Use tcpdump to Analyze Network Traffic
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The tcpdump tool provides a powerful command line option for network sniffing. With tcpdump, you can effectively capture and analyze network traffic, whether to diagnose network issues or to test network security.
In this tutorial, learn how to get started sniffing network traffic with tcpdump. See how to install tcpdump, how to read its output, and how to use it to capture and filter network packets.
Before You Begin
If you have not already done so, create a Linode account and Compute Instance. See our Getting Started with Linode and Creating a Compute Instance guides.
Follow our Setting Up and Securing a Compute Instance guide to update your system. You may also wish to set the timezone, configure your hostname, create a limited user account, and harden SSH access.
Update your system:
Debian / Ubuntusudo apt update && sudo apt upgradeAlmaLinux / CentOS Stream / Fedora / Rocky Linuxsudo dnf upgrade
sudo. If you’re not familiar with the sudo command, see the Users and Groups guide.How to Install tcpdump
The most straightforward way to install tcpdump is via your Linux system’s package manager. Fortunately, tcpdump is available by default on most Linux distributions.
sudo apt install tcpdumpsudo dnf install tcpdumpYou can verify your installation using the command below:
sudo tcpdump --versionThe command’s output may vary, but it should be similar to what you see here:
tcpdump version 4.9.3
libpcap version 1.9.1 (with TPACKET_V3)
OpenSSL 1.1.1k FIPS 25 Mar 2021How to Interpret tcpdump Output
Before getting started using tcpdump, it can be useful to familiarize yourself with how tcpdump displays packets. In tcpdump, each line corresponds to a single packet, displayed in distinct sections. Here is an example from later on in this tutorial:
20:26:16.902555 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 2957537435:2957537583, ack 850702985, win 1432, options [nop,nop,TS val 1668387813 ecr 3505486606], length 148Each packet’s display breaks down like this, using the example above as a guide:
20:26:16.902555is the Unix timestamp, representing when the packet was sent. Refer to the How to Use Advanced Display Options with tcpdump section further on to see how you can display more human-readable timestamps instead.IPis the network protocol. In this case, the packet is a TCP packet, which show up under theIPdesignation. An ICMP packet, by contrast, would showICMPhere.example.hostname-one.com.ssh > example.hostname-two.com.64024are the source and destination hostnames. The source is before the>symbol, and the destination is after.The last entry on each host above,
.sshfor the source and.64024for the destination, is the port.Refer to the How to Use Advanced Display Options with tcpdump section below to see how to display host IP addresses instead of hostnames.
The rest is query information.
In the above,
Flagindicates the TCP flag. The flags for TCP packets can be any combination of the following. Most often, the flag field contains a combination of one of these letter designations and the.symbol (like theP.in the example above):Sfor SYNFfor FINPfor PUSHRfor RSTUfor URGWfor ECN CWREfor ECN-Echo.for ACK- No flag, indicating that no flag is set for the packet
Explanations of each flag can be found in the Wikipedia page on TCP.
DNS queries, by contrast, replace the
Flagportion with the DNS query ID, something like44000+.
How to Start Reading Network Traffic with tcpdump
These next sections show you how to get started using tcpdump for capturing and analyzing network traffic.
The example commands and output in these sections use example.hostname-one.com (also as 192.0.2.0) and example.hostname-two.com (also as 198.51.100.0) for hosts throughout this tutorial. Replace these with your own relevant hostnames/IP addresses when reproducing these examples.
Finding Interfaces
tcpdump can provide a list of available network interfaces on your Linux system. In the next sections, these can be used to determine where you want to listen for packets.
