example tcp dump activity
Use ifconfig to identify the interfaces that are available:
sudo ifconfigexample output
analyst@b4aade4b3e15:~$ sudo ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1460
inet 172.18.0.2 netmask 255.255.0.0 broadcast 172.18.255.255
ether 02:42:ac:12:00:02 txqueuelen 0 (Ethernet)
RX packets 760 bytes 13683706 (13.0 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 523 bytes 44695 (43.6 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
loop txqueuelen 1000 (Local Loopback)
RX packets 53 bytes 8173 (7.9 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 53 bytes 8173 (7.9 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0The Ethernet network interface is identified by the entry with the eth prefix.
So, in this lab, you'll use eth0 as the interface that you will capture network packet data from in the following tasks.
Use tcpdump to identify the interface options available for packet capture:
sudo tcpdump -Dexample output
analyst@b4aade4b3e15:~$ sudo tcpdump -D
1.eth0 [Up, Running]
2.any (Pseudo-device that captures on all interfaces) [Up, Running]
3.lo [Up, Running, Loopback]
4.nflog (Linux netfilter log (NFLOG) interface)
5.nfqueue (Linux netfilter queue (NFQUEUE) interface)This command will also allow you to identify which network interfaces are available. This may be useful on systems that do not include the ifconfig command.
Inspect the network traffic of a network interface with tcpdump
In this task, you must use tcpdump to filter live network packet traffic on an interface.
- Filter live network packet data from the
eth0interface withtcpdump:
sudo tcpdump -i eth0 -v -c5-
This command will run
tcpdumpwith the following options:-i eth0: Capture data specifically from theeth0interface.-v: Display detailed packet data.-c5: Capture 5 packets of data.
Now, let's take a detailed look at the packet information that this command has returned.
Some of your packet traffic data will be similar to the following example output:
analyst@b4aade4b3e15:~$ sudo tcpdump -i eth0 -v -c5
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
13:34:41.849715 IP (tos 0x0, ttl 64, id 5124, offset 0, flags [DF], proto TCP (6), length 113)
b4aade4b3e15.5000 > nginx-us-central1-b.c.qwiklabs-terminal-vms-prod-00.internal.35886: Flags [P.], cksum 0x588b (incorrect -> 0x2db4), seq 824003473:824003534, ack 494355080, win 501, options [nop,nop,TS val 3565226228 ecr 381006698], length 61
13:34:41.849984 IP (tos 0x0, ttl 63, id 46029, offset 0, flags [DF], proto TCP (6), length 52)
nginx-us-central1-b.c.qwiklabs-terminal-vms-prod-00.internal.35886 > b4aade4b3e15.5000: Flags [.], cksum 0xf838 (correct), ack 61, win 507, options [nop,nop,TS val 381006741 ecr 3565226228], length 0
13:34:41.850797 IP (tos 0x0, ttl 64, id 56996, offset 0, flags [DF], proto UDP (17), length 69)
b4aade4b3e15.32961 > metadata.google.internal.domain: 15232+ PTR? 2.0.17.172.in-addr.arpa. (41)
13:34:41.853851 IP (tos 0x0, ttl 63, id 0, offset 0, flags [none], proto UDP (17), length 143)
metadata.google.internal.domain > b4aade4b3e15.32961: 15232 1/0/0 2.0.17.172.in-addr.arpa. PTR nginx-us-central1-b.c.qwiklabs-terminal-vms-prod-00.internal. (115)
13:34:41.854922 IP (tos 0x0, ttl 64, id 9599, offset 0, flags [DF], proto UDP (17), length 74)
b4aade4b3e15.56670 > metadata.google.internal.domain: 49479+ PTR? 254.169.254.169.in-addr.arpa. (46)
5 packets captured
6 packets received by filter
0 packets dropped by kerneThe specific packet data in your lab may be in a different order and may even be for entirely different types of network traffic. The specific details, such as system names, ports, and checksums, will definitely be different. You can run this command again to get different snapshots to outline how data changes between packets.
Exploring network packet details
In this example, you’ll identify some of the properties that tcpdump outputs for the packet capture data you’ve just seen.
