Tcpdump
tcpdump is a command that allows you to monitor your network traffic by ports and devices.
QuickStart
Let's set up a web server that has nothing but an index.html file saying "hello". Here is how we monitor traffic on it for localhost:
# tcpdump -Ai lo port 80
Once we start requesting an html page, tcpdump gets active: <source> tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes 21:46:20.363064 IP6 localhost.49816 > localhost.http: Flags [S], seq 1873285701, win 43690, options [mss 65476,sackOK,TS val 1957802 ecr 0,nop,wscale 7], length 0 `....(.@...................................Po..E.........0......... ............ 21:46:20.363089 IP6 localhost.http > localhost.49816: Flags [S.], seq 302679655, ack 1873285702, win 43690, options [mss 65476,sackOK,TS val 1957802 ecr 1957802,nop,wscale 7], length 0 `....(.@.................................P... .go..F.....0......... ............ 21:46:20.363109 IP6 localhost.49816 > localhost.http: Flags [.], ack 1, win 342, options [nop,nop,TS val 1957802 ecr 1957802], length 0 `.... .@...................................Po..F. .h...V.(..... ........ 21:46:20.363153 IP6 localhost.49816 > localhost.http: Flags [P.], seq 1:117, ack 1, win 342, options [nop,nop,TS val 1957802 ecr 1957802], length 116 `......@...................................Po..F. .h...V....... ........GET /index.htm HTTP/1.1 User-Agent: Wget/1.16 (linux-gnu) Accept: */* Host: localhost Connection: Keep-Alive
21:46:20.363173 IP6 localhost.http > localhost.49816: Flags [.], ack 117, win 342, options [nop,nop,TS val 1957802 ecr 1957802], length 0
`.... .@.................................P...
.ho......V.(.....
........
21:46:20.363500 IP6 localhost.http > localhost.49816: Flags [P.], seq 1:273, ack 117, win 342, options [nop,nop,TS val 1957803 ecr 1957802], length 272
`....0.@.................................P...
.ho......V.8.....
........HTTP/1.1 200 OK
Date: Wed, 18 Nov 2015 20:46:20 GMT
Server: Apache
Last-Modified: Fri, 30 Jan 2015 06:33:25 GMT
ETag: "6-50dd8c82254d0"
Accept-Ranges: bytes
Content-Length: 6
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
hallo
21:46:20.363518 IP6 localhost.49816 > localhost.http: Flags [.], ack 273, win 350, options [nop,nop,TS val 1957803 ecr 1957803], length 0 `.... .@...................................Po.... .x...^.(..... ........ 21:46:20.365359 IP6 localhost.49816 > localhost.http: Flags [F.], seq 117, ack 273, win 350, options [nop,nop,TS val 1957805 ecr 1957803], length 0 `.... .@...................................Po.... .x...^.(..... ........ 21:46:20.365417 IP6 localhost.http > localhost.49816: Flags [F.], seq 273, ack 118, win 342, options [nop,nop,TS val 1957805 ecr 1957805], length 0 `.... .@.................................P... .xo......V.(..... ........ 21:46:20.365430 IP6 localhost.49816 > localhost.http: Flags [.], ack 274, win 350, options [nop,nop,TS val 1957805 ecr 1957805], length 0 `.... .@...................................Po.... .y...^.(..... ........ ^C 10 packets captured 20 packets received by filter 0 packets dropped by kernel </source>
Saving in a file for wireshark
To save the output in a file for wireshark use
tcpdump -s 0 -Ali lo port 80 -w filename.txt
Examples
dhcp
You can watch out for dhcp communication on your network using:
tcpdump -i eth1 port 67 and port 68
SNMP
You can display incoming snmp traps using:
tcpdump -A port 162 -l | hexdump -C
