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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

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<LI><A NAME="tex2html175"
  HREF="node4.html#SECTION00041000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">1</SPAN> What is tcpprep?</A>
<LI><A NAME="tex2html176"
  HREF="node4.html#SECTION00042000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">2</SPAN> How does tcpprep work? </A>
<LI><A NAME="tex2html177"
  HREF="node4.html#SECTION00043000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">3</SPAN> Does tcpprep modify my libpcap file?</A>
<LI><A NAME="tex2html178"
  HREF="node4.html#SECTION00044000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">4</SPAN> Why use tcpprep?</A>
<LI><A NAME="tex2html179"
  HREF="node4.html#SECTION00045000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">5</SPAN> Can a cache file be used for multiple (different) libpcap files? </A>
<LI><A NAME="tex2html180"
  HREF="node4.html#SECTION00046000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">6</SPAN> Why would I want to use tcpreplay with two network cards? </A>
<LI><A NAME="tex2html181"
  HREF="node4.html#SECTION00047000000000000000"><SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">7</SPAN> How big are the cache files?</A>
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<H1><A NAME="SECTION00040000000000000000">
<SPAN CLASS="arabic">3</SPAN> Understanding tcpprep</A>
</H1>

<P>

<H2><A NAME="SECTION00041000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">1</SPAN> What is tcpprep?</A>
</H2>

<P>
Tcpreplay can send traffic out two network cards, however it requires
the calculations be done in real-time. These calculations can be expensive
and can significantly reduce the throughput of tcpreplay.

<P>
Tcpprep is a libpcap pre-processor for tcpreplay which enables using
two network cards to send traffic without the performance hit of doing
the calculations in real-time.

<P>

<H2><A NAME="SECTION00042000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">2</SPAN> How does tcpprep work? </A>
</H2>

<P>
Tcpprep reads in a libpcap (tcpdump) formatted capture file and does
some processing to generate a tcpreplay cache file. This cache file
tells tcpreplay which interface a given packet should be sent out
of. 

<P>

<H2><A NAME="SECTION00043000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">3</SPAN> Does tcpprep modify my libpcap file?</A>
</H2>

<P>
No. 

<P>

<H2><A NAME="SECTION00044000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">4</SPAN> Why use tcpprep?</A>
</H2>

<P>
There are three major reasons to use tcpprep:

<P>

<OL>
<LI>Tcpprep can split traffic based upon more methods and criteria then
tcpreplay.
</LI>
<LI>By pre-processing the pcap, tcpreplay has a higher theoretical maximum
throughput.
</LI>
<LI>By pre-processing the pcap, tcpreplay can be more accurate in timing
when replaying traffic at normal speed.
</LI>
</OL>

<P>

<H2><A NAME="SECTION00045000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">5</SPAN> Can a cache file be used for multiple (different) libpcap files? </A>
</H2>

<P>
Cache files have nothing linking them to a given libpcap file, so
there is nothing to stop you from doing this. However running tcpreplay
with a cache file from a different libpcap source file is likely to
cause a lot of problems and is not supported. 

<P>

<H2><A NAME="SECTION00046000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">6</SPAN> Why would I want to use tcpreplay with two network cards? </A>
</H2>

<P>
Tcpreplay traditionally is good for putting traffic on a given network,
often used to test a network intrusion detection system (NIDS). However,
there are cases where putting traffic onto a subnet in this manner
is not good enough- you have to be able to send traffic *through*
a device such as a router, firewall, or bridge.

<P>
In these cases, being able to use a single source file (libpcap) for
both ends of the connection solves this problem.

<P>

<H2><A NAME="SECTION00047000000000000000">
<SPAN CLASS="arabic">3</SPAN>.<SPAN CLASS="arabic">7</SPAN> How big are the cache files?</A>
</H2>

<P>
Very small. Actual size depends on the number of packets in the dump
file. Two bits of data is stored for each packet. On a test using
a 900MB dump file containing over 500,000 packets, the cache file
was only 150K. 

<P>

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<ADDRESS>
Aaron Turner
2005-06-28
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