## Configuration Tricks

Proficient users of `proftpd`, and site administrators who require fancy configurations, usually make use of a handful of useful tricks when it comes to configuring their FTP server. These tricks can help to make your `proftpd.conf` smaller, clearer, and easier to maintain.

Configuration File Variables
One juicy tidbit of configuration knowledge is the little known `%u` variable. It will be substituted, during the handling of an FTP session, with the name of the user who logged in.

The `%u` variable can be appear in the `proftpd.conf` like so:

      DefaultRoot /home/%u

Now, the above may not seem that useful, not much of an improvement over using the tilde (`~`) symbol, for restricting users to their home directories. But what if you wanted to restrict users to the `public_ftp/` subdirectory, within their home directories? Using the tilde character cannot achieve that. Instead, you might use:

      DefaultRoot /home/%u/public_ftp

This is also useful for setting up configurations for specific directories in all users' home directories, e.g.:

      <Directory /home/%u/public_ftp>
        ...
      </Directory>

This `%u` variable can appear in the following configuration directives:

- `DefaultChdir`
- `DefaultRoot`
- `<Directory>`

Simple Per-User Directory Hashing
Some sites find that they might have large number of users (on the order of hundreds to thousands), each with their own home directory. You might start off using a directory layout such as `/home/user/` for each user, but this quickly leads to such a large number of subdirectories in the `/home/`. With that number of entries, simple commands like `ls(1)` may never complete, or take minutes to hours.

To reduce the number of entries within a given directory, such large sites often look to use some form of directory hashing. The simplest form is to change from e.g. `/home/user/` to `/home/u/us/user/`. For example, if you had user "bobsmith", the home directory would be `/home/b/bo/bobsmith`.

The question then is: can `proftpd` be configured to use a directory hashing scheme like this, without having to change the home directory attribute for those thousands of users (which might be stored in an LDAP directory used by applications as well)? Answer: yes. The `%u` variable above supports way to handle simple directory hashing.

To use it, you would use something like the following in your `proftpd.conf`:

      DefaultRoot /home/%u[0]/%u

where the `%u` variable is the user name, and `%u[index]` can be used to reference the letters in the user name, at that index position. The above configuration would use a single level of directory hashing, resulting in e.g. `/home/b/bobsmith`. If your directory hashing gets more complex, using more levels, you can still use this syntax:

      DefaultRoot /home/%u[0]/%u[0]%u[1]/%u[0]%u[1]%u[2]/%u

which would result in `/home/b/bo/bob/bobsmith`.

Of course, more complex hashing schemes can be used, but `proftpd` does not currently implement them.

Environment Variables
Did you know that you can also substitute environment variables into your `proftpd.conf` as well? It's true! ProFTPD started supporting using environment variables in configuration files in the 1.2.10rc1 release. To use the environment variable, simply use:

      %{env:ENV_VAR_NAME}

Unlike the `%u` variable, you can use environment variables anywhere within your `proftpd.conf` file.

For example, you might use something like this if you wanted to control the default IP address on which `proftpd` listens using an environment variable:

      DefaultAddress %{env:PR_DEFAULT_ADDR}

Then whatever value the `PR_DEFAULT_ADDR` environment variable has, when starting `proftpd`, will be substituted in. Using:

      PR_DEFAULT_ADDR='127.0.0.1' /usr/local/sbin/proftpd ...

would mean that, effectively, the configuration would be:

      DefaultAddress 127.0.0.1

"What happens if the environment variable is not set?" you ask. Good question. Then the configuration file parser will substitute in an empty string, and `proftpd` will probably fail to start, citing a configuration error.

Conditional Configuration Files
Another common request or desire to make large sections of your configuration conditional. Maybe you only want some directives to apply to certain users, or maybe you want to ship a `proftpd.conf` that can be used by people with different modules loaded (e.g. for distributing a common `proftpd.conf`). Or maybe you simply want some sections of your configuration file to be used based on something else entirely.

The most common conditional you will probably see in various `proftpd.conf` files is:

      <IfModule moduleName>
        ...
      </IfModule>

This is way of making the configuration directives within that section only in effect if the specified module is present. It may not seem like much, but this is quite useful. (I have a single `proftpd.conf` with more than 20 such sections, for use while I develop new modules.)

I have seen one clever sysadmin use the above conditional section in conjunction with DSO modules in their `proftpd.conf` like so:

      <IfModule mod_dso.c>
        # If mod_dso is present, we know we can dynamically load DSO modules

        <IfModule !mod_sql.c>
          # If mod_sql is not yet loaded, load it!
          LoadModule mod_sql.c
        </IfModule>

        <IfModule !mod_sql_mysql.c>
          # If mod_sql_mysql is not yet loaded, load it!
          LoadModule mod_sql_mysql.c
        </IfModule>

      </IfModule>

You can see how, with the above, you can use provide the same `proftpd.conf` to sites which use shared modules as well as those which use static modules.

Users who wish to have entire sections of configuration only apply to specific users, or groups, or even classes of clients really should be aware of the `mod_ifsession` module, and its very handy `<IfUser>`, `<IfGroup>`, and `<IfClass>` sections.

