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Independent Verification of IPsec Functionality in FreeBSD
<email>honig@sprynet.com</email>
<personname><firstname>David</firstname><surname>Honig</surname></personname><affiliation> <_:address-1/> </affiliation>
1999-05-03
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$FreeBSD: head/en_US.ISO8859-1/articles/ipsec-must/article.xml 52227 2018-09-06 01:30:47Z ebrandi $
You installed IPsec and it seems to be working. How do you know? I describe a method for experimentally verifying that IPsec is working.
The Problem
First, lets assume you have <link linkend="ipsec-install"> installed <emphasis>IPsec</emphasis></link>. How do you know it is <link linkend="caveat">working</link>? Sure, your connection will not work if it is misconfigured, and it will work when you finally get it right. <citerefentry><refentrytitle>netstat</refentrytitle><manvolnum>1</manvolnum></citerefentry> will list it. But can you independently confirm it?
The Solution
First, some crypto-relevant info theory:
Encrypted data is uniformly distributed, i.e., has maximal entropy per symbol;
Raw, uncompressed data is typically redundant, i.e., has sub-maximal entropy.
Suppose you could measure the entropy of the data to- and from- your network interface. Then you could see the difference between unencrypted data and encrypted data. This would be true even if some of the data in <quote>encrypted mode</quote> was not encrypted---as the outermost IP header must be if the packet is to be routable.
MUST
Ueli Maurer's <quote>Universal Statistical Test for Random Bit Generators</quote>(<link xlink:href="https://web.archive.org/web/20011115002319/http://www.geocities.com/SiliconValley/Code/4704/universal.pdf"> <acronym>MUST</acronym></link>) quickly measures the entropy of a sample. It uses a compression-like algorithm. <link linkend="code">The code is given below</link> for a variant which measures successive (~quarter megabyte) chunks of a file.
Tcpdump
We also need a way to capture the raw network data. A program called <citerefentry><refentrytitle>tcpdump</refentrytitle><manvolnum>1</manvolnum></citerefentry> lets you do this, if you have enabled the <emphasis>Berkeley Packet Filter</emphasis> interface in your <link linkend="kernel">kernel's config file</link>.
The command:
<userinput>tcpdump -c 4000 -s 10000 -w <replaceable>dumpfile.bin</replaceable></userinput>
will capture 4000 raw packets to <replaceable>dumpfile.bin</replaceable>. Up to 10,000 bytes per packet will be captured in this example.
The Experiment

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