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Testing TCP/IP Network Stacks

When working on the network stack there is a need to test and verify the changes made.
While problems may be discovered by chance, it is hard to reproduce such situations.
The following sections show some methods to stress the target or generate some specific
traffic.

In the examples the target has the IP address 192.168.2.135

SYN Flood Attack

Flood the target with SYN packets to exhaust its resources.
It's a good way to test the network driver's buffer management.

sudo hping3 --flood -S -p 80 192.168.2.135

Building Packets with Scapy

A wonderful network testing tool is the Scapy lib.
It enables you to build pretty much any packet constellation you need for testing.

You have to add an iptables rule to prevent outgoing RST packets from
the OS's networking stack which does nothing know about our test connection.

...

sudo iptables -D OUTPUT -p tcp --tcp-flags RST RST -d 192.168.2.135 -j DROP

Testing Re-transmission behavior

When sending a 3-way handshake only, the target should time out
and reset the connection.

...

#!/usr/bin/env python

import logging
logging.getLogger("scapy.runtime").setLevel(logging.ERROR)
from scapy.all import *

get = 'GET / HTTP/1.1\r\n\r\n'

ip = IP(dst="192.168.2.135")
port = RandNum(1024, 65535)

# Create SYN packet
SYN = ip/TCP(sport=port, dport=80, flags="S", seq=42)

# Send SYN and receive SYN,ACK
SYNACK = sr1(SYN)

# Create ACK with GET request
ACK = ip/TCP(sport=SYNACK.dport, dport=80, flags="A", seq=SYNACK.ack, ack=SYNACK.seq + 1)

# SEND our ACK
send(ACK)

reply, err = sr(ip/TCP(sport=SYNACK.dport, dport=80, flags="A", seq=SYNACK.ack, ack=SYNACK.seq + 1) / get)

Simulating Packet Loss

With simulating packet loss one can test the retransmission behaviour re-transmission behavior of the
target stack.

To start packet loss:

...

# for the incoming packets:
sudo iptables -D INPUT -m statistic --mode random --probability 0.1 -j DROP

# and for the outgoing packets
sudo iptables -D OUTPUT -m statistic --mode random --probability 0.1 -j DROP

Fuzz-Testing

For fuzz testing network applications the excellent
SPIKE tool
tool  can be used.
To make it compile under Ubuntu 14.04 LTS you have to add
-fno-stack-protector to CFLAGS in src/Makefile.in.

SPIKE complained about missing SSL libs. I simply linked the
existing to the needed filenames:

...

Maybe those lib versions aren't the expected by SPIKE but if you don't
use SSL it works fine.

SPIKE provides a proxy server to record requests to your web
application. Based on these requests a application specific fuzz test
can be generated.

~/SPIKE/src$ mkdir requests && cd requests
# Record requests to the target at 192.168.2.135 on port 80
~/SPIKE/src/requests$ ../webmitm -t 192.168.2.135 -p 80

Now use your web applicaiton through localhost to record some
requests. Then you can generate your application specific fuzz test
from the recorded requests.

...