Tcpdump

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tcpdump is a command that allows you to monitor your network traffic by ports and devices.

Contents

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:

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

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

See also