ProFTPD module mod_ban

## ProFTPD module `mod_ban`

The `mod_ban` module is designed to add dynamic "ban" lists to `proftpd`. A ban prevents the banned user, host, or class from logging in to the server; it does not prevent the banned user, host, or class from connecting to the server. `mod_ban` is not a firewall. The module also provides automatic bans that are triggered based on configurable criteria.

This module is contained in the `mod_ban.c` file for ProFTPD 1.2.x/1.3.x, and is not compiled by default. Installation instructions are discussed here. Detailed documentation on `mod_ban` usage can be found here.

The most current version of `mod_ban` is distributed with the proftpd source.

## Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

## Directives

- BanCache
- BanCacheOptions
- BanControlsACLs
- BanEngine
- BanLog
- BanMessage
- BanOnEvent
- BanOptions
- BanTable

## Control Actions

- `ban`
- `permit`

### BanCache

Syntax: BanCache driver
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_ban
Compatibility: 1.3.4rc2 and later

The `BanCache` directive configures a driver to use for caching ban entries. Depending on the compile-time options, the following drivers are supported:

- memcache
- redis

### BanCacheOptions

Syntax: BanCacheOptions opt1 ... optN
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_ban
Compatibility: 1.3.4rc2 and later

The `BanCacheOptions` directive is used to configure various optional caching behavior of `mod_ban`. Note: all of the configured `BanCacheOptions` parameters must appear on the same line in the configuration; only the first `BanCacheOptions` directive that appears in the configuration is used.

Example:

      BanCacheOptions UseJSON

The currently implemented options are:

- `MatchServer`

  This option tells `mod_ban` to ignore cached entries unless they match the IP address/port of the server handling the FTP session. There might be other servers adding their own entries to a common cache, for example; it is for these situations that the `MatchServer` option should be used, to avoid false positives.

- `UseJSON`

  This option tells `mod_ban` to store the ban rules in the cache as JSON-formatted data. This makes it possible for other tools to discover/display the cached ban rules.

  Note that this option first appeared in `proftpd-1.3.6rc2`.

### BanControlsACLs

Syntax: BanControlsACLs actions|"all" "allow"|"deny" "user"|"group" list
Default: None
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanControlsACLs` directive configures access lists of users or groups who are allowed (or denied) the ability to use the actions implemented by `mod_ban`. The default behavior is to deny everyone unless an ACL allowing access has been explicitly configured.

If "allow" is used, then list, a comma-delimited list of users or groups, can use the given actions; all others are denied. If "deny" is used, then the list of users or groups cannot use actions all others are allowed. Multiple `BanControlsACLs` directives may be used to configure ACLs for different control actions, and for both users and groups.

The actions provided by `mod_ban` are "ban" and "permit".

Examples:

      # Allow only user root to ban and permit users
      BanControlsACLs all allow user root

### BanEngine

Syntax: BanEngine on|off
Default: off
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanEngine` directive enables or disables the banning of sessions by `mod_ban`. If it is set to off this module does no banning. Use this directive to disable the module instead of commenting out all `mod_ban` directives.

### BanLog

Syntax: BanLog path|"none"
Default: None
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanLog` directive is used to specify a log file for `mod_ban` reporting and debugging. The path parameter must be the full path to the file to use for logging. Note that this path must not be to a world-writeable directory and, unless `AllowLogSymlinks` is explicitly set to on (generally a bad idea), the path must not be a symbolic link.

If path is "none", no logging will be done at all.

### BanMessage

Syntax: BanMessage mesg
Default: None
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanMessage` directive configures a message that will be sent to clients that have been banned. This message may contain the following variables:

- `%a`: client IP address
- `%c`: client class (if none, will be empty)
- `%u`: USER name (if none, will be empty)

Example:

      BanMessage "Host %a has been banned"

### BanOnEvent

Syntax: BanOnEvent event freq expires [mesg]
Default: None
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanOnEvent` directive is used to configure a "rule" that is triggered whenever the named event occurs. The currently supported events are:

