Last updated on March 31, 2021 by Dan Nanni
When it comes to network troubleshooting and monitoring, what types of tools you are using make a world of difference. While required tools may vary depending on the types of network problems you are dealing with, there are a set of essential tools that every network administrator must be familiar with, and
tcpdump is definitely one of them.
tcpdump is a command-line tool packet sniffing that allows you to capture network packets based on packet filtering rules, interpret captured packet content, and display the result in a human-readable format. The main power of
tcpdump comes from its (1) flexible packet filtering rules and (2) versatile protocol dissection capability. Although GUI-based Wireshark provides equally powerful filtering/dissecting capabilities via a more user-friendly interface, its relatively high memory footprint (for buffering packets) and GUI-based operations make Wireshark unsuitable when you are troubleshooting directly from remote headless servers.
tcpdump, you can troubleshoot a wide range of network issues including but not limited to:
The rest of the post provides a comprehensive
tcpdump cheat sheet, which illustrates different types of packet capture scenarios using actual
tcpdump, typically you specify, with
-i option, which network interface you want to monitor traffic on. If you do not specify a network interface,
tcpdump will listen on a default network interface, which is the lowest-number interface in the network interface list.
tcpdump -D will show the network interface list.
$ sudo tcpdump -i docker0
Suppose there are multiple network interfaces on your system, and you want to capture traffic from all those interfaces simultaneously. Exactly for this purpose
tcpdump provides a special interface name called "
any". Thus, simply run
-i any option to capture traffic from all available network interfaces.
$ sudo tcpdump -i any
$ sudo tcpdump -e
13:27:18.002070 9c:b6:d0:fe:4d:95 (oui Unknown) > 48:d6:e5:7b:81:70 (oui Unknown), ethertype IPv4 (0x0800), length 66: xxxx.xxxx.xxxx.33800 > xxxx.xxxx.xxxx.8009: Flags [.], ack 111, win 501, options [nop,nop,TS val 1624743259 ecr 4014866], length 0
$ sudo tcpdump -n
13:30:50.832688 IP 184.108.40.206.443 > 192.168.1.236.35267: UDP, length 45
$ sudo tcpdump -v $ sudo tcpdump -vv $ sudo tcpdump -vvv
$ sudo tcpdump src 220.127.116.11
$ sudo tcpdump src net 192.168.100.0/24
$ sudo tcpdump dst 10.0.0.1
$ sudo tcpdump dst net 192.168.100.0/24
$ sudo tcpdump src 10.10.0.1 and dst 192.168.100.54
10.10.0.0/24to destination network
$ sudo tcpdump src net 10.10.0.0/24 and dst net 192.168.100.0/24
192.168.100.54in both directions:
$ sudo tcpdump host 10.10.0.1 and host 192.168.100.54
192.168.100.0./24in both directions:
sudo tcpdump net 10.10.0.0/24 and net 192.168.100.0/24
tcpdump allows you to capture network traffic with a specific network protocol. For well-known layer-3 or layer-4 protocols, you just need to specify their names. For other types of transport protocols (e.g., DHCP, DNS, SSH), you can filter them based on their port numbers (shown next).
$ sudo tcpdump ip
$ sudo tcpdump icmp
$ sudo tcpdump arp
$ sudo tcpdump ip6
$ sudo tcpdump tcp
$ sudo tcpdump udp
$ sudo tcpdump tcp and host 10.10.0.1 and host 192.168.100.54
$ sudo tcpdump dst port 80
$ sudo tcpdump udp src port 4001
$ sudo tcpdump port 22
$ sudo tcpdump port 53
$ sudo tcpdump port 80 or port 8000
$ sudo tcpdump 'tcp port 80 or udp port 4001'
$ sudo tcpdump portrange 800-900
$ sudo tcpdump tcp dst portrange 8000-8010
If my SSH session is originating from
$ sudo tcpdump port not 22 and not host 192.168.100.250
$ sudo tcpdump -c 100 tcp
$ sudo tcpdump greater 200
$ sudo tcpdump tcp dst port 80 and less 200
Note that the
less operators check the length of an entire packet, including all headers (e.g., Ethernet, IP, TCP headers).
$ sudo tcpdump not less 200 and not greater 500
You can filter packets by MAC address with the
$ sudo tcpdump ether dst ff:ff:ff:ff:ff:ff
9c:b6:d0:ee:fd:90in both directions:
$ sudo tcpdump ether host e8:2b:88:ef:55:11 and ether host 9c:b6:d0:ee:fd:90
$ sudo tcpdump ether host e8:2b:88:ef:55:11
$ sudo tcpdump -A tcp
Printing packet payload in ASCII format can be useful to inspect (unencrypted) HTTP protocol headers. For example:
$ sudo tcpdump -X
$ sudo tcpdump -c 1000 -w trace.pcap
$ tcpdump -r trace.pcap tcp
When you dump captured packets to a file, the capture file can grow quickly depending on the rate of incoming packets. So you want to rotate capture files regularly.
$ sudo tcpdump -w /tmp/trace -W 24 -G 3600 -C 500
This will create 24 capture files (/tmp/trace00, /tmp/trace01, ..., /tmp/trace23) in 24 hours. After that, it will overwrite the files from the beginning.
$ sudo tcpdump -w /tmp/trace-%Y-%m-%d_%H-%M.pcap -G 1800
This will create a new trace file every 30 minutes with the following names.
/tmp/trace-2021-23-29_12-00.pcap /tmp/trace-2021-23-29_12-30.pcap /tmp/trace-2021-23-29_13-00.pcap /tmp/trace-2021-23-29_13-30.pcap . . .
tcpdump use micro-second resolution for timestamping packets. However,
tcpdump version 4.6 or later supports nano-second resolution timestamp.
$ sudo tcpdump --time-stamp-precision nano
$ sudo tcpdump -tt
1617031894.463313 IP 192.168.1.236.35627 > 18.104.22.168.443: UDP, length 1350
$ sudo tcpdump -tttt
2021-03-29 11:33:57.181125 IP 192.168.1.236.52472 > 22.214.171.124.123: NTPv4, Client, length 48
$ sudo tcpdump -ttttt
00:00:00.000000 IP 192.168.1.228.49669 > 255.255.255.255.1947: UDP, length 40 00:00:00.205198 IP 126.96.36.199.443 > 192.168.1.236.35627: UDP, length 44 00:00:00.211631 IP 192.168.1.236.35627 > 188.8.131.52.443: UDP, length 33 00:00:02.356055 ARP, Request who-has 192.168.1.1 tell 192.168.1.204, length 46
tcpdump prints packet timestamp in the default timezone of your Linux system. If you want to show timestamp in a different timezone, you can specify the timezone in
TZ environment variable before calling
$ sudo TZ=America/New_York tcpdump $ sudo TZ=Europe/London tcpdump $ sudo TZ=Asia/Seoul tcpdump
tcpdump's expressive filter allows you to check for any arbitrary byte ranges in a packet. Using this capability, for example, you can capture TCP packets with particular flags in their TCP headers.
$ sudo tcpdump "tcp[tcpflags] & (tcp-syn) != 0"
That's the end of the
tcpdump cheat sheet. As you have seen here, only the sky is the limit when it comes to packet-level monitoring with
tcpdump. If you want to share your own
tcpdump examples, feel free to post them in the comment.
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