- In the example data at the start of the packet output,
tcpdumpreported that it was listening on theeth0interface, and it provided information on the link type and the capture size in bytes:tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes - On the next line, the first field is the packet's timestamp, followed by the protocol type, IP:
22:24:18.910372 IP - The verbose option,
-v, has provided more details about the IP packet fields, such as TOS, TTL, offset, flags, internal protocol type (in this case, TCP (6)), and the length of the outer IP packet in bytes:
The specific details about these fields are beyond the scope of this lab. But you should know that these are properties that relate to the IP network packet.(tos 0x0, ttl 64, id 5802, offset 0, flags [DF], proto TCP (6), length 134) - In the next section, the data shows the systems that are communicating with each other:
7acb26dc1f44.5000 > nginx-us-east1-c.c.qwiklabs-terminal-vms-prod-00.internal.59788:By default,
tcpdumpwill convert IP addresses into names, as in the screenshot. The name of your Linux virtual machine, also included in the command prompt, appears here as the source for one packet and the destination for the second packet. In your live data, the name will be a different set of letters and numbers.The direction of the arrow (>) indicates the direction of the traffic flow in this packet. Each system name includes a suffix with the port number (.5000 in the screenshot), which is used by the source and the destination systems for this packet.
- The remaining data filters the header data for the inner TCP packet:
Flags [P.], cksum 0x5851 (incorrect > 0x30d3), seq 1080713945:1080714027, ack 62760789, win 501, options [nop,nop,TS val 1017464119 ecr 3001513453], length 82The flags field identifies TCP flags. In this case, the P represents the push flag and the period indicates it's an ACK flag. This means the packet is pushing out data.
The next field is the TCP checksum value, which is used for detecting errors in the data.
This section also includes the sequence and acknowledgment numbers, the window size, and the length of the inner TCP packet in bytes.
Capture network traffic with tcpdump
In this task, you will use tcpdump to save the captured network data to a packet capture file.
In the previous command, you used tcpdump to stream all network traffic. Here, you will use a filter and other tcpdump configuration options to save a small sample that contains only web (TCP port 80) network packet data.
- Capture packet data into a file called
capture.pcap:sudo tcpdump -i eth0 -nn -c9 port 80 -w capture.pcap &analyst@b4aade4b3e15:~$ sudo tcpdump -i eth0 -nn -c9 port 80 -w capture.pcap & [1] 12811 tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytesYou may need to press the ENTER key to get your command prompt back after running this command.
This command will run
tcpdumpin the background with the following options:-i eth0: Capture data from theeth0interface.-nn: Do not attempt to resolve IP addresses or ports to names.This is best practice from a security perspective, as the lookup data may not be valid. It also prevents malicious actors from being alerted to an investigation.-c9: Capture 9 packets of data and then exit.port 80: Filter only port 80 traffic. This is the default HTTP port.-w capture.pcap: Save the captured data to the named file.&: This is an instruction to the Bash shell to run the command in the background.
This command runs in the background, but some output text will appear in your terminal. The text will not affect the commands when you follow the steps for the rest of the lab.
- Use
curlto generate some HTTP (port 80) traffic:
When thecurl opensource.google.comcurlcommand is used like this to open a website, it generates some HTTP (TCP port 80) traffic that can be captured.analyst@b4aade4b3e15:~$ curl opensource.google.com <HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8"> <TITLE>301 Moved</TITLE></HEAD><BODY> <H1>301 Moved</H1> The document has moved <A HREF="https://opensource.google/">here</A>. </BODY></HTML> analyst@b4aade4b3e15:~$ 9 packets captured 10 packets received by filter 0 packets dropped by kernel [1]+ Done sudo tcpdump -i eth0 -nn -c9 port 80 -w capture.pcap -
Verify that packet data has been captured:
ls -l capture.pcapNote: The "Done" in the output indicates that the packet was captured.
analyst@b4aade4b3e15:~$ ls -l capture.pcap -rw-r--r-- 1 root root 1445 Sep 13 13:51 capture.pcap
Filter the captured packet data
In this task, use tcpdump to filter data from the packet capture file you saved previously.
