ProFTPD: SQL and mod_sql

## ProFTPD: 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)
      );

      CREATE INDEX users_userid_idx ON users (userid);

(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)
      );

      CREATE INDEX groups_gid_idx ON groups (gid);

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

Recipes

Using SQLNamedConnectInfo
What is `SQLNamedConnectInfo`, and how can I use it? The `SQLNamedConnectInfo` directive appeared in proftpd-1.3.4rc2 (see Bug#3262); it allows logging to SQL tables in a database separate from e.g. your user database. Using `SQLNamedQuery` and `SQLLog`, you tell `mod_sql` to log information to your database. And now, with `SQLNamedConnectInfo`, you can tell `mod_sql` to log information to multiple different databases.

The following illustrates how `SQLNamedConnectInfo` can be used. We have two databases, a "userdb" containing the tables with our user data, and a "logdb" database containing various logging tables. First, in the `users` table schema in the `userdb` database, let's include a `users.last_accessed` column, indicating when that user last logged in successfully:

      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),
        last_accessed DATETIME
      );

In the `logdb` database, let's define a table that we will use for separately logging all successful logins:

      CREATE TABLE login_history (
        user VARCHAR NOT NULL,
        client_ip VARCHAR NOT NULL,
        server_ip VARCHAR NOT NULL,
        protocol VARCHAR NOT NULL,
        when DATETIME
      );

With these tables defined in their databases, we can now configure `mod_sql` to use the `users` table for authenticating users. In addition, when a user logs in successfully, update the `users.last_accessed` column and add a row to the `login_history` table in the separate logging database:

      <IfModule mod_sql.c>
        AuthOrder mod_sql.c

        # We need our "default" connection to the userdb database
        SQLConnectInfo userdb@dbhost:3306 user pass

        # Now that we have a default connection, we can create another connection, named "logdb" and using the "mysql" backend, to the logdb database
        SQLNamedConnectInfo logdb mysql logdb@dbhost:3306 user pass

        # Point mod_sql at our users/groups tables
        SQLUserInfo users ...
        SQLGroupInfo groups ...

        # Update the users.last_accessed column on successful login in the userdb
        SQLNamedQuery last_accessed UPDATE "last_accessed = NOW() WHERE userid='%u'" users
        SQLLog PASS last_accessed

        # Add a row to the login_history table on successful login in the logdb
        SQLNamedQuery log_sess FREEFORM "INSERT INTO login_history (user, client_ip, server_ip, protocol, when) VALUES ('%u', '%a', '%V', '%{protocol}', NOW())" logdb

        SQLLog PASS log_sess IGNORE_ERRORS
      </IfModule>

Notice how the "log_sess" `SQLNamedQuery` has "logdb" at the end of the directive, after the SQL to be run? That tells `mod_sql` that, when running that `SQLNamedQuery`, use the connection named "logdb".

Another interesting point in the example above is the use of the IGNORE_ERRORS modifier, when we call the "log_sess" `SQLNamedQuery`. Why is that used? Here, it means that if the logdb database is not available, or if there is any other problem when inserting the row into the `login_history` table, that `mod_sql` should ignore that error and keep processing the connection.

Question: Why does the `SQLNamedConnectInfo` documentation say that "`SQLNamedConnectInfo` directives will only be honored if a `SQLConnectInfo` directive is configured"?
Answer: The `mod_sql` module requires a `SQLConnectInfo` directive in order to define the "default" named connection. This will be the database connection that `mod_sql` uses for queries, unless explicitly told to use some other named connection. And if there is no `SQLConnectInfo` directive present, then `mod_sql` won't even bother to look for `SQLNamedConnectInfo` directives, since the "default" connection must be present in order to use any other named connection.

Using SQLUserPrimaryKey and SQLGroupPrimaryKey
What is a primary key in `mod_sql`, and how can I use it? To answer this question, let's look at a common database layout for a `proftpd` site. You have a `ftp_users` table containing the user information (i.e. username/password, UID, GID, etc). You have a `ftp_sessions` logging table that records all client sessions, keyed to the user that logged in. And you have another logging table, `ftp_transfers`, that tracks the files that are transferred in and out by clients; the entries in this table, too, are keyed to the logged-in user.

