Linux netfilter Hacking HOWTO: The Test Suite

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## 7. The Test Suite

Within the CVS repository lives a test suite: the more the test suite covers, the greater confidence you can have that changes to the code hasn't quietly broken something. Trivial tests are at least as important as tricky tests: it's the trivial tests which simplify the complex tests (since you know the basics work fine before the complex test gets run).

The tests are simple: they are just shell scripts under the testsuite/ subdirectory which are supposed to succeed. The scripts are run in alphabetical order, so `01test' is run before `02test'. Currently there are 5 test directories:

00netfilter/

  General netfilter framework tests.

01iptables/

  iptables tests.

02conntrack/

  connection tracking tests.

03NAT/

  NAT tests

04ipchains-compat/

  ipchains/ipfwadm compatibility tests

Inside the testsuite/ directory is a script called `test.sh'. It configures two dummy interfaces (tap0 and tap1), turns forwarding on, and removes all netfilter modules. Then it runs through the directories above and runs each of their test.sh scripts until one fails. This script takes two optional arguments: `-v' meaning to print out each test as it proceeds, and an optional test name: if this is given, it will skip over all tests until this one is found.

## 7.1 Writing a Test

Create a new file in the appropriate directory: try to number your test so that it gets run at the right time. For example, in order to test ICMP reply tracking (02conntrack/02reply.sh), we need to first check that outgoing ICMPs are tracked properly (02conntrack/01simple.sh).

It's usually better to create many small files, each of which covers one area, because it helps to isolate problems immediately for people running the testsuite.

If something goes wrong in the test, simply do an `exit 1', which causes failure; if it's something you expect may fail, you should print a unique message. Your test should end with `exit 0' if everything goes OK. You should check the success of every command, either using `set -e' at the top of the script, or appending `|| exit 1' to the end of each command.

The helper functions `load_module' and `remove_module' can be used to load modules: you should never rely on autoloading in the testsuite unless that is what you are specifically testing.

## 7.2 Variables And Environment

You have two play interfaces: tap0 and tap1. Their interface addresses are in variables `$TAP0` and `$TAP1` respectively. They both have netmasks of 255.255.255.0; their networks are in $TAP0NET and $TAP1NET respectively.

There is an empty temporary file in $TMPFILE. It is deleted at the end of your test.

Your script will be run from the testsuite/ directory, wherever it is. Hence you should access tools (such as iptables) using path starting with `../userspace'.

Your script can print out more information if $VERBOSE is set (meaning that the user specified `-v' on the command line).

## 7.3 Useful Tools

There are several useful testsuite tools in the "tools" subdirectory: each one exits with a non-zero exit status if there is a problem.

### gen_ip

You can generate IP packets using `gen_ip', which outputs an IP packet to standard input. You can feed packets in the tap0 and tap1 by sending standard output to /dev/tap0 and /dev/tap1 (these are created upon first running the testsuite if they don't exist).

gen_ip is a simplistic program which is currently very fussy about its argument order. First are the general optional arguments:

FRAG=offset,length

  Generate the packet, then turn it into a fragment at the following offset and length.

MF

  Set the `More Fragments' bit on the packet.

MAC=xx:xx:xx:xx:xx:xx

  Set the source MAC address on the packet.

TOS=tos

  Set the TOS field on the packet (0 to 255).

Next come the compulsory arguments:

source ip

  Source IP address of the packet.

dest ip

  Destination IP address of the packet.

length

  Total length of the packet, including headers.

protocol

  Protocol number of the packet, eg 17 = UDP.

Then the arguments depend on the protocol: for UDP (17), they are the source and destination port numbers. For ICMP (1), they are the type and code of the ICMP message: if the type is 0 or 8 (ping-reply or ping), then two additional arguments (the ID and sequence fields) are required. For TCP, the source and destination ports, and flags ("SYN", "SYN/ACK", "ACK", "RST" or "FIN") are required. There are three optional arguments: "OPT=" followed by a comma-separated list of options, "SYN=" followed by a sequence number, and "ACK=" followed by a sequence number. Finally, the optional argument "DATA" indicates that the payload of the TCP packet is to be filled with the contents of standard input.

### rcv_ip

You can see IP packets using `rcv_ip', which prints out the command line as close as possible to the original value fed to gen_ip (fragments are the exception).

This is extremely useful for analyzing packets. It takes two compulsory arguments:

wait time

  The maximum time in seconds to wait for a packet from standard input.

iterations

  The number of packets to receive.

There is one optional argument, "DATA", which causes the payload of a TCP packet to be printed on standard output after the packet header.

The standard way to use `rcv_ip' in a shell script is as follows:

    # Set up job control, so we can use & in shell scripts.
    set -m

    # Wait two seconds for one packet from tap0
    ../tools/rcv_ip 2 1 < /dev/tap0 > $TMPFILE &

    # Make sure that rcv_ip has started running.
    sleep 1

    # Send a ping packet
    ../tools/gen_ip $TAP1NET.2 $TAP0NET.2 100 1 8 0 55 57 > /dev/tap1 || exit 1

    # Wait for rcv_ip,
    if wait %../tools/rcv_ip; then :
    else
        echo rcv_ip failed:
        cat $TMPFILE
        exit 1
    fi

### gen_err

This program takes a packet (as generated by gen_ip, for example) on standard input, and turns it into an ICMP error.

It takes three arguments: a source IP address, a type and a code. The destination IP address will be set to the source IP address of the packet fed in standard input.

### local_ip

This takes a packet from standard input and injects it into the system from a raw socket. This give the appearance of a locally-generated packet (as separate from feeding a packet in one of the ethertap devices, which looks like a remotely-generated packet).

## 7.4 Random Advice

All the tools assume they can do everything in one read or write: this is true for the ethertap devices, but might not be true if you're doing something tricky with pipes.

dd can be used to cut packets: dd has an obs (output block size) option which can be used to make it output the packet in a single write.

Test for success first: eg. testing that packets are successfully blocked. First test that packets pass through normally, then test that some packets are blocked. Otherwise an unrelated failure could be stopping the packets...

Try to write exact tests, not `throw random stuff and see what happens' tests. If an exact test goes wrong, it's a useful thing to know. If a random test goes wrong once, it doesn't help much.

If a test fails without a message, you can add `-x' to the top line of the script (ie. `#! /bin/sh -x') to see what commands it's running.

If a test fails randomly, check for random network traffic interfering (try downing all your external interfaces). Sitting on the same network as Andrew Tridgell, I tend to get plagued by Windows broadcasts, for example.

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