- Use the
tcpdumpcommand to filter the packet header data from thecapture.pcapcapture file:sudo tcpdump -nn -r capture.pcap -vexample output:
analyst@b4aade4b3e15:~$ sudo tcpdump -nn -r capture.pcap -v reading from file capture.pcap, link-type EN10MB (Ethernet) 13:51:00.530684 IP (tos 0x0, ttl 64, id 58151, offset 0, flags [DF], proto TCP (6), length 60) 172.18.0.2.48328 > 64.233.182.139.80: Flags [S], cksum 0xa3b7 (incorrect -> 0xfeb3), seq 3977332362, win 65320, options [mss 1420,sackOK,TS val 109490055 ecr 0,nop,wscale 7], length 0 13:51:00.531500 IP (tos 0x60, ttl 126, id 0, offset 0, flags [DF], proto TCP (6), length 60) 64.233.182.139.80 > 172.18.0.2.48328: Flags [S.], cksum 0xddb4 (correct), seq 1521600211, ack 3977332363, win 65535, options [mss 1420,sackOK,TS val 4261937288 ecr 109490055,nop,wscale 8], length 0 13:51:00.531518 IP (tos 0x0, ttl 64, id 58152, offset 0, flags [DF], proto TCP (6), length 52) 172.18.0.2.48328 > 64.233.182.139.80: Flags [.], cksum 0xa3af (incorrect -> 0x0a5a), ack 1, win 511, options [nop,nop,TS val 109490056 ecr 4261937288], length 0 13:51:00.531593 IP (tos 0x0, ttl 64, id 58153, offset 0, flags [DF], proto TCP (6), length 137) 172.18.0.2.48328 > 64.233.182.139.80: Flags [P.], cksum 0xa404 (incorrect -> 0x790d), seq 1:86, ack 1, win 511, options [nop,nop,TS val 109490056 ecr 4261937288], length 85: HTTP, length: 85 GET / HTTP/1.1 Host: opensource.google.com User-Agent: curl/7.64.0 Accept: */* 13:51:00.531808 IP (tos 0x60, ttl 126, id 0, offset 0, flags [DF], proto TCP (6), length 52) 64.233.182.139.80 > 172.18.0.2.48328: Flags [.], cksum 0x0b03 (correct), ack 86, win 256, options [nop,nop,TS val 4261937289 ecr 109490056], length 0 13:51:00.533901 IP (tos 0x80, ttl 126, id 0, offset 0, flags [DF], proto TCP (6), length 634) 64.233.182.139.80 > 172.18.0.2.48328: Flags [P.], cksum 0xf3cd (correct), seq 1:583, ack 86, win 256, options [nop,nop,TS val 4261937291 ecr 109490056], length 582: HTTP, length: 582 HTTP/1.1 301 Moved Permanently Location: https://opensource.google/ Cross-Origin-Resource-Policy: cross-origin Content-Type: text/html; charset=UTF-8 X-Content-Type-Options: nosniff Date: Wed, 13 Sep 2023 13:51:00 GMT Expires: Wed, 13 Sep 2023 14:21:00 GMT Cache-Control: public, max-age=1800 Server: sffe Content-Length: 223 X-XSS-Protection: 0 <HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8"> <TITLE>301 Moved</TITLE></HEAD><BODY> <H1>301 Moved</H1> The document has moved <A HREF="https://opensource.google/">here</A>. </BODY></HTML> 13:51:00.533907 IP (tos 0x0, ttl 64, id 58154, offset 0, flags [DF], proto TCP (6), length 52) 172.18.0.2.48328 > 64.233.182.139.80: Flags [.], cksum 0xa3af (incorrect -> 0x07be), ack 583, win 507, options [nop,nop,TS val 109490058 ecr 4261937291], length 0 13:51:00.535313 IP (tos 0x0, ttl 64, id 58155, offset 0, flags [DF], proto TCP (6), length 52) 172.18.0.2.48328 > 64.233.182.139.80: Flags [F.], cksum 0xa3af (incorrect -> 0x07bb), seq 86, ack 583, win 507, options [nop,nop,TS val 109490060 ecr 4261937291], length 0 13:51:00.535704 IP (tos 0x80, ttl 126, id 0, offset 0, flags [DF], proto TCP (6), length 52) 64.233.182.139.80 > 172.18.0.2.48328: Flags [F.], cksum 0x08b4 (correct), seq 583, ack 87, win 256, options [nop,nop,TS val 4261937292 ecr 109490060], length 0This command will run
tcpdumpwith the following options:-nn: Disable port and protocol name lookup.-r: Read capture data from the named file.-v: Display detailed packet data.
