tcpdump : Linux Network Sniffer Tool
tcpdump: Linux Network Sniffer Tool
tcpdump is a most powerful and widely used command-line packets sniffer or package analyzer tool which is used to capture or filter TCP/IP packets that are received or transferred over a network on a specific interface. It is available under most of the Linux/Unix-based operating systems. tcpdump also gives us an option to save captured packets in a file for future analysis. It saves the file in a pcap format, that can be viewed by tcpdump command or an open-source GUI-based tool called Wireshark (Network Protocol Analyzier) that reads tcpdump pcap format files.
How to Install tcpdump in Linux
Many of Linux distributions already shipped with the tcpdump tool, if in case you don’t have it on systems, you can install it using the following Yum command.
# yum install tcpdump
Once the tcpdump tool is installed on systems, you can continue to browse the following commands with their examples.
Example:
1. Capture Packets from Specific Interface
The command screen will scroll up until you interrupt and when we execute the tcpdump command it will capture from all the interfaces, however with -i switch only capture from the desire interface.
# tcpdump -i eth0 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 11:33:31.976358 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 3500440357:3500440553, ack 3652628334, win 18760, length 196 11:33:31.976603 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 196, win 64487, length 0 11:33:31.977243 ARP, Request who-has tecmint.com tell 172.16.25.126, length 28
2. Capture Only N Number of Packets
When you run the tcpdump command it will capture all the packets for the specified interface, until you Hit the cancel button. But using -c option, you can capture a specified number of packets. The below example will only capture 6 packets.
# tcpdump -c 5 -i eth0 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 11:40:20.281355 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 3500447285:3500447481, ack 3652629474, win 18760, length 196 11:40:20.281586 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 196, win 65235, length 0 11:40:20.282244 ARP, Request who-has tecmint.com tell 172.16.25.126, length 28 11:40:20.282360 ARP, Reply tecmint.com is-at 00:14:5e:67:26:1d (oui Unknown), length 46 11:40:20.282369 IP 172.16.25.126.53216 > tecmint.com.domain: 49504+ PTR? 125.25.16.172.in-addr.arpa. (44) 11:40:20.332494 IP tecmint.com.netbios-ssn > 172.16.26.17.nimaux: Flags [P.], seq 3058424861:3058424914, ack 693912021, win 64190, length 53 NBT Session Packet: Session Message 6 packets captured 23 packets received by filter 0 packets dropped by kernel
3. Display Available Interfaces
To list the number of available interfaces on the system, run the following command with -D option.
# tcpdump -D 1.eth0 2.eth1 3.usbmon1 (USB bus number 1) 4.usbmon2 (USB bus number 2) 5.usbmon3 (USB bus number 3) 6.usbmon4 (USB bus number 4) 7.usbmon5 (USB bus number 5) 8.any (Pseudo-device that captures on all interfaces) 9.lo
4. Capture and Save Packets in a File
As we said, that tcpdump has a feature to capture and save the file in a .pcap format, to do this just execute command with -w option.
# tcpdump -w test.pcap -i eth0 tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 4 packets captured 4 packets received by filter 0 packets dropped by kernel
5. Read Captured Packets File
To read and analyze captured packet test.pcap file use the command with -r option, as shown below.
# tcpdump -r test.pcap reading from file test.pcap, link-type EN10MB (Ethernet) 09:59:34.839117 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 3353041614:3353041746, ack 4193563273, win 18760, length 132 09:59:34.963022 IP 192.168.0.1.nokia-ann-ch1 > 192.168.0.2.ssh: Flags [.], ack 132, win 65351, length 0 09:59:36.935309 IP 192.168.0.1.netbios-dgm > 192.168.0.255.netbios-dgm: NBT UDP PACKET(138) 09:59:37.528731 IP 192.168.0.1.nokia-ann-ch1 > 192.168.0.2.ssh: Flags [P.], seq 1:53, ack 132, win 65351, length 5
6. Capture IP address Packets
To capture packets for a specific interface, run the following command with option -n.
# tcpdump -n -i eth0 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 12:07:03.952358 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 3509512873:3509513069, ack 3652639034, win 18760, length 196 12:07:03.952602 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 196, win 64171, length 0 12:07:03.953311 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 196:504, ack 1, win 18760, length 308 12:07:03.954288 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 504:668, ack 1, win 18760, length 164 12:07:03.954502 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 668, win 65535, length 0
7. Capture only TCP Packets.
To capture packets based on TCP port, run the following command with option tcp.
# tcpdump -i eth0 tcp tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 12:10:36.216358 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 3509646029:3509646225, ack 3652640142, win 18760, length 196 12:10:36.216592 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 196, win 64687, length 0 12:10:36.219069 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 196:504, ack 1, win 18760, length 308 12:10:36.220039 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 504:668, ack 1, win 18760, length 164 12:10:36.220260 IP 172.16.25.125.apwi-rxspooler > 172.16.25.126.ssh: Flags [.], ack 668, win 64215, length 0 12:10:36.222045 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 668:944, ack 1, win 18760, length 276 12:10:36.223036 IP 172.16.25.126.ssh > 172.16.25.125.apwi-rxspooler: Flags [P.], seq 944:1
8. Capture Packet from Specific Port
Let’s say you want to capture packets for specific port 22, execute the below command by specifying port number 22 as shown below.
# tcpdump -i eth0 port 22 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 10:37:49.056927 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 3364204694:3364204890, ack 4193655445, win 20904, length 196 10:37:49.196436 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 4294967244:196, ack 1, win 20904, length 248 10:37:49.196615 IP 192.168.0.1.nokia-ann-ch1 > 192.168.0.2.ssh: Flags [.], ack 196, win 64491, length 0 10:37:49.379298 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 196:616, ack 1, win 20904, length 420 10:37:49.381080 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 616:780, ack 1, win 20904, length 164 10:37:49.381322 IP 192.168.0.1.nokia-ann-ch1 > 192.168.0.2.ssh: Flags [.], ack 780, win
9. Capture Packets from source IP
To capture packets from source IP, say you want to capture packets for 192.168.0.2, use the command as follows.
# tcpdump -i eth0 src 192.168.0.2 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 10:49:15.746474 IP 192.168.0.2.ssh > 192.168.0.1.nokia-ann-ch1: Flags [P.], seq 3364578842:3364579038, ack 4193668445, win 20904, length 196 10:49:15.748554 IP 192.168.0.2.56200 > b.resolvers.Level3.net.domain: 11289+ PTR? 1.0.168.192.in-addr.arpa. (42) 10:49:15.912165 IP 192.168.0.2.56234 > b.resolvers.Level3.net.domain: 53106+ PTR? 2.0.168.192.in-addr.arpa. (42) 10:49:16.074720 IP 192.168.0.2.33961 > b.resolvers.Level3.net.domain: 38447+ PTR? 2.2.2.4.
10. Capture Packets from destination IP
To capture packets from destination IP, say you want to capture packets for 50.117.62.221, use the command as follows.
# tcpdump -i eth0 dst 50.117.62.221 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 10:55:01.798591 IP 192.168.0.2.59896 > 50.116.66.139.http: Flags [.], ack 2480401451, win 318, options [nop,nop,TS val 7955710 ecr 804759402], length 0 10:55:05.527476 IP 192.168.0.2.59894 > 50.116.66.139.http: Flags [F.], seq 2521556029, ack 2164168606, win 245, options [nop,nop,TS val 7959439 ecr 804759284], length 0 10:55:05.626027 IP 192.168.0.2.59894 > 50.116.66.139.http: Flags [.], ack 2, win 245, opti
