Network utilities

This is a messy collection of commands and utilities that can be of use for troubleshooting network issues.

Network interfaces

ifconfig

To list all interfaces on your system and show their name, IP address and MAC address:

$ ifconfig | grep -e '^[a-z]\|inet \|ether'

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
    inet 127.0.0.1 netmask 0xff000000
...
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ether f8:ff:c2:5d:02:a7
    inet 192.168.1.119 netmask 0xffffff00 broadcast 192.168.1.255
...

Set the MTU of an interface:

$ ifconfig en1 mtu 9000

netstat

Check for udp services (ports that are in use):

$ netstat -anp udp
...

Show protocol stack counters for udp

$ netstat -s -p udp
...

Network capture

With packet capture tools you can capture the raw network data - including some that would be dropped by the Operating System’s protocol stack.

tcpdump

A command line based capturing tool.

To capture to console in increasing order of specificity:

$ tcpdump -i lo0
$ tcpdump -i lo0 udp
$ tcpdump -i lo0 udp port 9000
$ tcpdump -i lo0 udp port 9000 and greater 7000

To capture a specific number of packets and write them to file:

$ tcpdump -i lo0 udp port 9000 -c 1000 -w capture.pcap

Wireshark

Wireshark is a GUI based network capture tool. It can be loaded with custom made plugins for specific protocols/data formats. This makes it valuable for advanced debugging.

It does however, require a graphical desktop environment and may not always be available. Hence for simply capturing traffic that need to be emailed for support we recommend using tcpdump.