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How do I find out whether a network connection is full or half duplex?

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You can use mii-tool :

$ sudo mii-tool 
eth0: negotiated 100baseTx-FD flow-control, link ok

'FD' is full duplex. 'HD' would represent half. For those who haven't seen this, it's because it generally represents a network problem -- you should always see and expect full.

If you don't have mii-tool installed, it is in the net-tools package:

$ dpkg -S mii-tool
net-tools: /usr/share/man/man8/mii-tool.8.gz
net-tools: /sbin/mii-tool

I believe it is included with a base install, however.

In some cases, dmesg will be sufficient.
Identify your local interface and check dmesg: 

$ dmesg | grep -i duplex
[   31.920900] e1000e: eth0 NIC Link is Up 100 Mbps Full Duplex, Flow Control: Rx/Tx
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  • An article that gives more details: cyberciti.biz/faq/…
    – N.N.
    Oct 20, 2011 at 18:01
  • @N.N. Ah, it does also talk about ethtool . Not installed by default, but the package is also named ethtool, for those wanting to check it out.
    – belacqua
    Oct 20, 2011 at 18:07

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