle the various *`pwent()` and *`grent()` calls. When `SQLAuthenticate` is told not to handle userset or groupset, `mod_sql` simply passes the request on to whatever authentication handlers exist in the system. Keep in mind that using `SQLAuthenticate` in this way means that the `proftpd` daemon is not using the same information to authenticate the user as it is to control the user's actions during their session.

For those of you who have used `mod_sql` in the past, these lookups should probably be set to off. Versions of `mod_sql` prior to 3.2.0 (or thereabouts) did not handle the `{set|get|end}{pw|gr}ent` functions at all, so by setting these lookups to off, you lose no functionality. Those of you new to `mod_sql` should to consider your needs: is the cost of iterating through every user stored in the database worth the ability to limit access based on users/groups from the database? If not, you will need to re-evaluate the way you are using the database, and where you should be storing your user/group information.

Question: When I use an `SQLAuthTypes` that includes "OpenSSL", what do the values in the database need to look like?
Answer: The value that `mod_sql` expects is the base64-encoded value of the digested password string. To get a list of the message digest algorithms supported by your OpenSSL installation, you can execute the following command:

      openssl list-message-digest-commands

To generate the string to put into the SQL tables, using MD5 as the digest algorithm and "password" as the password:

      /bin/echo "{md5}"`/bin/echo -n "password" | openssl dgst -binary -md5 | openssl enc -base64`

The "{md5}" prefix is necessary, so that `mod_sql` knows what digest algorithm was used.

Here's a quick and dirty example of generating database-ready strings using every digest algorithm supported by the installed OpenSSL:

      for c in `openssl list-message-digest-commands`; do
        /bin/echo "{$c}"`/bin/echo -n "password" | openssl dgst -binary -$c | openssl enc -base64`
      done

which generates:

      {md2}8DiBqIxuORNfDsxg79YJuQ==
      {md4}ip0JPxT4cB3xdzKyuxgsdA==
      {md5}X03MO1qnZdYdgyfeuILPmQ==
      {mdc2}HA0SCu32vhW+exItsGK4lQ==
      {rmd160}LAjo9YhHUKe5n28vNC/GONsl/zE=
      {sha}gAclaL6zshAjJesgP20P+S9c744=
      {sha1}W6ph5Mm5Pz8GgiULbPgzG37mj9g=

Note: the `{digest}` prefix syntax is only needed for passwords that will be handled as OpenSSL-style passwords. This prefix is not needed for any of the other `SQLAuthType` types.

There are a lot of PHP applications which might want to generate passwords suitable for use with `mod_sql`. For example, to generate a base64-encoded MD5 password with PHP, which can then be read by `mod_sql`, use the following PHP code:

      // $password contains the cleartext password before
      $password = "{md5}".base64_encode(pack("H*", md5($password)));

      // $password now contains the encrypted, encoded password

Or if you're using Python, you might try the following:

      password = "{md5}" + base64.b64encode(md5.new("password").digest())

If this prefixed format is not sufficient for your needs, you can also use the `mod_sql_passwd` module, which knows how to handle "raw" MD5, SHA1, and other encoding schemes.

Question: Why do I see "error deleting scoreboard entry: Invalid argument"?
Answer: This log message almost always denotes use of `mod_sql`, and a problem in the `mod_sql` configuration. To debug the problem, define an `SQLLogFile` (making sure the path is not in a world-writable directory), to which `mod_sql` will write debugging information.

Question: I am trying to compile `proftpd` with MySQL support, and get the following error when I run `make`. Why?

      /usr/bin/ld: cannot find -lz
      collect2: ld returned 1 exit status
      make: *** [proftpd] Error 1

Answer: This error means that the linker cannot find the `libz` library. For many systems, this library is provided by the `zlib` package; you may need to install this zlib package. The `libz` library is required by the MySQL client libraries.

Question: I was able to compile `proftpd` with `mod_sql` without problems, but now when I try to start `proftpd`, I get the following error. Why?

      proftpd: error while loading shared libraries: libpq.so.5: cannot open shared object file: No such file or directory

Answer: Errors about "loading shared libraries", especially when they happen when starting `proftpd`, come from the dynamic/shared library loader. The linker was able to find that library at compile time, but not at run time. If you were to look for the specified library, you would see it. So why can't the shared library loader find it?

Updating the directories searched automatically by the shared library loader is a platform-specific task. On Linux, for example, you will need to edit your `/etc/ld.so.conf` file to include the directory containing the SQL client libraries; then run `/sbin/ldconfig` in order update the shared library loader cache file. Once the shared library loader has been updated, `proftpd` should start without problems.

Question: What is the difference between the userset and usersetfast `SQLAuthenticate` options, and when would I use them?
Answer: It depends, of course.

First, the use of these options is only relevant when the session is chrooted, either via the `DefaultRoot` directive or because it is an `<Anonymous>` login. (Why? When chrooting a session, `proftpd` calls the `setpwent()` function in order to "reset" or "rewind" the user info database; this ensures that all of the info in the user info database is accessible in the chrooted session.)

The userset `SQLAuthenticate` option says that the info for all users should be looked up and cached when `setpwent()` is called. Specifically, the list of names for every user in the database (and just the names!) is looked up. For each user name, a SELECT query (using the `SQLUserInfo` configuration) is used to retrieve all of the details for that user.

The usersetfast `SQLAuthenticate` option goes even further, Rather than having a SELECT per user name, usersetfast says to lookup all the info for all the users when `setpwent()` is called, in a single SELECT query (again, based on the `SQLUserInfo` configuration).

Knowing the above, the choice of when to use userset or usersetfast becomes one of efficiency. Some efficiency (in terms of ID-to-name lookups during the session) can be gained by using userset, since the info for all users is cached (using a SELECT query per user). Even more efficiency is gained by using usersetfast to lookup the info for all users using a single SELECT query. In both cases, the price you pay for the efficiency is increased memory usage in the session process, of course; the info for all users will be held in memory in the process for every session.

The above holds true for the groupset and groupsetfast `SQLAuthenticate` options as well.

Question: How do I configure `mod_sql` so that it will use encrypted connections (e.g. SSL/TLS) to the backend database server?
Answer: This sort of configuration depends on the backend database server you are using.

If you are using MySQL, then you can configure this in the "[client]" section of your `my.cnf` configuration file.

If you are using Postgres, then this will happen automatically, by default, as long as your `libpq` Postgres client library has been compiled with SSL support.

Last Updated: $Date: 2011/03/16 19:07:48 $