Good schema design principles suggest that which this schema, the `ftp_sessions` and `ftp_transfers` tables should have foreign key constraints on columns in the `ftp_users` table, to enforce the fact that entries in the `ftp_sessions` and `ftp_transfers` tables must be related to a row in the `ftp_users` table. And for efficient database storage, you want the foreign key constraint to use a small column. But there's a problem. The obvious choice of primary key for an `ftp_user` row would be the UID, a numeric value. But you reuse UIDs for your users; this means that a single UID does not uniquely identify a user. In fact, the only unique identifier you have in `ftp_users` is the actual username, a string. And some of your usernames are quite large.

To demonstrate this more clearly, let's look at the `mod_sql` configuration that would be used to add rows to the `ftp_sessions` table:

      # Insert a row on successful login (i.e. successful PASS)
      SQLLog PASS start-session IGNORE_ERRORS

      SQLNamedQuery start-session FREEFORM "INSERT INTO ftp_sessions (userid, session_id, ...) VALUES ('%u', '%{env:UNIQUE_ID}', ...)"

Instead of `userid` (which is a `VARCHAR(255)`), you would prefer to use the `ftp_users.id` column value, which you added to your `ftp_users` table specifically to use as a primary key. How to do you tell `mod_sql` to use that as the primary key for the logged-in user?

The `SQLUserPrimaryKey` and `SQLGroupPrimaryKey` directives appeared in proftpd-1.3.5rc3 (see Bug#3864) for just this reason; they configure the columns that `mod_sql` to retrieve/use as primary keys. When these directives were present, whenever `mod_sql` does a successful lookup of a user by name (or of a group by name), `mod_sql` will then run another query, i.e.:

      SELECT column-name FROM user/group-table WHERE name = user/group-name

And to support the case where the tables/columns are customised, these new directives would of course support custom queries:

      SQLUserPrimaryKey custom:/named-query
      SQLGroupPrimaryKey custom:/named-query

So that you could do:

      SQLNamedQuery get-user-key SELECT "id FROM ftp_users WHERE userid = '%U'"
      SQLUserPrimaryKey custom:/get-user-key

      SQLNamedQuery get-group-key SELECT "id FROM ftp_groups WHERE groupname = '%g'"
      SQLGroupPrimaryKey custom:/get-group-key

Now that `mod_sql` can look up the value for the primary key for the user/group, we need to then be able to use that value in other queries. For this, you would use the "notes" `SQLLog` variable:

      %{note:sql.user-primary-key}
      %{note:sql.group-primary-key}

Putting this altogether, the above `mod_sql` configuration for adding rows to the `ftp_sessions` table, using `ftp_users.id` as the primary key, becomes:

      # Tell mod_sql to use the ftp_users table for user data
      SQLUserInfo ftp_users ...

      # Use ftp_users.id as the user primary key
      SQLUserPrimaryKey id

      # Insert a row on successful login (i.e. successful PASS)
      SQLLog PASS start-session IGNORE_ERRORS

      SQLNamedQuery start-session FREEFORM "INSERT INTO ftp_sessions (userid, session_id, ...) VALUES (%{note:sql.user-primary-key}, '%{env:UNIQUE_ID}', ...)"

Question: Why use new configuration directives, rather than changing the existing `SQLUserInfo`/`SQLGroupInfo` directives to include a primary key column?
Answer: New directives were added for this functionality for two reasons: 1) not every site needs/uses primary keys and foreign key constraints in their schema, and 2) we wanted to make sure that existing configurations/schema did not break and behave unexpectedly (i.e. to preserve backward compatibility).

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.

This problem also presents itself in terms of queries using UID/GID of 65533, e.g. when you see queries like this against your database:

      SELECT groupname FROM ftp_groups WHERE (gid = 65533) LIMIT 1

If the UID/GID retrieved from the database is below the minimum UID/GID, then `mod_sql` will use the default UID/GID value, which is 65533. These defaults can be changed using `SQLDefaultUID` and `SQLDefaultGID`.

