Packet Analyzer_ 15 TCPDUMP Command Examples

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All Command Examples of TCPDUMP
  136 Mi piaceMi piace ≡  MenuHomeFree eBook Start HereContactAbout Packet Analyzer: 15 TCPDUMP Command Examples bySasikalaonAugust 25, 2010   Tweet tcpdump command is also called as packet analyzer.tcpdump command will work on most flavors of unix operating system. tcpdump allows us to save the packets that are captured, so that we can use it forfuture analysis. The saved file can be viewed by the same tcpdump command. We can also use open source software like wireshark to read the tcpdumppcap files.In this tcpdump tutorial, let us discuss some practical examples on how to use the tcpdump command. 1. Capture packets from a particular ethernet interface using tcpdump -i When you execute tcpdump command without any option, it will capture all the packets flowing through all the interfaces. -i option with tcpdumpcommand, allows you to filter on a particular ethernet interface. $ tcpdump -i eth114:59:26.608728 IP > . ack 540 win 1655414:59:26.610602 IP > 4278 1/0/0 (73)14:59:26.611262 IP > 26364+ PTR? (45) In this example, tcpdump captured all the packets flows in the interface eth1 and displays in the standard output. Note : Editcap utility is used to select or remove specific packets from dump file and translate them into a given format. Packet Analyzer: 15 TCPDUMP Command Examples 1 di 1224/01/2017 14:52  2. Capture only N number of packets using tcpdump -c When you execute tcpdump command it gives packets until you cancel the tcpdump command. Using -c option you can specify the number of packets tocapture. $ tcpdump -c 2 -i eth0listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes14:38:38.184913 IP > P 1457255642:1457255758(116) ack 1561463966 win 6365214:38:38.690919 IP > P 116:232(116) ack 1 win 636522 packets captured13 packets received by filter0 packets dropped by kernel The above tcpdump command captured only 2 packets from interface eth0. Note:  Mergecap and TShark : Mergecap is a packet dump combining tool, which will combine multiple dumps into a single dump file. Tshark is apowerful tool to capture network packets, which can be used to analyze the network traffic. It comes with wireshark network analyzer distribution. 3. Display Captured Packets in ASCII using tcpdump -A The following tcpdump syntax prints the packet in ASCII. $ tcpdump -A -i eth0tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes14:34:50.913995 IP > P 1457239478:1457239594(116) ack 1561461262 win 63652E.....@.@..]..i...9...*.V...]...P....h....E...>{..U=...g.......G..7\+KA....A...L.14:34:51.423640 IP > P 116:232(116) ack 1 win 63652E.....@.@..\..i...9...*.V..*]...P....h....7......X..!....Im.S.g.u:*..O&....^#Ba...E..(R.@.|.....9...i.*...]...V..*P..OWp........ Note: Ifconfig command is used to configure network interfaces 4. Display Captured Packets in HEX and ASCII using tcpdump -XX Some users might want to analyse the packets in hex values. tcpdump provides a way to print packets in both ASCII and HEX format. $tcpdump -XX -i eth018:52:54.859697 IP > . ack 232 win 16511 0x0000: 0050 569c 35a3 0019 bb1c 0c00 0800 4500 .PV.5.........E. 0x0010: 0028 042a 4000 7906 c89c 10b5 aaf6 0f9a .(.*@.y......... 0x0020: 69c4 f999 0016 57db 6e08 c712 ea2e 5010 i.....W.n.....P. 0x0030: 407f c976 0000 0000 0000 0000 @..v........18:52:54.877713 IP > igmp query v3 [max resp time 1s] 0x0000: 0050 569c 35a3 0000 0000 0000 0800 4600 .PV.5.........F. 0x0010: 0024 0000 0000 0102 3ad3 0a00 0000 e000 .$......:....... 0x0020: 0001 9404 0000 1101 ebfe 0000 0000 0300 ................ 0x0030: 0000 0000 0000 0000 0000 0000 ............ 5. Capture the packets and write into a file using tcpdump -w tcpdump allows you to save the packets to a file, and later you can use the packet file for further analysis. $ tcpdump -w 08232010.pcap -i eth0tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes32 packets captured32 packets received by filter0 packets dropped by kernel -w option writes the packets into a given file. The file extension should be .pcap, which can be read by any network protocolanalyzer. Packet Analyzer: 15 TCPDUMP Command Examples 2 di 1224/01/2017 14:52  6. Reading the packets from a saved file using tcpdump -r You can read the captured pcap file and view the packets for analysis, as shown below. $tcpdump -tttt -r data.pcap2010-08-22 21:35:26.571793 00:50:56:9c:69:38 (oui Unknown) > Broadcast, ethertype Unknown (0xcafe), length 74: 0x0000: 0200 000a ffff 0000 ffff 0c00 3c00 0000 ............<... 0x0010: 0000 0000 0100 0080 3e9e 2900 0000 0000 ........>.)..... 