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s Note The tcpdump command in the process of gathering these packet headers will place the interface
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it is capturing packets from into promiscuous mode unless you specifically specify otherwise.
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Most distributions come with the tcpdump package installed; if not, it is usually available on your distribution s installation media, or you can download and install it from the tcpdump home page at http://www.tcpdump.org/. If you run tcpdump on the command line without any options, as you can see in Listing 2-75, it will print all packet headers from all interfaces on your host to the screen until stopped with a SIGINT signal such as Control+C. Listing 2-75. Basic tcpdump kitten# tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes 00:18:39.980445 IP puppy.yourdomain.com.ssh > kitten.yourdomain.com.3717: P 900077725:900077841(116) ack 260615777 win 9648 1 packets captured 1 packets received by filter 0 packets dropped by kernel You can also display more information on the packet using the -v and -vv flags, which increase the verbosity of the capture. You can also limit the number of packet headers captured using the -c flag and specifying the number of packet headers you would like to capture. You can see both these flags in operation in Listing 2-76. Listing 2-76. Verbose tcpdump with Packet Count kitten# tcpdump -v -c 1 tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes 00:28:09.202191 IP (tos 0x10, ttl 64, id 41395, offset 0, flags [DF], proto 6, length: 92) puppy.yourdomain.com.ssh > kitten.yourdomain.com.3717: P 900095437:900095489(52) ack 260624565 win 9648 1 packets captured 1 packets received by filter 0 packets dropped by kernel In Listing 2-76 I have captured another packet showing a ssh connection from host puppy to host kitten but with the verbose flag enabled and additional information contained in the capture, including the TOS, TTL, and the packet s flags. Other flags are available to you on the tcpdump command line, and Table 2-12 describes some of the more useful flags.
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Table 2-12. tcpdump Command-Line Flags
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-i interface -N -p -q -r file -t -vv | -vvv -w file
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Listen on a particular interface. Use any to listen on all interfaces. Do not print domain information (puppy instead of puppy.yourdomain.com). Do not put the interface in promiscuous mode. Quiet mode that prints less protocol information. Read in packets from a file. Do not print a time stamp. More verbose and even more verbose. Prints increasing amounts of information. Write the packets to a file; use - for writing to standard out.
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With testing iptables using tcpdump, the objective is to monitor the incoming and outgoing traffic on your host to ensure traffic is correctly being allowed and denied using your rules. Obviously, most interfaces generate a huge volume of traffic, so tcpdump offers the capability to filter that traffic and display only those packets you want to monitor. The tcpdump command offers three key filtering selection criteria: types, directions, and protocols. For example, Table 2-13 shows the list of possible type-filtering criteria. Table 2-13. tcpdump Type Selectors
Selector
host net port
Description
Selects only traffic from a particular host Selects only traffic from a particular network Selects only traffic on a particular port
I discuss some of the other filtering criteria shortly, or you can refer to the tcpdump man page for more information. Listing 2-77 shows tcpdump selection at its most basic selecting only traffic from a particular host using the Type selector, host. Listing 2-77. Basic tcpdump Selection kitten# tcpdump -v -c 1 host puppy In Listing 2-77 the tcpdump command selects only packets that contain a reference to the host puppy. This will include both packets to and from the host puppy. In addition to single hosts, you can also capture only that traffic from a particular network using the net selector. Enter the following to capture traffic only from the 192.168.0.0/24 network: kitten# tcpdump net 192.168.0.0 mask 255.255.255.0 The tcpdump command also allows Boolean operators to be used with its selectors. In Listing 2-78 I am selecting all traffic between the host puppy and either the host kitten or the host duckling using the and / or Boolean operators.
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