Directive | Class/Host/User Ban
`AnonRejectPasswords` | Host ban
`BadProtocol` | Host ban
`ClientConnectRate` | Host ban
`EmptyPassword` | Host ban
`MaxClientsPerClass` | Class ban
`MaxClientsPerHost` | Host ban
`MaxClientsPerUser` | User ban
`MaxCommandRate` | Host ban
`MaxConnectionsPerHost` | Host ban
`MaxHostsPerUser` | User ban
`MaxLoginAttempts` | Host ban
`RootLogin` | Host ban
`TimeoutIdle` | Host ban
`TimeoutLogin` | Host ban
`TimeoutNoTransfer` | Host ban
`TLSHandshake` | Host ban
`UnhandledCommand` | Host ban

An event is generated whenever one of these limits is reached by a client.

The freq parameter should be formatted as:

      N/hh:mm:ss

where N is the number of occurrences, and `hh:mm:ss` specifies a number of hours, minutes, and seconds. This parameter says that if N occurrences of event happen within the given time interval, then a ban is automatically added. The IP address of the connecting client is banned when the following event rules are triggered: `AnonRejectPasswords`, `BadProtocol`, `MaxCommandRate`, `MaxClientsPerHost`, `MaxConnectionsPerHost`, `MaxLoginAttempts`, `TimeoutIdle`, `TimeoutNoTransfer`, and `UnhandledCommand`. The class of the connected client, if any, is banned when a rule for `MaxClientsPerClass` is triggered. Rules for `MaxClientsPerUser` and `MaxHostsPerUser` will cause the connected username to be banned.

The duration should be formatted as:

      hh:mm:ss

and specifies the numbers of hours, minutes, and seconds that the automatic ban generated a `BanOnEvent` rule lasts. Unlike bans added via the ban `ftpdctl` control action, automatic bans do not have infinite lifetimes.

An optional message, to be displayed to banned clients, can be configured. If no such optional message is configured using the `BanOnEvent` directive, then any `BanMessage` message will be displayed.

For example:

      # Configure a rule to automatically ban scripts looking for anonymous
      # servers to which they can upload
      BanOnEvent AnonRejectPasswords 1/01:00:00 99:99:99

      # Ban clients which connect too frequently.  This rule bans clients
      # which connect more than 5 times within one minute.  Include a special
      # message just for them.
      BanOnEvent ClientConnectRate 5/00:01:00 04:00:00 "Stop connecting frequently"

See also: `BanMessage`

### BanOptions

Syntax: BanOptions opt1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_ban
Compatibility: 1.3.7rc4 and later

The `BanOptions` directive is used to configure various optional behavior of `mod_ban`.

Example:

      BanOptions MatchAnyServer

The currently implemented options are:

- `MatchAnyServer`

  By default, any ban entries are valid only for the `<VirtualHost>` handling the banned clients. This is intentional; the assumption is that there will be different clients, different use cases for the different `<VirtualHost>` sections.

  However, some sites would like ban entries to be valid across `<VirtualHost>`, to match, or apply to any server. Use this option to enable this behavior.

### BanTable

Syntax: BanTable path
Default: None
Context: server config
Module: mod_ban
Compatibility: 1.2.10rc1 and later

The `BanTable` directive configures a path to a file that `mod_ban` uses for handling its ban data. The given path must be an absolute path. Note: this directive is required for `mod_ban` to function. It is recommended that this file not be on an NFS mounted partition.

Note that ban data is not kept across daemon stop/starts. That is, once `proftpd` is shutdown, all current ban data is lost.

## Control Actions

### `ban`

Syntax: ftpdctl ban class|host|info|user [-s address#port] name1 [name2 ...]
Purpose: Add a ban or display ban information

The `ban` action is used to add bans to the `mod_ban` lists. For example, to ban a user:

      ftpdctl ban user dave

This will create a ban rule for user 'dave' for all virtual servers in your `proftpd.conf`. If you want to create such a ban rule, but only for one specific `<VirtualHost>`, use the `-s` command-line option, e.g.:

      ftpdctl ban user -s 1.2.3.4#21 dave

This example will create the user 'dave' ban rule for the `<VirtualHost>` handling IP address 1.2.3.4, port 21. The `-s` command-line option applies to host and class bans as well.