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 compatibility 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: I've upgraded to MySQL 5.7, and now I am unable to login using my MySQL users. The `SQLLogFile` shows something like this:

      mod_sql/4.3[9097]: checking password using SQLAuthType 'Backend'
      mod_sql/4.3[9097]: entering     mysql cmd_checkauth
      mod_sql/4.3[9097]: MySQL client library used MySQL SHA256 password format, and Backend SQLAuthType cannot succeed; consider using MD5/SHA1/SHA256 SQLAuthType using mod_sql_passwd
      mod_sql/4.3[9097]: MySQL server used MySQL 4.1 password format for PASSWORD() value
      mod_sql/4.3[9097]: password mismatch

and my `mod_sql` configuration has not changed; it uses:

      SQLAuthTypes Backend

Is this a bug?

Answer: The short answer is that no, it is not a bug. But it is a regression caused by changes in the MySQL API, and the only fix is to change the password values stored in your MySQL tables.

First, the background. The "Backend" `SQLAuthType` is only implemented by `mod_sql_mysql`, for MySQL databases; Postgres, SQLite, et al do not have an equivalent. And in MySQL, the "Backend" `SQLAuthType` is there specifically for the use case where an admin uses MySQL's `PASSWORD()` function to generate the hashed value stored in the `users` table for ProFTPD users. The way that the "Backend" `SQLAuthType` is implemented is that `mod_sql_mysql` gets the hashed value stored in the row from the MySQL server, and then `mod_sql_mysql` uses a function in the `libmysqlclient` library to try to generate the same hash (using the password from the client); it then compares the two hashes to see if they match.

MySQL has had issues with its `PASSWORD()` generated format over the years. They initially generated a 16 byte string, but this was deemed insecure in MySQL 4.1, and changed to a 41 byte string. But even that 41 byte string was found to be vulnerable to e.g. rainbow table attacks, and thus the format was changed again to be much longer, to use SHA256, and (importantly for us) to always include a randomly generated salt. (It is this salt which mitigates the rainbow table attack scenario.) This sequence of events means that a MySQL database table used by `mod_sql_mysql` might contain a `PASSWORD()`-generated hashed value that is in the pre-4.1 format (16 bytes), the 4.1 format (41 bytes), and/or the sha256 format.

For the pre-4.1 and 4.1 hashed value formats, the MySQL `PASSWORD()` function and the `libmysqlclient` library functions used by `mod_sql_mysql` would generate the same hashed value for the same password, and thus worked as expected. But for sha256 hashed value formats, `PASSWORD()` and the `libmysqlcient` library functions will not generate the same hashed value for the same password. Why not? Each time those functions are called, they internally generate and use a random salt value; there is no way to explicitly provide the salt value to the function. This means that the sha256 hashed value will be different, each time. If `PASSWORD()` is used to generate an sha256 formatted hashed value in the database table using password "test", and if the `libmysqlclient` library function is called to generate an sha256 formatted hashed using password "test", those functions will generate different hashed values. Calling those functions again will generate different hashes, each and every time; this means that they cannot be compared/matched.

If sha256 hashed values are different each time, how can MySQL use them to authenticate? The process used for authenticating the client (the database client, `mod_sql_mysql`, not the FTP client connecting to ProFTPD) is all internal to MySQL, and is slightly different; it assumes that the client being authenticated is defined in the `mysql.users` system table, and only there. That internal support does not extend to any other table which might have used the `PASSWORD()` function for its columns. (This is why that `PASSWORD()` function is slated for deprecation, as of MySQL 5.7, and later removal.) This means that you can use sha256 formatted hashed values for authenticating `mod_sql_mysql`, but not for authenticating the ProFTPD users stored in the MySQL database tables.

