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Code Editor : netfilter-hacking-HOWTO-7.html

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 <TITLE>Linux netfilter Hacking HOWTO: The Test Suite</TITLE>
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<H2><A NAME="s7">7.</A> <A HREF="netfilter-hacking-HOWTO.html#toc7">The Test Suite</A></H2>

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:

<DL>
<DT>00netfilter/<DD>
General netfilter framework tests.
<DT>01iptables/<DD>
iptables tests.
<DT>02conntrack/<DD>
connection tracking tests.
<DT>03NAT/<DD>
NAT tests
<DT>04ipchains-compat/<DD>
ipchains/ipfwadm compatibility tests
</DL>

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.

<H2><A NAME="ss7.1">7.1</A> <A HREF="netfilter-hacking-HOWTO.html#toc7.1">Writing a Test</A>
</H2>

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.

<H2><A NAME="ss7.2">7.2</A> <A HREF="netfilter-hacking-HOWTO.html#toc7.2">Variables And Environment</A>
</H2>

You have two play interfaces: tap0 and tap1. Their interface
addresses are in variables <CODE>$TAP0</CODE> and <CODE>$TAP1</CODE>
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).

<H2><A NAME="ss7.3">7.3</A> <A HREF="netfilter-hacking-HOWTO.html#toc7.3">Useful Tools</A>
</H2>

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

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:

<DL>

<DT>FRAG=offset,length<DD>
Generate the packet, then turn it into a
fragment at the following offset and length.

<DT>MF<DD>
Set the `More Fragments' bit on the packet.

<DT>MAC=xx:xx:xx:xx:xx:xx<DD>
Set the source MAC address on the
packet.

<DT>TOS=tos<DD>
Set the TOS field on the packet (0 to 255).

</DL>

Next come the compulsory arguments:

<DL>
<DT>source ip<DD>
Source IP address of the packet.

<DT>dest ip<DD>
Destination IP address of the packet.

<DT>length<DD>
Total length of the packet, including headers.

<DT>protocol<DD>
Protocol number of the packet, eg 17 = UDP.

</DL>

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.

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:

<DL>
<DT>wait time<DD>
The maximum time in seconds to wait for a packet
from standard input.

<DT>iterations<DD>
The number of packets to receive.
</DL>

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:

<PRE>
# Set up job control, so we can use &amp; in shell scripts.
set -m

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

# 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
</PRE>

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.

<H3>local_ip</H3>

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).

<H2><A NAME="ss7.4">7.4</A> <A HREF="netfilter-hacking-HOWTO.html#toc7.4">Random Advice</A>
</H2>

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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