To ban specific hosts, you can use either IP addresses or DNS names:

      ftpdctl ban host 1.2.3.4 5.6.7.8
      ftpdctl ban host gw.evil.com

Banning a class works the same way:

      ftpdctl ban class anonftp

The info parameter is used to view information on current bans. Example listing:

      # ftpdctl ban info
      ftpdctl: Banned Hosts:
      ftpdctl:   127.0.0.1

Or, if you wish to see more information, use the `-v` option:

      # ftpdctl ban info -v
      ftpdctl: Banned Hosts:
      ftpdctl:   127.0.0.1
      ftpdctl:     Reason: MaxLoginAttempts autoban at Wed May 19 14:59:25 2004
      ftpdctl:     Expires: Wed May 19 14:59:55 2004 (in 24 seconds)
      ftpdctl:     <VirtualHost> ServerName (1.2.3.4#21)

It is also possible to see the state of ban event rules, using the `-e` option:

      # ftpdctl ban info -e
      ftpdctl: No bans
      ftpdctl:
      ftpdctl: Ban Event Entries:
      ftpdctl:   Event: MaxLoginAttempts
      ftpdctl:   Source: 127.0.0.1
      ftpdctl:     Occurrences: 1/2
      ftpdctl:     Entry Expires: 589 seconds

This shows that no bans are currently in effect, and that a `BanOnEvent` has been configured for the `MaxLoginAttempts` event. That event has occurred once, and will need to happen one more time within 589 seconds in order for an automatic ban to be added.

See also: `permit`

### `permit`

Syntax: ftpdctl permit class|host|user [-s address#port] name1 [name2 ...]
Purpose: Permit banned clients to connect

The `permit` action is used to remove a ban for users, hosts, and classes:

      # ftpdctl permit user dave
      # ftpdctl permit user -s 1.2.3.4#21 dave
      # ftpdctl permit host 1.2.3.4 gw.evil.com
      # ftpdctl permit class anonftp

See also: `ban`

## Installation

The `mod_ban` module is distributed with ProFTPD. Simply follow the normal steps for using third-party modules in ProFTPD, making sure to include the `--enable-ctrls` configure option, which `mod_ban` requires:

      $ ./configure --enable-ctrls --with-modules=mod_ban

To build `mod_ban` as a DSO module:

      $ ./configure --enable-ctrls --enable-dso --with-shared=mod_ban

Then follow the usual steps:

      $ make
      $ make install

For those with an existing ProFTPD installation, you can use the `prxs` tool to add `mod_ban`, as a DSO module, to your existing server:

      $ prxs -c -i -d mod_ban.c

## Usage

The `mod_ban` module implements its bans by checking if a client is banned, either by host or by class, when that client connects to the server. Banned clients are immediately disconnected. Banned users are checked after the client has sent the `USER` and `PASS` commands; if that user has been banned, the client is immediately disconnected.

Here is an example `mod_ban` configuration, demonstrating how to configure an automatic ban for `MaxLoginAttempts`:

      MaxLoginAttempts 1

      <IfModule mod_ban.c>
        BanEngine on
        BanLog /var/log/proftpd/ban.log
        BanTable /var/data/proftpd/ban.tab

        # If the same client reaches the MaxLoginAttempts limit 2 times
        # within 10 minutes, automatically add a ban for that client that
        # will expire after one hour.
        BanOnEvent MaxLoginAttempts 2/00:10:00 01:00:00

        # Allow the FTP admin to manually add/remove bans
        BanControlsACLs all allow user ftpadm
      </IfModule>

By default, the `mod_ban` module allocate size for 512 bans. In practice, this has seemed to work well. However, if you need to allocate space for more bans, you will need to recompile proftpd, and use the `CFLAGS` environment variable like so:

        ./configure CFLAGS="-DBAN_LIST_MAXSZ=1024" ...

or whatever your necessary ban list size is.

Logging
The `mod_ban` module supports trace logging, via the module-specific log channels:

- ban

Thus for trace logging, to aid in debugging, you would use the following in your `proftpd.conf`:

      TraceLog /path/to/ftpd/trace.log
      Trace ban:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

Frequently Asked Questions
Question: Why does `mod_ban` not store ban data across daemon stop/starts?
Answer: The `mod_ban` module was not designed to add yet another ACL mechanism to `proftpd`. For persistent access control, there is the `<Limit LOGIN>` section, the `mod_wrap` module, the `/etc/ftpusers` file, and various other mechanisms. The purpose of `mod_ban` is to provide dynamic, short-lived bans.