You must specify the
-nnswitch again here, as you want to make suretcpdumpdoes not perform name lookups of either IP addresses or ports, since this can alert threat actors.This returns output data similar to the following:
As in the previous example, you can see the IP packet information along with information about the data that the packet contains.reading from file capture.pcap, link-type EN10MB (Ethernet) 20:53:27.669101 IP (tos 0x0, ttl 64, id 50874, offset 0, flags [DF], proto TCP (6), length 60) 172.17.0.2:46498 > 146.75.38.132:80: Flags [S], cksum 0x5445 (incorrect), seq 4197622953, win 65320, options [mss 1420,sackOK,TS val 610940466 ecr 0, nop,wscale 7], length 0 20:53:27.669422 IP (tos 0x0, ttl 62, id 0, offset 0, flags [DF], proto TCP (6), length 60) 146.75.38.132:80: > 172.17.0.2:46498: Flags [S.], cksum 0xc272 (correct), seq 2026312556, ack 4197622953, win 65535, options [mss 1420,sackOK,TS val 155704241 ecr 610940466, nop,wscale 9], length 0 - Use the
tcpdumpcommand to filter the extended packet data from thecapture.pcapcapture file:
example output:sudo tcpdump -nn -r capture.pcap -Xexample output
analyst@b4aade4b3e15:~$ sudo tcpdump -nn -r capture.pcap -X reading from file capture.pcap, link-type EN10MB (Ethernet) 13:51:00.530684 IP 172.18.0.2.48328 > 64.233.182.139.80: Flags [S], seq 3977332362, win 65320, options [mss 1420,sackOK,TS val 109490055 ecr 0,nop,wscale 7], length 0 0x0000: 4500 003c e327 4000 4006 b40b ac12 0002 E..<.'@.@....... 0x0010: 40e9 b68b bcc8 0050 ed11 468a 0000 0000 @......P..F..... 0x0020: a002 ff28 a3b7 0000 0204 058c 0402 080a ...(............ 0x0030: 0686 af87 0000 0000 0103 0307 ............ 13:51:00.531500 IP 64.233.182.139.80 > 172.18.0.2.48328: Flags [S.], seq 1521600211, ack 3977332363, win 65535, options [mss 1420,sackOK,TS val 4261937288 ecr 109490055,nop,wscale 8], length 0 0x0000: 4560 003c 0000 4000 7e06 58d3 40e9 b68b E`.<..@.~.X.@... 0x0010: ac12 0002 0050 bcc8 5ab1 c6d3 ed11 468b .....P..Z.....F. 0x0020: a012 ffff ddb4 0000 0204 058c 0402 080a ................ 0x0030: fe08 0088 0686 af87 0103 0308 ............ 13:51:00.531518 IP 172.18.0.2.48328 > 64.233.182.139.80: Flags [.], ack 1, win 511, options [nop,nop,TS val 109490056 ecr 4261937288], length 0 0x0000: 4500 0034 e328 4000 4006 b412 ac12 0002 E..4.(@.@....... 0x0010: 40e9 b68b bcc8 0050 ed11 468b 5ab1 c6d4 @......P..F.Z... 0x0020: 8010 01ff a3af 0000 0101 080a 0686 af88 ................ 0x0030: fe08 0088 .... 13:51:00.531593 IP 172.18.0.2.48328 > 64.233.182.139.80: Flags [P.], seq 1:86, ack 1, win 511, options [nop,nop,TS val 109490056 ecr 4261937288], length 85: HTTP: GET / HTTP/1.1 0x0000: 4500 0089 e329 4000 4006 b3bc ac12 0002 E....)