Why is it only recently (as of late 2016) that users are encountering this issue? MySQL 5.5 (the default MySQL version for Debian/Ubuntu for a while) did not support the sha256 formatted hashed values, only the pre-4.1 and 4.1 formatted hashed values. So no issue with the "Backend" `SQLAuthType` there. The introduction/use of sha256 formatted hashed values started in MySQL 5.6.

But ProFTPD users using MySQL 5.6 did not encounter this problem, either. Why not? Turns out that the `libmysqlclient` library in 5.6 had a couple of different functions for making hashed values (of different formats). The `mod_sql_mysql` module would try all of those different functions, for backward compatibility. And it worked, as one of the older `libmysqlclient` library functions (the `make_scrambled_password()` function, specifically) would generate the 4.1 formatted hash value, which would match the values stored for ProFTPD users in the `users` table successfully.

However, in MySQL 5.7, those older `libmysqlclient` library functions were removed (they had been deprecated for a while anyway), so now, the only `libmysqlclient` library function for generating hashed values is the sha256 one. And that, as mentioned above, uses an internally-generated random salt each time, and its output is not consistent/repeatable. And that means that starting with MySQL 5.7, the "Backend" `SQLAuthType` will no longer work. As mentioned by the log message in the `SQLLogFile`.

The alternatives are to use different `SQLAuthTypes`; this will require that you change the `users.passwd` value for your ProFTPD users. First, make sure that the `mod_sql_passwd` module is compiled/loaded into your ProFTPD. If you are building from source:

      $ ./configure --enable-dso --with-shared=mod_sql:mod_sql_mysql:mod_sql_passwd ...
      $ make
      $ make install

And in your `proftpd.conf`:

      # Make sure that mod_sql_passwd is loaded, if available
      LoadModule mod_sql_passwd.c

      <IfModule mod_sql.c>
        SQLBackend mysql

        <IfModule !mod_sql_passwd.c>
          # If mod_sql_passwd is not available, then consider using the OpenSSL
          # SQLAuthType
          SQLAuthTypes OpenSSL Backend
        </IfModule>

        <IfModule mod_sql_passwd.c>
          # If mod_sql_passwd is available, try other SQLAuthTypes, too
          SQLPasswordEngine on

          # MySQL uses lowercase hex-encoded strings for MD5() et al
          SQLPasswordEncoding hex

          SQLBackend SHA256 SHA1 MD5 Backend
        </IfModule>
      </IfModule>

When populating the `users.passwd` field for your users, you can use MD5 passwords:

      mysql> UPDATE users SET passwd = MD5('password') WHERE userid = 'myuser';

or SHA1 passwords:

      mysql> UPDATE users SET passwd = SHA1('password') WHERE userid = 'myuser';

or SHA256 passwords:

      mysql> UPDATE users SET passwd = SHA2('password', 256) WHERE userid = 'myuser';

And the `SQLAuthTypes` configuration will try all of those types. The "Plaintext" and "Empty" `SQLAuthTypes` should never be used except for debugging/development; they are too insecure for production systems.

Question: I see the following SSL-related error message in my `SQLLogFile` recently; what happened?

      mod_sql/4.3[30]: unrecoverable backend error
      mod_sql/4.3[30]: error: '2026'
      mod_sql/4.3[30]: message: 'SSL connection error: unable to use random device'

Answer: This mostly likely occurs for MySQL connections, but can occur for any database connection that uses TLS. And specifically, it can happen when the `DefaultRoot` directive is used to `chroot` the sessions.

Newer database client libraries, such as for MySQL, now automatically try to use SSL/TLS for connecting to the database server whenever possible. When a session is `chrooted`, however, any SSL/TLS reconnects or renegotiations can fail with the above "unable to use random device" error. Why? The SSL/TLS library in use, such as OpenSSL, needs a source of randomness for its cryptographic needs; it usually uses a device such as `/dev/random` or `/dev/urandom`, provided by most Unix platforms. However, the `chroot` means that those devices are no longer accessible from within the chrooted filesystem.

To prevent this, you can disable use of SSL/TLS (not preferable), or use something like `SQLKeepAlive` to help ensure that your database connections stay alive, and thus avoid reconnects.

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.

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