Question: What about having `proftpd` delay sending responses to clients, say by 30 seconds or so?
Answer: This is a bad idea. It would allow malicious clients, who knew they were banned, to tie up your `proftpd` processes, since those processes would be taking up space, waiting before sending responses back to the client. This makes it possible for those clients to use the delaying as a Denial of Service attack, eventually tying up your available system resources with waiting `proftpd` processes.

Question: I have the following, taken from the example config above, configured for `mod_ban`, but it does not appear to be working. Why not?

      # If the same client reaches the MaxLoginAttempts limit 2 times
      # within 10 minutes, automatically add a ban for that client that
      # will expire after one hour.
      BanOnEvent MaxLoginAttempts 2/00:10:00 01:00:00

Answer: The most common reason is that the `MaxLoginAttempts` directive does not function the way that many assume it does.

The above `BanOnEvent` rule says that the same client (i.e. source IP address) which reaches the `MaxLoginAttempts` limit 2 times within 10 minutes will be banned. If you test by connecting to `proftpd`, trying a bad password once, disconnecting, then connecting again and trying the same bad password again, you will not be hitting the `MaxLoginAttempts` limit. Remember that the `MaxLoginAttempts` directive configures a limit to the number of bad login attempts for the same connection; it does not configure limit on the number of bad login attempts across multiple connections.

The default `MaxLoginAttempts` value for `proftpd` is 3. Which means that simply connecting, trying a bad login attempt once, then disconnecting, will not trigger the `MaxLoginAttempts` limit. This is why the example configuration given above explicitly configures the `MaxLoginAttempts` limit to be lower:

      MaxLoginAttempts 1

in order to make `mod_ban` behave the way that most administrators assume it will. If you have not explicitly configured `MaxLoginAttempts` lower, then this is probably why your `BanOnEvent` rule is not taking effect as you expect.

Question: It looks like the `BanTable` file I configured is untouched:

      -rw-r--r-- 1 ftp  ftp     0 2007-01-13 23:18 ban.table

Does this mean that something is wrong, or that `mod_ban` is not using that file?

Answer: A `BanTable` file is needed, just not in an obvious way. The `mod_ban` module stores its ban information in a SysV shared memory segment. The configured `BanTable` file is used a) to generate a unique name/ID for the shared memory segment to use (and to see if a segment for that name/ID already exists), and b) for locking, as when multiple sessions are trying to update the shared memory segment at the same time. But `mod_ban` does not write any data into that file itself, and hence the last-modified-time of the file, and the file size, does not change much.

Question: How can I configure a whitelist of IP addresses, to be excluded from the effects of `mod_ban`?
Answer: This can be done using a combination of Classes and the `mod_ifsession` module. For example:

      <Class whitelist>
        From ...
      </Class>

      <IfClass whitelist>
        # Turn the mod_ban module off for whitelisted clients
        BanEngine off
      </IfClass>

      <IfClass !whitelist>
        # Make sure the mod_ban module is on for clients that are not whitelisted
        BanEngine on
      </IfClass>

Note that it is important to have both `<IfClass>` sections in your configuration.

It is also important that for the above to work, when using shared/DSO modules, that `mod_ban` be loaded in your configuration before `mod_ifsession`:

      LoadModule mod_ban.c
      ...
      LoadModule mod_ifsession.c

Otherwise, when parsing the configuration, `mod_ban` might not set all of the proper internal state for implementing the whitelists.

Question: I would like to ban clients which try to login as root. How would I do this?
Answer: You would use the `BanOnEvent` directive, with an event name of "mod_auth.root-login", thus:

      # Ban clients which try to login as root.  This bans clients which
      # try root logins twice within 10 minutes, banning them for 6 hours.
      BanOnEvent mod_auth.root-login 2/00:10:00 06:00:00

© Copyright 2004-2020 TJ Saunders
All Rights Reserved