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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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