ethtool is for people who need to manage physical networks

server, represented by a sad rectangle: why no internet??
smiling stick figure with short curly hair: oops! it would help if you ethernet cable was plugged in!

ethtool eth0

(name of network interface)

this tells you:

• is it even connected? (“link detected”)
• speed
• lots more

--show-offload --offload

your network card can do a lot for you! Like computing checksums. This is called “offloading”. This lets you see/change configured offloads.

--identify INTERFACE

blink the light on the ethernet port. good if you have multiple ports! and cute. (heart)

-S INTERFACE

show statistics like bytes sent. works for wifi interfaces too.

-s

change speed/duplex/other settings of an interface.

$ ethtool - eth0 speed 100

-i INTERFACE

show firmware info

iw dev wlan0 link

ethtool is mostly for Ethernet.

To see the speed (and more) of a wireless connection, use iw.