The tcpdump utility allows you to capture packets that flow within your network to assist in network troubleshooting. The following are several examples of using tcpdump with different options. Traffic is captured based on a specified filter. A variety of options exist, including:
| Options | Description |
|---|---|
| -D | Print a list of network interfaces. |
| -i | Specify an interface on which to capture. |
| -c | Specify the number of packets to receive. |
| -v, -vv, -vvv | Increase the level of detail (verbosity). |
| -w | Write captured data to a file. |
| -r | Read captured data from a file. |
Installing tcpdump utility
On most of the unix/linux systems you would not find the tcpdump package already installed. To install the latest version use the appropriate package manager on your system. For example, In case of CentOS/RHEL servers:
# yum install tcpdump
Examples of using tcpdump for network troubleshooting
1. Display list of network interfaces
To print a list of network interfaces available on which tcpdump can capture packets:
# tcpdump -D 1.eth0 2.nflog (Linux netfilter log (NFLOG) interface) 3.nfqueue (Linux netfilter queue (NFQUEUE) interface) 4.any (Pseudo-device that captures on all interfaces) 5.lo [Loopback]
2. Capturing on a specific interface
As seen from the ‘tcpdump -D’ command, for each network interface, a number and an interface name is printed. The interface name or the number can be supplied to the -i flag to specify an interface on which to capture. For example, to capture the packets on the interface eth0:
# tcpdump -i 1 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 01:26:51.118038 IP ec2-54-159-106-120.compute-1.amazonaws.com.48021 > geeklab.31297: Flags [S], seq 3960153353, win 26883, options [mss 1460,sac kOK,TS val 2229362922 ecr 0,nop,wscale 7], length 0 01:26:51.118072 IP geeklab.31297 > ec2-54-159-106-120.compute-1.amazonaws.com.48021: Flags [S.], seq 547340507, ack 3960153354, win 26847, optio ns [mss 8961,sackOK,TS val 5714985 ecr 2229362922,nop,wscale 7], length 0
In this example, the output is continuous until terminated by pressing Ctrl + C.
3. Capture a specific number of packets only
To exit tcpdump after receiving a specific number of packets, use the -c (count) option followed by the number of packets to receive. The following example captures two packets:
# tcpdump -i 1 -c 2 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 01:37:08.956549 IP 182.100.67.76.38819 > geeklab.ssh: Flags [P.], seq 542149092:542149176, ack 774431931, win 271, options [nop,nop,TS val 26493 51 ecr 6332468], length 84 01:37:08.956575 IP geeklab.ssh > 182.100.67.76.38819: Flags [.], ack 84, win 230, options [nop,nop,TS val 6332824 ecr 2649351], length 0 2 packets captured 6 packets received by filter 0 packets dropped by kernel
As shown in this example, when tcpdump finishes capturing packets, it reports the following:
- packets captured: This is the number of packets that tcpdump has received and processed.
- packets received by filter: A filter can be specified on the command line and only those packets that match the defined filter are processed by tcpdump and counted.
- packets dropped by kernel: This is the number of packets that were dropped due to a lack of buffer space. Use the -B option to set the buffer size.
4. Increase the details (verbosity) of the output
To increase the detail (verbosity) of the output, use the -v option, or -vv for even more verbose output, or -vvv for the most verbose level of output:
# tcpdump –i 1 –v # tcpdump –i 1 -vv # tcpdump –i 1 –vvv
5. Capture the data to a file
Using the tcpdump utility with the -w option allows you to write captured data to a file. This allows the captured data to be read by other network analysis tools, such as Wireshark. The following example captures data to a file named capture.out:
# tcpdump –i 1 –v –c2 –w capture.out
6. reading captured data
You can also read captured data from a file by using the –r option:
# tcpdump –r capture_file
Some more examples of tcpdump command
Many other options and arguments can be used with tcpdump. The following are some specific examples of the power of the tcpdump utility.
1. Display traffic between 2 hosts
To display all traffic between two hosts (represented by variables host1 and host2):
# tcpdump host host1 and host2
2. Display traffic from a source or destination host only
To display traffic from only a source (src) or destination (dst) host:
# tcpdump src host # tcpdump dst host
3. Display traffic for a specific protocol
Provide the protocol as an argument to display only traffic for a specific protocol, for example tcp, udp, icmp, arp:
# tcpdump protocol
For example to display traffic only for the tcp traffic :
# tcpdump tcp
4. Filtering based on source or destination port
To filter based on a source or destination port:
# tcpdump src port ftp # tcpdump dst port http
The tcpdump utility also accepts Boolean operators (AND, NOT, OR) and grouping of operators, allowing you to create complex filters for capturing network data.