To get a list of available interfaces, use the tcpdump command with the -D option:
sudo tcpdump -D1.eth0 [Up, Running]
2.lo [Up, Running, Loopback]
3.any (Pseudo-device that captures on all interfaces) [Up, Running]
4.bluetooth-monitor (Bluetooth Linux Monitor) [none]
5.nflog (Linux netfilter log (NFLOG) interface) [none]
6.nfqueue (Linux netfilter queue (NFQUEUE) interface) [none]
7.usbmon0 (Raw USB traffic, all USB buses) [none]Capturing Packets
You can begin capturing packets by simply executing the tcpdump command. By default, the command uses the lowest-numbered interface, which you can see from the command above would be eth0. However, it can be good practice to explicitly provide the interface, which you can accomplish with the -i option.
sudo tcpdump -i eth0dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
16:45:52.001714 IP example.hostname-one.com.ssh > example.hostname-two.com.49165: Flags [P.], seq 1317852016:1317852228, ack 2902976470, win 317, options [nop,nop,TS val 1605028389 ecr 2281811401], length 212
16:45:52.002503 IP example.hostname-one.com.47687 > resolver08.atlanta.linode.com.domain: 65233+ PTR? 173.174.209.73.in-addr.arpa. (45)
16:45:52.033269 IP example.hostname-two.com.49165 > example.hostname-one.com.ssh: Flags [.], ack 212, win 2044, options [nop,nop,TS val 2281811472 ecr 1605028389], length 0
16:45:52.118992 IP resolver08.atlanta.linode.com.domain > example.hostname-one.com.47687: 65233 2/0/0 PTR example.hostname-two.com., PTR example.hostname-two.com. (134)
16:45:52.119259 IP example.hostname-one.com.48245 > resolver08.atlanta.linode.com.domain: 43515+ PTR? 148.150.187.170.in-addr.arpa. (46)
[...]You can stop capturing packets using the Ctrl + C key combination. And, as you may notice, the output from tcpdump can accumulate quickly. This is especially the case for broad, unfiltered searches like the one above.
You can make the output more manageable for the purposes of getting to know tcpdump by using the -c option. This option allows you to set a number of lines (packets) of output:
sudo tcpdump -i eth0 -c 5dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
19:45:50.085053 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 2957513879:2957513939, ack 850693945, win 1432, options [nop,nop,TS val 1665960996 ecr 3503059910], length 60
19:45:50.086314 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 60:176, ack 1, win 1432, options [nop,nop,TS val 1665960997 ecr 3503059910], length 116
19:45:50.086463 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 176:292, ack 1, win 1432, options [nop,nop,TS val 1665960997 ecr 3503059910], length 116
19:45:50.086859 IP example.hostname-one.com.53728 > resolver06.atlanta.linode.com.domain: 43700+ PTR? 173.174.209.73.in-addr.arpa. (45)
19:45:50.087197 IP resolver06.atlanta.linode.com.domain > example.hostname-one.com.53728: 43700 2/0/0 PTR example.hostname-two.com., PTR example.hostname-two.com. (134)
5 packets captured
12 packets received by filter
0 packets dropped by kerneltcpdump also gives you the option of capturing packets on all available interfaces, by specifying any as the interface. Doing so lets you cast an even wider net when observing network traffic:
sudo tcpdump -i anyApplying Filters
By itself, tcpdump produces a lot of output. It can accumulate quickly, making it difficult to find what you are looking for. That is why, most of the time, you are likely to want to use tcpdump with specific filters. These allow you to limit the captured packets based on certain criteria, keeping out many of the results that you do not need.
Here, you can see the primary filtering options available in tcpdump. In the next section, you can also see how to combine filters and use negative filters, allowing you to even further refine your packet capturing.
Port filtering can be accomplished using the
portoption followed by a specific port you want to observe network traffic on:sudo tcpdump -i eth0 -c 1 port 80dropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 20:12:53.530741 IP example.hostname-two.com.64251 > example.hostname-one.com.http: Flags [S], seq 3536229667, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 2559116808 ecr 0,sackOK,eol], length 0 1 packet captured 1 packet received by filter 0 packets dropped by kernelHost filtering can be accomplished using the
hostoption followed by the IP address for a host you want to observe network traffic for:sudo tcpdump -i eth0 -c 5 host 170.187.150.148dropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 20:20:18.850561 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 2957531627:2957531743, ack 850700453, win 1432, options [nop,nop,TS val 1668029761 ecr 3505128549], length 116 20:20:18.850695 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 116:232, ack 1, win 1432, options [nop,nop,TS val 1668029761 ecr 3505128549], length 116 20:20:18.851206 IP example.hostname-one.com.43356 > resolver06.atlanta.linode.com.domain: 55916+ PTR? 173.174.209.73.in-addr.arpa. (45) 20:20:18.851759 IP resolver06.atlanta.linode.com.domain > example.hostname-one.com.43356: 55916 2/0/0 PTR example.hostname-two.com., PTR example.hostname-two.com. (134) 20:20:18.852052 IP example.hostname-one.com.54556 > resolver06.atlanta.linode.com.domain: 9603+ PTR? 148.150.187.170.in-addr.arpa. (46) 5 packets captured 10 packets received by filter 0 packets dropped by kernelProtocol filtering can be used to narrow results to only a given protocol type. You can accomplish this by simply appending the protocol designation to the tcpdump command:
sudo tcpdump -i eth0 -c 5 icmpdropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 20:21:13.452381 IP example.hostname-one.com > zg-0421d-148.stretchoid.com: ICMP host example.hostname-one.com unreachable - admin prohibited filter, length 48 20:21:18.979723 IP example.hostname-one.com > 45.155.204.63: ICMP host example.hostname-one.com unreachable - admin prohibited filter, length 48 20:21:32.953615 IP example.hostname-one.com > 45.143.203.88: ICMP host example.hostname-one.com unreachable - admin prohibited filter, length 48 20:21:36.452595 IP ec2-54-197-15-194.compute-1.amazonaws.com > example.hostname-one.com: ICMP echo request, id 6, seq 10080, length 16 20:21:36.452732 IP example.hostname-one.com > ec2-54-197-15-194.compute-1.amazonaws.com: ICMP echo reply, id 6, seq 10080, length 16 5 packets captured 5 packets received by filter 0 packets dropped by kernel
Combining Filters
Filters can be made even more effective by combining them using the logical operators and, or, and not. Here’s an example that combines the host and port options:
sudo tcpdump -i eth0 -c 5 host 192.0.2.0 and port 80The not operator can be used on its own as well:
sudo tcpdump -i eth0 -c 5 not port 80Saving Results
Often, the results from tcpdump are extensive, and sometimes you may want to save the results for deeper analysis at a later time. For that, you can use the tcpdump feature for saving results to a file.
This uses the -w option followed by the name of the file to save the results to:
sudo tcpdump -i eth0 -c 5 -w example-packet-dump.pcapYou can then read the results again right in tcpdump, using the -r option:
sudo tcpdump -r example-packet-dump.pcapreading from file example-packet-dump.pcap, link-type EN10MB (Ethernet)
dropped privs to tcpdump
20:26:16.902555 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 2957537435:2957537583, ack 850702985, win 1432, options [nop,nop,TS val 1668387813 ecr 3505486606], length 148
20:26:16.923433 IP 107.189.4.117.51676 > example.hostname-one.com.personal-agent: Flags [S], seq 46350988, win 65535, options [mss 536], length 0
20:26:16.923532 IP example.hostname-one.com > 107.189.4.117: ICMP host example.hostname-one.com unreachable - admin prohibited filter, length 52
20:26:16.932689 IP example.hostname-two.com.64024 > example.hostname-one.com.ssh: Flags [.], ack 148, win 65532, options [nop,nop,TS val 3505486685 ecr 1668387813], length 0
20:26:17.357029 IP6 example.hostname-one.com > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28How to Use Advanced Display Options with tcpdump
tcpdump provides a number of options to control how results display. These range from making output easier to read to providing additional information about packets.