0x0020: 0000 0000 ffff ffff ad00 996b 0600 0050 ...........k...P 0x0030: 569c 6938 0000 0000 8e07 0000 V.i8........2010-08-22 21:35:26.571797 IP > P 800464396:800464448(52) ack 203316566 win 712010-08-22 21:35:26.571800 IP > P 52:168(116) ack 1 win 712010-08-22 21:35:26.584865 IP > NBT UDP PACKET(137): QUERY; REQUEST; BROADC 7. Capture packets with IP address using tcpdump -n In all the above examples, it prints packets with the DNS address, but not the ip address. The following example captures the packets and it will displaythe IP address of the machines involved. $ tcpdump -n -i eth015:01:35.170763 IP > P 105:157(52) ack 18060 win 1654915:01:35.170776 IP > P 23988:24136(148) ack 157 win 11315:01:35.170894 IP > P 24136:24380(244) ack 157 win 113 8. Capture packets with proper readable timestamp using tcpdump -tttt $ tcpdump -n -tttt -i eth02010-08-22 15:10:39.162830 IP > . ack 49800 win 163902010-08-22 15:10:39.162833 IP > . ack 50288 win 166602010-08-22 15:10:39.162867 IP > . ack 50584 win 16586 9. Read packets longer than N bytes You can receive only the packets greater than n number of bytes using a filter ‘greater’ through tcpdump command $ tcpdump -w g_1024.pcap greater 1024 10. Receive only the packets of a specific protocol type You can receive the packets based on the protocol type. You can specify one of these protocols — fddi, tr, wlan, ip, ip6, arp, rarp, decnet, tcp and udp. Thefollowing example captures only arp packets flowing through the eth0 interface. $ tcpdump -i eth0 arptcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes19:41:52.809642 arp who-has tell valh9.lell.net19:41:52.863689 arp who-has tell valh6.lell.net19:41:53.024769 arp who-has tell 11. Read packets lesser than N bytes You can receive only the packets lesser than n number of bytes using a filter ‘less’ through tcpdump command $ tcpdump -w l_1024.pcap less 1024 12. Receive packets flows on a particular port using tcpdump port If you want to know all the packets received by a particular port on a machine, you can use tcpdump command as shown below. $ tcpdump -i eth0 port 2219:44:44.934459 IP > P 18932:19096(164) ack 105 win 7119:44:44.934533 IP > P 19096:19260(164) ack 105 win 7119:44:44.934612 IP > P 19260:19424(164) ack 105 win 71 13. Capture packets for particular destination IP and Port The packets will have source and destination IP and port numbers. Using tcpdump we can apply filters on source or destination IP and port number. Thefollowing command captures packets flows in eth0, with a particular destination ip and port number 22. $ tcpdump -w xpackets.pcap -i eth0 dst and port 22 14. Capture TCP communication packets between two hosts If two different process from two different machines are communicating through tcp protocol, we can capture those packets using tcpdump as shown Packet Analyzer: 15 TCPDUMP Command Examples 3 di 1224/01/2017 14:52  Tweet136 Mi piaceMi piace below. $tcpdump -w comm.pcap -i eth0 dst and port 22 You can open the file comm.pcap using any network protocol analyzer tool to debug any potential issues. 15. tcpdump Filter Packets – Capture all the packets other than arp and rarp In tcpdump command, you can give “and”, “or” and “not” condition to filter the packets accordingly. $ tcpdump -i eth0 not arp and not rarp20:33:15.479278 IP > 26929 1/0/0 (73)20:33:15.479890 IP > 56556+ PTR? (45)20:33:15.480197 IP > P 540:1504(964) ack 1 win 9620:33:15.487118 IP > . ack 540 win 1648620:33:15.668599 IP > igmp query v3 [max resp time 1s] > Add your comment If you enjoyed this article, you might also like.. 50 Linux Sysadmin Tutorials1. 50 Most Frequently Used Linux Commands (With Examples)2. Top 25 Best Linux Performance Monitoring and Debugging Tools3. Mommy, I found it! – 15 Practical Linux Find Command Examples4. Linux 101 Hacks 2nd Edition eBook 5. Awk Introduction – 7 Awk Print ExamplesAdvanced Sed Substitution Examples8 Essential Vim Editor Navigation Fundamentals25 Most Frequently Used Linux IPTables Rules ExamplesTurbocharge PuTTY with 12 Powerful Add-OnsTagged as:aix tcpdump, dhcp tcpdump, FreeBSD tcpdump, TCP Dump, tcpdump command, tcpdump DNS, tcpdump for Windows, tcpdump format, tcpdump how to, tcpdump icmp, tcpdump ip, tcpdump not, tcpdump windows, tcpdump wireshark  {41 comments… add one}JeffreyAugust 25, 2010, 2:50 amWhy not use wireshark?Link Zigor Alcañiz EigurenAugust 25, 2010, 2:57 amHi!Nice tips, good work.I think there is a glitch in number 14.It should be something like:$tcpdump -w comm.pcap -i eth0 tcp host or host specify that you just want TCP traffic and you don’t specify whether you want those hosts as src or dst, so it captures traffic in both directions.Bye…Link Zigor Alcañiz EigurenAugust 25, 2010, 3:05 amSorry,… I correct myself (this happens for no trying the command prior to writting it):There is a lacking “and”: Packet Analyzer: 15 TCPDUMP Command Examples 4 di 1224/01/2017 14:52
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