ts3init_linux_netfilter_module

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ts3init_linux_netfilter_module

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A Linux netfilter module to aid in (d)dos protection

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ts3init Linux netfilter module
==============================
A Linux netfilter module to help filter ts3init floods on TeamSpeak 3 servers

TeamSpeak 3 servers are a popular (D)Dos target. They are harder to protect than
other servers because the TeamSpeak 3 protocol is based on UDP. A popular
method, which is hard to mitigate by hosters is called *init floods*. This is
where the attacker sends many connection request to the server, usually spoofing
the source address, to make it harder to block them. The TeamSpeak 3 protocol
has an anti spoof check as the first stage of the connection, but the server
program can not keep up with the flood of packets.

This set of plugins is designed to let the Linux kernel, or rather netfilter,
handle the anti spoofing stages of a new TeamSpeak 3 connection. This could be
done on a different machine than the one the TeamSpeak 3 server is executing
on.

Current version
===============
The current release is version 0.1. This version is a beta and has by no means
been tested in a production environment. At this stage this software is merely
a proof of concept. We invite the TeamSpeak community and of course the Linux
community to test this tool and to contribute improvements and new ideas.

prerequisites
=============
In order to compile and install this software you need the following things:
* Linux kernel headers (version 2.6.35 or greater)
* Iptables development headers
* GCC (version 4.6.0 or greater)

Usually these can be installed using your package manager (yum/apt-get/etc).
Usually these packages resemble the names "Linux-header-<..>", "iptables-dev",
"GCC"

How to install
==============

```
make
sudo make install
sudo depmod -a
sudo modprobe xt_ts3init
```

Protocol background and module description
==========================================
When a TeamSpeak 3 client attempts to connect to a TeamSpeak 3 server, it sends
out a *get cookie* packet. The server then replies with a *set cookie* packet.
This packet has some secret information about the connection details. The client
then response with a *get puzzle* packet. This packet includes the cookie that
it got previously. The server now validates this cookie and if it is valid, the 
server continues with the rest of the connection packets.

This software packages comes with two netfilter match extensions, and three
netfilter target extensions which we will discuss next. Combined these
extensions and some other netfilter modules, can handle the initial connection
phase for a TeamSpeak 3 server. This prevents any packet, with a spoofed source
ip address, to reach the TeamSpeak 3 server.

Match extensions
================

ts3init_get_cookie
------------------
Matches if the packet in question is a valid TeamSpeak 3 *get cookie* packet
from the client.
There are additional parameters that can be set:
```
$ iptables -m ts3init_get_cookie -h
<..>
ts3init_get_cookie match options:
  --min-client n                The client needs to be at least version n.
  --check-time sec              Check packet send time request.
                                May be off by sec seconds.
```
* `min-client` checks that the client version in the packet is at least the
  version specified. 
* `check-time` compares the unix-timestamp in the client packet to the unix-time
  on the server. If they differ too much, the packet is not matched.

ts3init_get_puzzle
------------------
Matches if the packet in question is a valid TeamSpeak 3 *get puzzle* packet
from the client.
There are additional parameters that can be set:
```
$ iptables -m ts3init_get_puzzle -h
<..>
ts3init_get_puzzle match options:
  --min-client n               The client needs to be at least version n.
  --check-cookie               Check that the cookie was generated by same seed.
  --random-seed <seed>         Seed is a 60 byte hex number.
                               A source could be /dev/random.
  --random-seed-file <file>    Read the seed from a file.
```
* `min-client` checks that the client version in the packet is at least the
  version specified. 
* `check-cookie` matches if it matches the cookie that was generated in the
  netfilter target extension `TS3INIT_SET_COOKIE`. To match the seed needs to be
  exactly the same. It is possible to check cookies that were generated on a
  different machine, provided that those machines have the same date and time,
  and the same seed specified. In other words: The cookie is created in a
  deterministic way, depending only on the current time and the seed. If
  `check-cookie` is specified, either `random-seed` or `random-seed-file` needs
  to be specified too.

ts3init
-------
Matches a ts3init packet, by checking if the packet starts with the *TS3INIT1*.
Additional header checks for client and server packets can be specified:
```
$ iptables -m ts3init -h
<..>
ts3init match options:
  --client                     Match ts3init client packets.
  --server                     Match ts3init server packets.
  --command <command>          Match packets with the specified command.
```
* `client` checks that the packet has a valid ts3init client header
* `server` checks that the packet has a valid ts3init server header
* `command` checks that the packet has the specified command in its header.
  Requires either --client or --server.
  
