ProFTPD module mod_proxy_protocol

## ProFTPD module `mod_proxy_protocol`

The purpose of the `mod_proxy_protocol` module is to handle protocols which are used by proxies, e.g. `haproxy`, for conveying information about the real origin/client to the backend server. Protocols like HTTP often have their own mechanism for doing so, via headers such as "X-Forwarded-For". Unfortunately, FTP does not have such a mechanism, nor does SSH.

However, there are protocols which an provide this information without impacting FTP. HAproxy's `PROXY` protocol is one such mechanism. The `mod_proxy_protocol` module uses these mechanisms to change the information about the "remote" client so that it is the real client, not the proxy, whose IP address/port are logged and used for e.g. network ACLs.

This module is contained in the `mod_proxy_protocol.c` file for ProFTPD 1.3.x, and is not compiled by default. Installation instructions are discussed here; detailed notes on best practices for using this module are here.

The most current version of `mod_proxy_protocol` can be found at:

      https://github.com/Castaglia/proftpd-mod_proxy_protocol.git

## Author

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

## Directives

- ProxyProtocolEngine
- ProxyProtocolIgnore
- ProxyProtocolOptions
- ProxyProtocolTimeout
- ProxyProtocolVersion

### ProxyProtocolEngine

Syntax: ProxyProtocolEngine on|off
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_proxy_protocol
Compatibility: 1.3.5rc4 and later

The `ProxyProtocolEngine` directive enables the expectation and handling of protocols which provide information on proxied connections; support for these protocols is provided by `mod_proxy_protocol`.

### ProxyProtocolIgnore

Syntax: ProxyProtocolIgnore on|off
Default: off
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_proxy_protocol
Compatibility: 1.3.5rc4 and later

The `ProxyProtocolIgnore` directive determines whether the `mod_proxy_protocol` module will use the information it reads, in the PROXY protocol data that may be sent by a client.

This is useful for cases where clients may be sending/using the PROXY protocol, but you may not want `mod_proxy_protocol` to use the data from some clients. For example:

      <Class untrusted-senders>
        From 1.2.3.4
        From 5.6.7.8
      </Class>

      <IfModule mod_proxy_protocol.c>
        # We always to read PROXY protocol headers from clients...
        ProxyProtocolEngine on

        # ...but we don't always trust the senders
        <IfClass untrusted-senders>
          ProxyProtocolIgnore on
        </IfClass>
      </IfModule>

### ProxyProtocolOptions

Syntax: ProxyProtocolOptions opt1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_proxy_protocol
Compatibility: 1.3.5a and later

The `ProxyProtocolOptions` directive is used to configure various optional behavior of `mod_proxy_protocol`. For example:

      ProxyProtocolOptions UseProxiedServerAddress

The currently implemented options are:

- `UseProxiedServerAddress`

  The `PROXY` protocol sends both the original source (client) IP address/port, and the original destination (server) address/port. The `mod_proxy_protocol` module only uses the client IP address/port in the `PROXY` protocol message by default. Use this option to tell `mod_proxy_protocol` to use the provided server address/port as well, by looking for any matching `<VirtualHost>` configurations, and using them if found. Note that if no matching `<VirtualHost>` configurations are found, the proxied server address/port information will be ignored.

### ProxyProtocolTimeout

Syntax: ProxyProtocolTimeout seconds
Default: 3sec
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_proxy_protocol
Compatibility: 1.3.5rc4 and later

The `ProxyProtocolTimeout` directive is used to configure the amount of time, in seconds, that `mod_proxy_protocol` will wait to receive the full expected proxy information. If the full information is not received within the given number of seconds, the connection to the client is closed.

### ProxyProtocolVersion

Syntax: ProxyProtocolVersion protocolVersion
Default: haproxyV1
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_proxy_protocol
Compatibility: 1.3.5rc4 and later

The `ProxyProtocolVersion` directive is used to configure the protocol that `mod_proxy_protocol` expects to handle. The currently supported values are:

- `haproxyV1`
- `haproxyV2`

## Usage

Example Configuration

      <IfModule mod_proxy_protocol.c>
        ProxyProtocolEngine on
        ProxyProtocolTimeout 3sec

        # Necessary to allow data transfers
        AllowForeignAddress on
      </IfModule>

Module Load Order
In order for `mod_proxy_protocol` to work its magic, it must the first module in line to handle the bytes coming in from the client. If some other module (such as `mod_sftp` or `mod_tls`) tries to handle the incoming bytes first, Bad Things will happen, since those modules will expect different protocols than the `PROXY` protocol.

For `mod_proxy_protocol` to be the first module called, it must the last module loaded. To do this as a static module, you would use something like this when building proftpd:

      # ./configure --with-modules=...:mod_proxy_protocol

ensuring that `mod_proxy_protocol` is the last module in your `--with-modules` list.

As a shared module, configuring `mod_proxy_protocol` to be the last module loaded is much easier. Your configuration will have a list of `LoadModule` directives; the last of which would be:

      LoadModule mod_proxy_protocol.c

Note that using `mod_proxy_protocol` as a shared module is required in cases where you want to use both `mod_proxy_protocol` and `mod_ifsession`. For example, perhaps you want to use `mod_ifsession` to change the behavior of some module, e.g. `mod_ban`, based on the IP address of the original client. This means that `mod_proxy_protocol` would need to hande the connection first, so that it can decode the `PROXY` protocol and set the correct client IP address. However, the ProFTPD build system is hardcoded to ensure that the `mod_ifsession` will always be first -- if using static modules. By using shared modules, you can enforce the proper ordering using the `LoadModule` directive, like so:

      <IfModule mod_dso.c>
        ...
        LoadModule mod_ifsession.c
        LoadModule mod_proxy_protocol.c
      </IfModule>

The last module loaded will be the first module called.