@.@....... 0x0010: 40e9 b68b bcc8 0050 ed11 468b 5ab1 c6d4 @......P..F.Z... 0x0020: 8018 01ff a404 0000 0101 080a 0686 af88 ................ 0x0030: fe08 0088 4745 5420 2f20 4854 5450 2f31 ....GET./.HTTP/1 0x0040: 2e31 0d0a 486f 7374 3a20 6f70 656e 736f .1..Host:.openso 0x0050: 7572 6365 2e67 6f6f 676c 652e 636f 6d0d urce.google.com. 0x0060: 0a55 7365 722d 4167 656e 743a 2063 7572 .User-Agent:.cur 0x0070: 6c2f 372e 3634 2e30 0d0a 4163 6365 7074 l/7.64.0..Accept 0x0080: 3a20 2a2f 2a0d 0a0d 0a :.*/*.... 13:51:00.531808 IP 64.233.182.139.80 > 172.18.0.2.48328: Flags [.], ack 86, win 256, options [nop,nop,TS val 4261937289 ecr 109490056], length 0 0x0000: 4560 0034 0000 4000 7e06 58db 40e9 b68b E`.4..@.~.X.@... 0x0010: ac12 0002 0050 bcc8 5ab1 c6d4 ed11 46e0 .....P..Z.....F. 0x0020: 8010 0100 0b03 0000 0101 080a fe08 0089 ................ 0x0030: 0686 af88 .... 13:51:00.533901 IP 64.233.182.139.80 > 172.18.0.2.48328: Flags [P.], seq 1:583, ack 86, win 256, options [nop,nop,TS val 4261937291 ecr 109490056], length 582: HTTP: HTTP/1.1 301 Moved Permanently 0x0000: 4580 027a 0000 4000 7e06 5675 40e9 b68b E..z..@.~.Vu@... 0x0010: ac12 0002 0050 bcc8 5ab1 c6d4 ed11 46e0 .....P..Z.....F. 0x0020: 8018 0100 f3cd 0000 0101 080a fe08 008b ................ 0x0030: 0686 af88 4854 5450 2f31 2e31 2033 3031 ....HTTP/1.1.301 0x0040: 204d 6f76 6564 2050 6572 6d61 6e65 6e74 .Moved.Permanent 0x0050: 6c79 0d0a 4c6f 6361 7469 6f6e 3a20 6874 ly..Location:.ht 0x0060: 7470 733a 2f2f 6f70 656e 736f 7572 6365 tps://opensource 0x0070: 2e67 6f6f 676c 652f 0d0a 4372 6f73 732d .google/..Cross- 0x0080: 4f72 6967 696e 2d52 6573 6f75 7263 652d Origin-Resource- 0x0090: 506f 6c69 6379 3a20 6372 6f73 732d 6f72 Policy:.cross-or 0x00a0: 6967 696e 0d0a 436f 6e74 656e 742d 5479 igin..Content-Ty 0x00b0: 7065 3a20 7465 7874 2f68 746d 6c3b 2063 pe:.text/html;.c 0x00c0: 6861 7273 6574 3d55 5446 2d38 0d0a 582d harset=UTF-8..X- 0x00d0: 436f 6e74 656e 742d 5479 7065 2d4f 7074 Content-Type-Opt 0x00e0: 696f 6e73 3a20 6e6f 736e 6966 660d 0a44 ions:.nosniff..D 0x00f0: 6174 653a 2057 6564 2c20 3133 2053 6570 ate:.Wed,.13.Sep 0x0100: 2032 3032 3320 3133 3a35 313a 3030 2047 .2023.13:51:00.G 0x0110: 4d54 0d0a 4578 7069 7265 733a 2057 6564 MT..Expires:.Wed 0x0120: 2c20 3133 2053 6570 2032 3032 3320 3134 ,.13.Sep.2023.14 0x0130: 3a32 313a 3030 2047 4d54 0d0a 4361 6368 :21:00.GMT..Cach 0x0140: 652d 436f 6e74 726f 6c3a 2070 7562 6c69 e-Control:.publi 0x0150: 632c 206d 6178 2d61 6765 3d31 3830 300d c,.max-age=1800. 