Use
-vfor verbose results, providing additional information for each packet. You can make the results even more verbose with-vv:sudo tcpdump -i eth0 -c 5 -vvdropped privs to tcpdump tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 20:27:34.957840 IP (tos 0x48, ttl 64, id 13252, offset 0, flags [DF], proto TCP (6), length 200) example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], cksum 0x3a89 (incorrect -> 0xebae), seq 2957544691:2957544839, ack 850706093, win 1432, options [nop,nop,TS val 1668465868 ecr 3505564654], length 148 20:27:34.958582 IP (tos 0x0, ttl 64, id 59922, offset 0, flags [DF], proto UDP (17), length 73) example.hostname-one.com.47790 > resolver06.atlanta.linode.com.domain: [bad udp cksum 0xbf7f -> 0x3f97!] 6992+ PTR? 173.174.209.73.in-addr.arpa. (45) 20:27:34.958655 IP (tos 0x48, ttl 64, id 13253, offset 0, flags [DF], proto TCP (6), length 184) example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], cksum 0x3a79 (incorrect -> 0xa704), seq 148:280, ack 1, win 1432, options [nop,nop,TS val 1668465869 ecr 3505564654], length 132 20:27:34.960232 IP (tos 0x0, ttl 59, id 23535, offset 0, flags [none], proto UDP (17), length 162) resolver06.atlanta.linode.com.domain > example.hostname-one.com.47790: [udp sum ok] 6992 q: PTR? 173.174.209.73.in-addr.arpa. 2/0/0 173.174.209.73.in-addr.arpa. PTR example.hostname-two.com., 173.174.209.73.in-addr.arpa. PTR example.hostname-two.com. (134) 20:27:34.960494 IP (tos 0x0, ttl 64, id 59923, offset 0, flags [DF], proto UDP (17), length 74) example.hostname-one.com.53590 > resolver06.atlanta.linode.com.domain: [bad udp cksum 0xbf80 -> 0x2e92!] 17994+ PTR? 148.150.187.170.in-addr.arpa. (46) 5 packets captured 9 packets received by filter 0 packets dropped by kernelSometimes results are easier to interpret when hostnames are rendered as IP addresses. Alternatively, rendering hostnames as IP addresses may be essential when DNS resolution is unavailable. In these cases, you can use the
-noption. This option shows hosts by their IP addresses:sudo tcpdump -i eth0 -n -c 5dropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 20:15:28.908978 IP6 fe80::f03c:92ff:fe4d:2fb0 > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28 20:15:28.918797 IP 192.0.2.0.ssh > 198.51.100.0.64024: Flags [P.], seq 2957522283:2957522471, ack 850697205, win 1432, options [nop,nop,TS val 1667739829 ecr 3504838648], length 188 20:15:28.918915 IP 192.0.2.0.ssh > 198.51.100.0.64024: Flags [P.], seq 188:352, ack 1, win 1432, options [nop,nop,TS val 1667739829 ecr 3504838648], length 164 20:15:28.919162 IP 192.0.2.0.ssh > 198.51.100.0.64024: Flags [P.], seq 352:748, ack 1, win 1432, options [nop,nop,TS val 1667739830 ecr 3504838648], length 396 20:15:28.919278 IP 192.0.2.0.ssh > 198.51.100.0.64024: Flags [P.], seq 748:952, ack 1, win 1432, options [nop,nop,TS val 1667739830 ecr 3504838648], length 204 5 packets captured 6 packets received by filter 0 packets dropped by kernelTo get more human-readable timestamps, as opposed to the default Unix-formatted timestamps, you can use the
-ttttoption:sudo tcpdump -i eth0 -c 5 -ttttdropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 2022-06-09 20:28:27.636762 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 2957547039:2957547099, ack 850706389, win 1432, options [nop,nop,TS val 1668518547 ecr 3505617361], length 60 2022-06-09 20:28:27.636812 IP example.hostname-one.com.ssh > example.hostname-two.com.64024: Flags [P.], seq 60:248, ack 1, win 1432, options [nop,nop,TS val 1668518547 ecr 3505617361], length 188 2022-06-09 20:28:27.637105 IP example.hostname-one.com.35506 > resolver06.atlanta.linode.com.domain: 17406+ PTR? 173.174.209.73.in-addr.arpa. (45) 2022-06-09 20:28:27.638663 IP resolver06.atlanta.linode.com.domain > example.hostname-one.com.35506: 17406 2/0/0 PTR example.hostname-two.com., PTR example.hostname-two.com. (134) 2022-06-09 20:28:27.638766 IP example.hostname-one.com.33127 > resolver06.atlanta.linode.com.domain: 13406+ PTR? 148.150.187.170.in-addr.arpa. (46) 5 packets captured 11 packets received by filter 0 packets dropped by kernel
Conclusion
With that, you have a basis to start using tcpdump to capture and analyze traffic on your system’s network. Building on the examples shown in this tutorial, you can effectively monitor traffic for issues or sniff packets to verify security.
To further enhance your tcpdump usage, take a look at the official tcpdump man pages for a full account of the available command line options.
More Information
You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.
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