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Following are examples of commands used to run the tcpdump utility:
Selecting an Interface or VLAN
The tcpdump utility is able to sniff for packets on only one interface or VLAN. By default, it will select the lowest numbered interface.
To select an interface, use the -i flag as follows:
tcpdump -i
For example:
tcpdump -i exp1
tcpdump -i 1.10
tcpdump -i internal
Disabling name resolution
By default, tcpdump will attempt to look up IP addresses and use names, rather than numbers, in the output. BIG-IP must wait for a response from the DNS server, so the lookups can be time consuming and the output may be confusing.
To disable name resolution, use the -n flag as in the following examples:
tcpdump -n
tcpdump -ni internal
Saving tcpdump output to a file
You can save the tcpdump data to one of the following file formats:
• A binary file that contains all the information collected by the tcpdump and is readable by the tcpdump utility as well as many other traffic analysis packages.
• A text file that contains a subset of the full tcpdump data, but is readable only as plain text.
Binary file
To save the tcpdump output to a binary file, type the following command:
tcpdump -w
tcpdump -w dump1.bin
Note: The tcpdump utility will not print data to the screen while it is capturing to a file. To stop the capture, press CTRL-C.
Text file
To save the tcpdump output to a text file, type the following command:
tcpdump >
For example:
tcpdump >dump1.txt
Reading tcpdump binary file output
To read data from a binary tcpdump file (that you saved by using the tcpdump -w command), type the following command:
tcpdump -r
For example:
tcpdump -r dump1.bin
In this mode, the tcpdump utility reads stored packets from the file, but otherwise operates just as it would reading from the network interface. As a result, you can use formatting commands and filters.
Filters
The tcpdump utility allows you to use filters to, among other things, restrict the output to specified addresses and ports and specified tcp flags.
Filtering on a host address
• To view all packets that are traveling to or from a specific IP address, type the following command:
tcpdump host
For example:
tcpdump host 10.90.100.1
• To view all packets that are traveling from a specific IP address, type the following command:
tcpdump src host
For example:
tcpdump src host 10.90.100.1
• To view all packets that are traveling to a particular IP address, type the following command:
tcpdump dst host
For example:
tcpdump dst host 10.90.100.1
Filtering on a port
• To view all packets that are traveling through the BIG-IP system and are either sourced from or destined to a specific port, type the following command:
tcpdump port
For example:
tcpdump port 80
• To view all packets that are traveling through the BIG-IP system and sourced from a specific port, type the following command:
tcpdump src port
For example:
tcpdump src port 80
• To view all packets that are traveling through the BIG-IP system and destined to a specific port, type the following command:
tcpdump dst port
For example:
tcpdump dst port 80
Filtering on a tcp flag
• To view all packets that are traveling through the BIG-IP system that contain the SYN flag, type the following command:
tcpdump 'tcp[tcpflags] & (tcp-syn) != 0'
• To view all packets that are traveling through the BIG-IP system that contain the RST flag, type the following command:
tcpdump 'tcp[tcpflags] & (tcp-rst) != 0'
Combining filters with the and operator
You can use the and operator to filter for a mixture of output.
Following are some examples of useful combinations:
tcpdump host 10.90.100.1 and port 80
tcpdump src host 172.16.101.20 and dst port 80
tcpdump src host 172.16.101.20 and dst host 10.90.100.1
Capturing Packet Data
The tcpdump utility provides an option which allows you to specify the amount of each packet to capture.
You can use the -s (snarf/snaplen) option to specify the amount of each packet to capture. To capture the entire packet, use a value of 0 (zero). For example:
tcpdump -s0 src host 172.16.101.20 and dst port 80
Alternatively, you can specify a length large enough to capture the packet data you need to examine. For example:
tcpdump -s200 src host 172.16.101.20 and dst port 80
If you are using the tcpdump utility to examine the output on the console during capture or by reading from an input file with the -r option, you should also use the -X flag to display ASCII encoded output along with the default HEX encoded output. For example:
tcpdump -X -s200 src host 172.16.101.20 and dst port 80
Supressing hostname and port resolution
The tcpdump utility provides an option which allows you to specify whether IP addresses and service ports are translated to their corresponding hostnames and service names.
Since performing multiple name lookups during a packet capture may be resource intensive, you should disable name resolution while capturing on a busy system using the -n option. For example:
tcpdump -n src host 172.16.101.20 and dst port 80
Service port lookups incur less overhead than DNS-based name resolutions, but still are usually unnecessary while capturing. You can disable both name and service port resolution while capturing by using the -nn option. For example:
tcpdump -nn src host 172.16.101.20 and dst port 80
Combining tcpdump options
This Solution contains the most essential tcpdump options. You will generally need to use most of the options in combination.
Following are examples of how to combine the tcpdump options to provide the most meaningful output:
tcpdump -ni internal -w dump1.bin
tcpdump -ni internal -r dump1.bin host 10.90.100.1
tcpdump -ni exp1 host 10.90.100.1 and port 80
tcpdump -ni 1.10 src host 172.16.101.20 and dst port 80 >dump1.txt
tcpdump -Xs200 -nni eth0 -w /var/tmp/mgmt.cap dst host 172.16.101.20 and dst port 162
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