Target extensions
=================

TS3INIT_GET_COOKIE
------------------
Rewrites the packet into a *get_cookie* packet and then accepts it.
It is assumed that the packet is a ts3init packet of any kind, any other packet
may or may not result in a valid *get_cookie* packet. Used for pre 3.1 clients,
as an alternative to `TS3INIT_RESET`. It takes no parameters.

TS3INIT_SET_COOKIE
------------------
Generates a *set_cookie* packet to the matched *get_cookie* packet. The orginal
*get_cookie* packet is dropped. It is assumed that the orginal packet is a 
*get_cookie* packet, no attempt is made to check if that is true. It should
always be used with `ts3init_get_puzzle` match.

```
$ iptables -j TS3INIT_SET_COOKIE -h
<..>
TS3INIT_SET_COOKIE target options:
  --zero-random-sequence       Always return 0 as random sequence.
  --random-seed <seed>         Seed is a 60 byte hex number in.
                               A source could be /dev/random.
  --random-seed-file <file>    Read the seed from a file.
```

* `zero-random-sequence` forces the returned *random-sequence* to be always
  zero. This allows the target to not look at the payload of the packet.
* `random-seed` is used to generate the cookie returned in the *set-cookie*
  packet. *seed* must be a 120 character long hexstring.
* `random-seed-file` read the `random-seed` from a file. The file must contain
  a 120 character long hexstring, without any newlines.

TS3INIT_RESET
-------------
Drops the packet and sends a *reset* packet back to the sender. The
sender should always be the TeamSpeak 3 client. Starting with the TeamSpeak 3.1
client, the client will react to the reset packet by resending the *get cookie*
to the server. Older clients do not handle this packet. It takes no parameters.

How to use
==========
The idea for which these extensions were developed was to create a few iptables
rules that do the anti spoofing phase for a TeamSpeak server. This can be done
as follows:
* [recommended] Disable connection tracking with the help of raw table
* Create ipset: ts3_authorized with a timeout of 30 seconds
* If a source ip address is in the ipset ts3_authorized, renew the entry in the
  set to update the timeout then accept the packet.
* Use `ts3init_get_cookie` matches to get connection requests and reply with
  `TS3INIT_SET_COOKIE`.
* Use `ts3init_get_puzzle` matches to get the cookie replies from the client.
  If they match, add the source ip address to the ts3_authorizing ipset and then
  reply with `TS3INIT_RESET`
* Drop all other packets

It is possible to make a more detailed firewall.

Example iptables setup
======================
There are two examples included: _simple_ and _complex_. Both use ipset to
create a set of whitelisted ip addresses that are allowed to send packets to the
TeamSpeak3 server. The simple example does the bare minimum to do the ip
addresss authentication on the firewall, and to protect the file transfer (tcp)
port from traffic from unverified ip addresses.

The complex example is a bit more advanced. It keeps three ipsets. Authorizing
authorized and authorized_ft.

The autorizing set stores ip addresses and ports for connections that have
verified ip addresses, but did not yet complete the puzzle phase on the ts3
server.

The authorized set keeps ip addresses and ports that have completed the puzzle
phase on the server and are assumed to be authorized. It is technically not
true that the server has accepted this connection. It could still reject it
because the password is wrong, or the server is full, or other reasons. But for
this example, it is good enough.

The authorized_ft set keeps a list of authorized ip addresses (not ports). Only
these ip addresses are allowed to send traffic to the file transfer
port. Since there is no way to know in advance what source port the TeamSpeak 3
client is going to use for file transfer, this is the best we can do.

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