Say I make a DNS request:
$ dig +short @188.8.131.52 google.com 184.108.40.206
What went over the network?
We can find out with a program called
tcpdump is a command-line program
which can prints everything that goes over a network interface on a UNIX box.
Let’s see an example, getting all DNS traffic.
DNS requests go over UDP port 53.
tcpdump for this using the expression
udp and port 53:
$ sudo tcpdump -n 'udp and port 53' tcpdump: data link type PKTAP tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on pktap, link-type PKTAP (Apple DLT_PKTAP), capture size 262144 bytes 23:21:50.909488 IP 192.168.1.4.61988 > 192.168.1.254.53: 61163+ [1au] A? google.com. (39) 23:21:50.929583 IP 192.168.1.254.53 > 192.168.1.4.61988: 61163 1/0/1 A 220.127.116.11 (55)
The output shows exactly two UDP packets. One for the DNS request, the next for the response.
You’ll notice that
tcpdump is terribly named!
It does not just dump TCP; it can dump all manner of network activity:
UDP, IP, ICMP, Ethernet, and many others.
Tagged #programming, #unix. All content copyright James Fisher 2018. This post is not associated with my employer.