0x0160: 0a53 6572 7665 723a 2073 6666 650d 0a43 .Server:.sffe..C 0x0170: 6f6e 7465 6e74 2d4c 656e 6774 683a 2032 ontent-Length:.2 0x0180: 3233 0d0a 582d 5853 532d 5072 6f74 6563 23..X-XSS-Protec 0x0190: 7469 6f6e 3a20 300d 0a0d 0a3c 4854 4d4c tion:.0....<HTML 0x01a0: 3e3c 4845 4144 3e3c 6d65 7461 2068 7474 ><HEAD><meta.htt 0x01b0: 702d 6571 7569 763d 2263 6f6e 7465 6e74 p-equiv="content 0x01c0: 2d74 7970 6522 2063 6f6e 7465 6e74 3d22 -type".content=" 0x01d0: 7465 7874 2f68 746d 6c3b 6368 6172 7365 text/html;charse 0x01e0: 743d 7574 662d 3822 3e0a 3c54 4954 4c45 t=utf-8">.<TITLE 0x01f0: 3e33 3031 204d 6f76 6564 3c2f 5449 544c >301.Moved</TITL 0x0200: 453e 3c2f 4845 4144 3e3c 424f 4459 3e0a E></HEAD><BODY>. 0x0210: 3c48 313e 3330 3120 4d6f 7665 643c 2f48 <H1>301.Moved</H 0x0220: 313e 0a54 6865 2064 6f63 756d 656e 7420 1>.The.document. 0x0230: 6861 7320 6d6f 7665 640a 3c41 2048 5245 has.moved.<A.HRE 0x0240: 463d 2268 7474 7073 3a2f 2f6f 7065 6e73 F="https://opens 0x0250: 6f75 7263 652e 676f 6f67 6c65 2f22 3e68 ource.google/">h 0x0260: 6572 653c 2f41 3e2e 0d0a 3c2f 424f 4459 ere</A>...</BODY 0x0270: 3e3c 2f48 544d 4c3e 0d0a ></HTML>.. 13:51:00.533907 IP 172.18.0.2.48328 > 64.233.182.139.80: Flags [.], ack 583, win 507, options [nop,nop,TS val 109490058 ecr 4261937291], length 0 0x0000: 4500 0034 e32a 4000 4006 b410 ac12 0002 E..4.*@.@....... 0x0010: 40e9 b68b bcc8 0050 ed11 46e0 5ab1 c91a @......P..F.Z... 0x0020: 8010 01fb a3af 0000 0101 080a 0686 af8a ................ 0x0030: fe08 008b .... 13:51:00.535313 IP 172.18.0.2.48328 > 64.233.182.139.80: Flags [F.], seq 86, ack 583, win 507, options [nop,nop,TS val 109490060 ecr 4261937291], length 0 0x0000: 4500 0034 e32b 4000 4006 b40f ac12 0002 E..4.+@.@....... 0x0010: 40e9 b68b bcc8 0050 ed11 46e0 5ab1 c91a @......P..F.Z... 0x0020: 8011 01fb a3af 0000 0101 080a 0686 af8c ................ 0x0030: fe08 008b .... 13:51:00.535704 IP 64.233.182.139.80 > 172.18.0.2.48328: Flags [F.], seq 583, ack 87, win 256, options [nop,nop,TS val 4261937292 ecr 109490060], length 0 0x0000: 4580 0034 0000 4000 7e06 58bb 40e9 b68b E..4..@.~.X.@... 0x0010: ac12 0002 0050 bcc8 5ab1 c91a ed11 46e1 .....P..Z.....F. 0x0020: 8011 0100 08b4 0000 0101 080a fe08 008c ................ 0x0030: 0686 af8c .... analyst@b4aade4b3e15:~$This command will run
tcpdumpwith the following options:-nn: Disable port and protocol name lookup.-r: Read capture data from the named file.-X: Display the hexadecimal and ASCII output format packet data. Security analysts can analyze hexadecimal and ASCII output to detect patterns or anomalies during malware analysis or forensic analysis.