Packet Capture, part 3: Analysis Tools - Page 2
The program tcpdpriv is another program for removing sensitive information from tcpdump files. There are several major differences between tcpdpriv and sanitize. First, as a shell script, sanitize should run on almost any Unix system. As a compiled program, this is not true of tcpdpriv. On the other hand, tcpdpriv supports the direct capture of data as well as the analysis of existing files. The captured packets are written as a tcpdump file, which can be subsequently processed.
Also, tcpdpriv allows you some degree of control over how much of the original data is removed or scrambled. For example, it is possible to have an IP address scrambled but retain its class designation. If the -C4 option is chosen, an IP address such as 126.96.36.199 might be replaced with 188.8.131.52. Notice that address classes are preserved -- a class C address is replaced with a class C address.
There are a variety of command-line options that control how data is rewritten, several of which are mandatory. Many of the command-line options will look familiar to tcpdump users. The program does not allow output to be written to a terminal, so it must be written directly to a file or redirected. While a useful program, the number of required command-line options can be annoying. There is some concern that if the options are not selected properly, it may be possible to reconstruct the original data from the scrambled data. In practice, this should be a minor concern.
bsd1# tcpdpriv -P99 -C4 -M20 -r tracefile -w outfile
The -P99 option preserves (doesn't scramble) the port numbers, -C4 preserves the class identity of the IP addresses, and -M20 preserves multicast addresses. If you want the data output to your terminal, you can pipe the output to tcpdump:
bsd1# tcpdpriv -P99 -C4 -M20 -r tracefile -w- | tcpdump -r-
The last options look a little strange, but they will work.
Another useful tool is tcpflow, written by Jeremy Elson. This program allows you to capture individual TCP flows or sessions. If the traffic you are looking at includes, say, three different Telnet sessions, tcpflow will separate the traffic into three different files so you can examine each individually. The program can reconstruct data streams regardless of out-of-order packets or retransmissions but does not understand fragmentation.
tcpflow stores each flow in a separate file with names based on the source and destination addresses and ports. For example, SSH traffic (port 22) between 172.16.2.210 and 184.108.40.206 might have the filename 172.016.002.210.00022-205.153.063.030.01071, where 1071 is the ephemeral port created for the session.
Since tcpflow uses libpcap, the same packet capture library tcpdump uses, capture filters are constructed in exactly the same way and with the same syntax. It can be used in a number of ways. For example, you could see what cookies are being sent during an HTTP session. Or you might use it to see if SSH is really encrypting your data. Of course, you could also use it to capture passwords or read email, so be sure to set permissions correctly.