Opensvp is a security tool implementing "attacks" to be able to test the resistance of firewall to protocol level attack.
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Opensvp is a security tool implementing "attacks" to be able to test the resistance of firewall to protocol level attack. It implements classic attacks as well as some new kind of attacks against application layer gateway (called helper in the Netfilter world).

For example, opensvp is able under some conditions (see explanation below for details) to open a pin hole in a firewall protecting a ftp server: even if the filtering policy garantee that only the 21 port is open to the server, you can open 'any' port on the server by using opensvp.

Lets have a server running ftp, placed behind a firewall. If the user, as root, runs:

opensvp --attacker -t --helper ftp --port 23 -v -i eth0

Then he will have a temporary access on port 23 of the server independantly of the firewall rules.

The document "Secure use of iptables and connection tracking helpers" describe the protection method against this type of attack.

Implemented attacks

Spoofed attack on helpers

See the following chapter for a precise description of the implemented attack.

Being on a network directly connected to the firewall via the eth0 interface, the attacker can run the following command

opensvp --attacker -t --helper ftp --port 23 -v -i eth0 is the address of the FTP server and 23 is the port we want to open on the server.

It is then possible to connect to on port 23 after a successful attack.

Abusive usage of helpers

It is possible for a client to send a forged command message which is interpreted as possible dynamic connection opening by the firewalls.

It is possible to use a standard server to send the attack but with a custom server you will know the transformation made by the possible NAT gateway.

A typical session is the following. On the server which has IP address, you can run

$ opensvp --server --helper irc -v

On the client, you can then run

$ opensvp --client -t --helper irc --port 23 -v should be opened from outside

On the server, the following message is displayed

You should be able to connect to

Here is the public address of the client.

TTL attack on DPI solution

On the attacker, you need to start the opensvp and indicate what is the used Netfilter queue and what is the output interface

# opensvp -n -q 0 -i eth1

You then need to use iptables to userspace the trafic you want to hide to protocol recognition mechanism

iptables -I INPUT -p tcp --sport 443 -j NFQUEUE
iptables -I OUTPUT -p tcp --dport 443 -j NFQUEUE

When you're done, press CTRL+C to interrupt the attack process.

Description of the attack against helper


Some network protocols are using multiple connections for the exchange between a client and a server. The most known example is ftp where command goes through a connection on port 21 and where data exchange are done with two different mode (connection from port 20 or dynamic connection).

Some firewall implementation implement application layer gateway (ALG) to be able to detect this parallel connection and be able to autorize them dynamically. Other solutions are to use application relay (transparent proxy) or to open all the possible flow (read almost everything).

The ALG analyse the traffic and detect and parse the command sent between the peers to declare the parameters of the parallel connections. Once done they open temporary pin hole in the firewall to let the probable traffic goes through.

The idea of this attack is to forge this type of messages to open pin hole in the firewall but pin hole that should not have been open.

  • Attacker computer is on a network directly connected to the firewall.
  • Firewall is sensible to the attack (for example, Netfilter with rp_filter set to 0)
  • Attacker is able to sniff data packet (or by pcap sniffing or by running himself a data connection)
The cinematic is the following :
  1. Sniffer on the attacker network capture one packet from the protocol flow

    • it reverse the ethernet dst and src
    • it increase id in IP and seq for TCP
    • it set payload to the wanted command (with selected port)
  2. The forged packet is sent on the interface connected to the firewall

  3. Firewall transmit the packet back to the client and is now expecting a packet with caracteristic based on attacker input

Attacking IRC

This attack is a direct application of the described principle. Once data packet is received, the attacker send a forged DCC command.

Attacking FTP

In this attack, the client connection is open by the attacker. He connect to the ftp server behind a firewall and initiate a real connection. Once the session is setup, he launch the attack by sending a forged 227 command.

If IPv6 is used, the same attack is done with a forged 229 command.

Impact of the attack

Possible target

The main contraint about these attack is that the attacker has to be on a network directly connected to the firewall.

Thus, the main possibilities are:
  • Attack from a user LAN
  • Attack in a hosting farm

Both case can lead to severe information exposure by giving the attacker access to unprotected services.


This attack is known to work on IPv4 Netfilter firewall if rp_filter is set to 0 (this is hopefully not the default value).

There is currently no reverse path filtering implementation for IPv6, the firewall is thus not protected and the protection has to be setup in the firewall rules (see next chapter).

Some firewall software are known to be vulnerable:
  • fwbuilder: a specific policy has to be set up
  • shorewall: recent version fix the issue
  • edenwall: vulnerable

The attack works for both gateway and local firewall. On a local firewall, FORWARD filtering has to be activated and a ESTABLISHED ACCEPT rules has to be set up on this chain. This could be the case of system running virtual machine.

Defense against the attack


See the following document which is dedicated to the subject:

Other OS and devices

The basic requirement is to activate strict anti-spoofing and to control the loading of ALG is possible.