Trusting Senders of Proxy Data
Use of these proxy protocols means changes in audit trails and/or client access permissions (e.g. different `mod_wrap2` and/or `mod_geoip` rules will apply). Unscrupulous senders may try to actively lie to your server about the original client using these protocols. Thus you must trust the upstream machines before enabling the `mod_proxy_protocol` module.

Put another way: do not use the `mod_proxy_protocol` module if your server handles connections from the open Internet. Doing so means that any machine can use the proxy protocol to hide their activities, or make it look like the connection is coming from someone else. Only accept proxy information from trusted sources.

If you need to selectively support the `PROXY` protocol from some IPs, but not others, then you can use classes, and the `mod_ifsession` module, like this:

      <Class expect-proxy-protocol>
        From 192.168.18.33/24
        From 192.168.33.2
        From 10.0.0.0/8
      </Class>

      <IfClass expect-proxy-protocol>
        <IfModule mod_proxy_protocol.c>
          # Enable PROXY protocol support for clients in this class
          ProxyProtocolEngine on

          # Necessary to allow data transfers from this class
          AllowForeignAddress expect-proxy-protocol
        </IfModule>
      </IfClass>

Note that per-user or per-group enabling of the `PROXY` protocol cannot be done; we only know the user/group of the connection after the `PROXY` protocol has been used.

Why `AllowForeignAddress` Is Needed
One of the consequences of allowing `mod_proxy_protocol` to change the remote IP address is that security checks performed on data transfers will cause problems. For active data transfers (i.e. for clients which send the `PORT` or `EPRT` commands), `proftpd` requires that the IP address sent in the command matches the IP address of the client which sends the command. Otherwise, a message like the following is logged:

      Refused PORT 127,0,0,1,218,225 (address mismatch)

and the command is rejected.

Similarly for passive data transfers (i.e. for clients which send the `PASV` or `EPSV` commands), `proftpd` requires that the remote IP address of the client which connects to the data transfer address must match the remote IP address of the client on the control connection. If the addresses do no match, then the following is logged:

      SECURITY VIOLATION: Passive connection from 127.0.0.1 rejected.

and the control connection is closed.

These security measures are done to prevent abuses of FTP data transfers such as the FTP bounce attack. However, the very fact that `mod_proxy_protocol` changes the remote IP address means that to allow data transfers when using this module, you need to use:

      AllowForeignAddress on

in the same virtual host section in which the `ProxyProtocolEngine` directive appears.

A more secure configuration for `AllowForeignAddress` is possible when you use the `<Class>` and `<IfClass>` configurations, where you can limit the scope of `AllowForeignAddress` to just that class, using:

      # Allow foreign addresses, but only from the "expect-proxy-protocol" class
      AllowForeignAddress expect-proxy-protocol

rather than allowing any IP address to connect to the data sockets.

Session Notes
The `mod_proxy_protocol` module may provide the following session notes for e.g. PROXY protocol V2 Type-Length-Value (TLVs) as well:

- `mod_proxy_protocol.alpn`
- `mod_proxy_protocol.authority`
- `mod_proxy_protocol.tls.cipher`
- `mod_proxy_protocol.tls.version`
- `mod_proxy_protocol.unique-id`

These note variables are intended for informational purposes, e.g. `ExtendedLog` or `SQLLog` directives via the `%{note:...}` syntax.

## Installation

To install `mod_proxy_protocol`, copy the `mod_proxy_protocol.c` file into:

      proftpd-dir/contrib/

after unpacking the latest proftpd-1.3.x source code. For including `mod_proxy_protocol` as a staticly linked module:

      $ ./configure --with-modules=...:mod_proxy_protocol

To build `mod_proxy_protocol` as a DSO module:

      $ ./configure --enable-dso --with-shared=...:mod_proxy_protocol

Then follow the usual steps:

      $ make
      $ make install

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

      $ prxs -c -i -d mod_proxy_protocol.c

Frequently Asked Questions

Question: We are using HAProxy in front of ProFTPD, and we see `PROXY` protocol headers appended to our upload files. What is wrong?
Answer: Usually this happens because of the specific HAProxy configuration used -- and can be fixed by adjusting that HAProxy configuration.

Let's assume your HAProxy configuration looks something like this:

      listen ftp
        bind 0.0.0.0:21,:60000-60250
        mode tcp
        balance source
        server services-a 192.168.1.71 send-proxy-v2 check port 21
        server services-b 192.168.1.72 send-proxy-v2 check port 21

This causes HAProxy to use the PROXY protocol (via `send-proxy-v2`) for control and data connections, which is probably not what you intended.

The solution is to use separate HAProxy configurations for the FTP control and FTP data addresses/ports, and to not use `send-proxy-v2` (or `send-proxy`) for the FTP data ports:

      listen ftp
        bind 0.0.0.0:21
        mode tcp
        balance source
        server services-a 192.168.1.71 send-proxy-v2 check port 21
        server services-b 192.168.1.72 send-proxy-v2 check port 21

      listen ftp-data
        bind :60000-60250
        mode tcp
        balance source
        server services-a 192.168.1.71 check port 21
        server services-b 192.168.1.72 check port 21

© Copyright 2013-2021 TJ Saunders
All Rights Reserved