And, for those admins who want even more control over large sections of their `proftpd.conf` on a conditional basis, there is:

      <IfDefine value>
        ...
      </IfDefine>

The enclosed section of the configuration file will be parsed only if value is defined. For multiple directives, this trick is better than using multiple environment variables; the latter is better for single parameters.

"But how you define value?", you say. There are two ways of defining the values that `<IfDefine>` looks for: using the `-D` command-line option, or by using the `Define` directive.

Let us assume that your `proftpd.conf` contains a section like:

      <IfDefine USE_SQL>
        LoadModule mod_sql.c
        LoadModule mod_sql_mysql.c
      </IfDefine>

You can then make sure your `proftpd` loads those modules by starting it using `-D`, i.e.:

      /usr/local/sbin/proftpd -DUSE_SQL ...

Or, if later you decide that you don't want to use `-D` anymore, you can simply add a `Define` to the `proftpd.conf`:

      Define USE_SQL

      ...

      <IfDefine USE_SQL>
        LoadModule mod_sql.c
        LoadModule mod_sql_mysql.c
      </IfDefine>

If you are really ambitious, you can use devious combinations of `Define`, environment variables, `<IfModule>` and `<IfDefine>` sections in your configuration file to achieve some terse but powerful files.

Multiple Daemons on Same Host
What if you wanted to run multiple instances of `proftpd` on the same host? This is actually a prudent idea, for running one production `proftpd` while running a different, possibly newer `proftpd` for testing, side-by-side. Is it possible? Of course!

Different `proftpd` daemons can coexist on the same machine, but they cannot all use the same configuration file. There is a small list of directives that need to have different parameters for each `proftpd` instance, i.e.:

- `PidFile`
- `ScoreboardFile`
- `ServerLog`
- `SystemLog`

Failure to assign different `PidFile`s and `ScoreboardFile`s for each `proftpd` will cause the multiple instances to overwrite each other's files, and inevitably cause problems. Keeping separate log files (`ServerLog`, `SystemLog`, etc) for each daemon is simply a good idea.

What about the `PassivePorts` directive? Do the different `proftpd` instances each need their own range? No. When a passive data transfer is requested, `proftpd` will choose a random port from within a `PassivePorts` range, but not before then. If the port happens to be in use, `proftpd` will try another random port within the range, and so on, until the range is exhausted. Thus multiple `proftpd` instances should be able to share the same `PassivePorts` range without issue (assuming it is a decently wide range).

There is one final setting which can cause problems: `Port`. An actual incident can help illustrate the issue:

> I tried to setup another instance of proftpd. I copied my existing config file and changed the port information. My production FTP server runs on port 1979. In the test config file I specified 1980. I started the testing instance on the command line by executing the following command: ./proftpd -d 5 -n -c /etc/proftpd/proftpd.test.conf The testing instance started up without any problems. Unfortunately, when a client connected it gave the error message that the server is already bound to port 1979. This is very strange, as the client connected successfully to port 1980 in the first instance. How did the test server get the port information of the other production server?

In reality, the test server did not "get" the port information from the production instance. Instead, this admin was encountering the "L-1" issue. The FTP RFCs state that for active data transfers, the source port of the TCP connection (the source port of the TCP connection from the server to the client) must be L-1, where L is the control port number.

So if your control port (configured by the `Port` directive in `proftpd.conf`) is 1980, then the source port that `proftpd` has to use, for any active data transfers (e.g. for clients which use the `PORT` or `EPRT` commands), is 1979. If that port 1979 is already in use by another daemon (such as another `proftpd` instance as its control port), you have a collision, and will likely see the "Address already in use" error message.

In the case, the test server should work if the following was used:

      Port 1981

i.e. a port number that is the existing `proftpd`'s port number plus two (or more).

Using the above configuration tricks, the same `proftpd.conf` file could be used by both the production and the test daemons, using something like:

      # These directives need to differ depending on whether the test server
      # or the production server is reading this configuration.
      <IfDefine TEST>
        Port 2121
        PidFile /var/ftpd/proftpd.test.pid
        ScoreboardFile /var/ftpd/proftpd.test.scoreboard
      </IfDefine>

      <IfDefine !TEST>
        Port 2123
        PidFile /var/ftpd/proftpd.pid
        ScoreboardFile /var/ftpd/proftpd.scoreboard
      </IfDefine>

Then, starting `proftpd` with the proper command-line invocation e.g.:

      /usr/local/sbin/proftpd -DTEST ...

will use the test server configuration. Omitting the `-D` option on the command-line will cause `proftpd` to use the production configuration.

Reducing FTP Traffic Impact on Host System
Some sites run `proftpd` on a system which is used to host other services as well (e.g. Apache, a file server, etc). These sites might notice that when many FTP downloads or uploads are happening, the other services start to be impacted: slower/stuttering web pages or streaming video/music, for example.

Wouldn't it be nice if you could somehow tell `proftpd` to use a lower priority, i.e. in the manner of the `nice(1)` command, for such data transfers? Fortunately, you can do exactly this using the `TransferPriority` directive. For example, a good default configuration to use is:

      # Use a low process priority during all data transfers
      TransferPriority APPE,RETR,STOR,STOU low

One user reporting always having stuttering streaming video when FTP data transfers were happening. After using the `TransferPriority` directive, that user saw no more video stream stuttering while still approaching full LAN network speed for the FTP data transfer.

$Date: 2011/03/30 22:27:20 $