ProFTPD module mod_rlimit

## ProFTPD module `mod_rlimit`

The `mod_rlimit` module handles setting and enforcement of resource limits such as CPU, memory, and file descriptor usage.

By default, the `mod_rlimit` module will impose one resource restriction on new session processes: the `RLIMIT_NPROC` (see `getrlimit(2)` limit, which controls the number of processes which can be forked from this process, is set to zero. There is absolutely no reason for a `proftpd` session process to need to create a new process via `fork(2)`.

This module is contained in the `mod_rlimit.c` file for ProFTPD 1.3.x, and is compiled by default. See the installation section for more details.

The most current version of `mod_rlimit` can be found in the ProFTPD source distribution:

      http://www.proftpd.org/

## Directives

- RLimitChroot
- RLimitCPU
- RLimitMemory
- RLimitOpenFiles

### RLimitChroot

Syntax: RLimitChroot on|off
Default: RLimitChroot on
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_rlimit
Compatibility: 1.3.5rc5

The `RLimitChroot` directive is used to enable/disable checks for modifications to "sensitive" directories when a session is chrooted. These checks are designed to mitigate and guard against attacks such as the "Roaring Beast" attack; see:

- https://auscert.org.au/15286
- https://auscert.org.au/15526

When a session is chrooted, e.g. via the `DefaultRoot` directive or by `<Anonymous>` login, the checks for the "sensitive" directories are automatically enabled. To disable these checks, use:

      RLimitChroot off

Note: We strongly recommend that you do not disable these checks.

The checks in question will specifically prevent any attempts to upload files into the `/etc` and `/lib` directories, or attempts to delete, create, rename, link, or otherwise try to change anything in these directories. All attempts to make modifications will be rejected with "Permission denied" errors. In addition, the following message will be logged (at debug level 2):

      WARNING: attempt to use sensitive path '/etc/file' within chroot '/home/user', rejecting

The `RLimitChroot` directive is not intended to prevent "Roaring Beast" style attacks entirely; the guarded `/etc` and `/lib` directories might be created via other means, outside of ProFTPD, which would also allow for the attack. The `RLimitChroot` directive is meant to mitigate (not prevent) the attacks by making sure it cannot be done using just ProFTPD.

### RLimitCPU

Syntax: RLimitCPU [scope] soft-limit|"max" [hard-limit|"max"]
Default: System defaults
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_rlimit
Compatibility: 1.3.5rc2

The `RLimitCPU` directive is used to set a limit on the CPU usage, expressed as a maximum number of seconds.

`RLimitCPU` takes from one to three parameters. The first parameter is an optional scope parameter, indicating the scope of the resource limit. The scope parameter may be one of:

- "daemon" (applies the resource limit only to the daemon process)
- "session" (which applies the limit only to session processes handling each client)
- "none" (which disables any possibly inherited limits)

If none of these keywords are used, the resource limit is assumed to apply to both daemon and session processes.

The next parameters indicate the actual resource limits, both the so-called "soft limit" and any "hard limit". These parameters may be a number (indicating the number of seconds), or the value "max" to indicate that maximum resource limit value allowed is to be used.

Example:

      # Limit a given session to 30 minutes of CPU time (which can take
      # considerably longer than 30 minutes of wall time, due to CPU scheduling)
      RLimitCPU 1800

### RLimitMemory

Syntax: RLimitMemory [scope] soft-limit|"max" [hard-limit|"max"]
Default: System defaults
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_rlimit
Compatibility: 1.3.5rc2

The `RLimitMemory` directive is used to set a limit on the memory usage, expressed as a maximum number of bytes.

`RLimitMemory` takes from one to three parameters. The first parameter is an optional scope parameter, indicating the scope of the resource limit. The scope parameter may be one of:

- "daemon" (applies the resource limit only to the daemon process)
- "session" (which applies the limit only to session processes handling each client)
- "none" (which disables any possibly inherited limits)

If none of these keywords are used, the resource limit is assumed to apply to both daemon and session processes.

The next parameters indicate the actual resource limits, both the so-called "soft limit" and any "hard limit". These parameters may be a number (indicating the number of bytes), or the value "max" to indicate that maximum resource limit value allowed is to be used.

Example:

      # Limit a given session to 128MB minutes of memory
      RLimitMemory 128MB

Note: If you use `RLimitMemory`, e.g.:

      <IfModule mod_rlimit.c>
        RLimitMemory session 64M
      </IfModule>

and you use `mod_tls` for FTPS transfers:

      <IfModule mod_tls.c>
        ...
      </IfModule>

then your transfers are likely to fail. Why? Because OpenSSL will need to allocate memory for the TLS support, in addition to the memory that ProFTPD already allocates for data transfers. Depending on the specific ciphersuites negotiated, and the specific memory limit configured, you are very likely to hit the `RLimitMemory` limit. In short, your `RLimitMemory` might be too low, and not allowing ProFTPD and OpenSSL enough memory for the transfer.

### RLimitOpenFiles

Syntax: RLimitOpenFiles [scope] soft-limit|"max" [hard-limit|"max"]
Default: System defaults
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_rlimit
Compatibility: 1.3.5rc2

The `RLimitOpenFiles` directive is used to set a limit on the file descriptors, expressed as counts.

`RLimitOpenFiles` takes from one to three parameters. The first parameter is an optional scope parameter, indicating the scope of the resource limit. The scope parameter may be one of:

- "daemon" (applies the resource limit only to the daemon process)
- "session" (which applies the limit only to session processes handling each client)
- "none" (which disables any possibly inherited limits)

If none of these keywords are used, the resource limit is assumed to apply to both daemon and session processes.

The next parameters indicate the actual resource limits, both the so-called "soft limit" and any "hard limit". These parameters may be a number, or the value "max" to indicate that maximum resource limit value allowed is to be used.

Example:

      # Limit a given session to 12 open file descriptors
      RLimitOpenFiles session 12

## Installation

The `mod_rlimit` module is compiled into `proftpd` by default.

FAQ Frequently Asked Questions

Question: Why can't I create directories named "lib" or "etc" in the root directory? For example, my FTP client fails like so: `Command: MKD lib Response: 550 lib: Permission denied Command: MKD /lib Response: 550 /lib: Permission denied` Although I don't have anything in my `proftpd.conf` that would block these commands, and the filesystem permissions are OK. Why does this happen?
Answer: For the answer to this, see the description for the `RLimitChroot` directive.

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