I cannot get this laptop, Craptop, to connect to the Internet using the eth0 interface, but it can connect using the wlan0 interface. NetworkManager, managing eth0, times out when trying to find a DHCP server (present as my home router, 192.168.0.1).
A different laptop, Deltique, can connect and negotiate DHCP just fine using the same Ethernet cable.
After asking Wireshark to monitor eth0 on both machines, I found that Deltique negotiated an IP address with 192.168.0.1 after sending a broadcast on 255.255.255.255, but Craptop sent four DHCP broadcasts without receiving any reply.
It seems that the Ethernet port on Craptop is functional, since it can detect when I plug in the wire, but why can't Craptop receive any packets?
Is Craptop missing a driver? How can I troubleshoot this further?
I've included details on my setup below:
Logs and Command Outputs
NetworkManager (from /var/log/syslog)
lshw
Wireshark: DHCP packets
![[Screenshot of Wireshark]](https://i.stack.imgur.com/QDyRH.png)
Note that this Super User question is unrelated, since it talks about being unable to connect to wlan0.
Best Answer
The probable cause was a defective Ethernet port on the router. I didn't find the evidence to support this until 30 April 2015, and I didn't make the connection until 10 August 2015.
What I Should Have Done
A few additional troubleshooting steps:
I should have run
sudo ethtool -S eth0to see if there were any errors in the network interface controller (NIC) statistics.Since I was able to send but not receive over Ethernet, I might have seen something like the
rx_crc_errorsstatistic increment on each receive attempt.I should have tried plugging in the router end of the Ethernet cable into a different router port.
Sure, I plugged in the computer end of the Ethernet cable into two different computers, but that led me to believe that a computer (Craptop) was faulty because the other (Deltique) was able to connect to the router.
On the working computer, Deltique, I should have run
sudo ethtool eth0to see if a good link was established.Deltique has a 1Gbps capacity, and so does the router, so
ethtoolshould have shownSpeed: 1000Mb/sandDuplex: Full. I would later discover on 30 April 2015 that the defective port would have causedethtoolto return different, more concerning values.I should have run a speed benchmark to test the Ethernet link to make sure that I was getting the full performance expected between Deltique and the router.
Had I done a speed test, I likely would have seen a 1MiB/s transfer speed, indicating a link problem, even on Deltique.
To rule out a cable problem, I should have tried using a different Gigabit Ethernet cable.
It turns out that the cable was fine, but this is a good troubleshooting step anyway.
Evidence and Explanation
On 30 April 2015, I noticed that my trusty
rsync -avzHXShPscommand was maxing out at about 1MiB/s. I ruled out other traffic by monitoringiftopand by attempting an Internet download on a different computer, which transferred data ten times faster.I checked the network link with
ethtool:Aha! 10Mbps at half duplex. That's not normal and is very slow. For reference, this was the expected (good) output, captured by switching Ethernet ports on the router:
The computer and the router tried to negotiate slower and slower speeds until a reliable one could be established. The slowest one, 10BASE-T half duplex, was picked.
Forcing the full 1000BASE-T full duplex (
ethtool -s eth0 speed 1000 duplex full) expected performance caused the affected computer to drop from the network, strongly suggesting that the router didn't want to talk on a faster link.Craptop, which is limited to 100Mbps full duplex in the first place, probably didn't have a chance to negotiate 10Mbps half duplex.
Now that I have identified the bad Ethernet port, all I need to do is not use that port or get a new router.
On 27 July 2015, I returned the defective router after installing a new router.
Today, Craptop is running on a fault-tolerant network bond with a 100Mbps full duplex Ethernet connection: