Managing your network interface with ethtool

NIC Check

© Lead Image © Sean Gladwell, Fotolia.com

© Lead Image © Sean Gladwell, Fotolia.com

Article from Issue 182/2016
Author(s):

If ping won't solve your network configuration issues, try ethtool, a powerful utility that lets you manage configuration settings for your network interface card.

The sheer volume of traffic on a super-busy and ever-changing production network makes any oddities very tricky to diagnose. Many of the more serious networking headaches are devastatingly disruptive to business operations. Additionally, the constantly changing nature of busy networks, coupled with the 24/7 attacks of varying competency, mean you need to know your stuff and maintain eagle-eyed diligence in order to catch issues.

If you're like me, though, you probably try to solve these kinds of issues in a simple and elegant way. To alter the network settings on a NIC (Network Interface Card), you have almost certainly used the now deprecated ifconfig command and its successor the ip command.

Another useful utility you can use to troubleshoot network problems is ethtool. The malleable ethtool isn't concerned with IP addresses, VLANs, and subnets. Instead, ethtool lets you manage and configure software drivers and hardware settings that control network interfaces.

If you want to know what the main network interface, eth0, is up to at this moment, you can enter the following command as root:

ethtool eth0

The output is shown in Listing 1.

Listing 1

Standard Info for etho

 

As you can see, the output in Listing 1 provides information on the network configuration, including the speed of the network card, as well as details such as the MDI-X setting, which allows a network interface to figure out if a "straight-through" or "crossover" cable might be necessary to complete the connection.

Probably of greatest benefit of the information presented in Listing 1 is that you can determine the network card settings without breaking a sweat. The following facts are quite useful for understanding the network connection:

Speed: 10000Mb/s
Duplex: Full
Auto-negotiation: off
Link detected: yes

Note that this system is managing to speak to the next device in the chain at a massive 10Gb/s. That device might be some kind of networking kit, such as a switch.

This super-fast speed occurs simultaneously in both directions because it's a full-duplex link, which is just what I'm looking for to achieve the highest data transfers speed possible on the NIC. Finally, the link is detected as being up, and the settings are forced onto the card, rather than acquired through the occasionally-problematic auto-negotiation.

Changes

In addition to telling you what the NIC is doing, ethtool also lets you change the configuration settings. If you want to make sure your precious primary interface doesn't try to connect to the upstream switch using auto-negotiation, enter the following:

# ethtool -s eth0 autoneg off

The -s option specifies that you're making a change. In case it's a stumbling block in the future, the -s switch is needed to apply any new settings that you make, so feel free to use it readily (but with care). The other parts of the command should be self-explanatory.

The capitalized version of the -s option, the uppercase -S, is nothing like its baby brother and is used to request some exceptionally useful statistics.

Listing 2 shows the abbreviated output of the ethtool -S command ( the full output is actually about two thirds longer). The statistics shown in Listing 2 are invaluable when troubleshooting a malfunctioning NIC. I've even created a cron job just to email these verbose statistics to me on a daily basis. That way, when I'm doing my preflight checks in the morning, I can scan the error sections for peace of mind that a change I made previously is working as expected.

Listing 2

Abbreviated -S Statistics Request

 

The most common use of ethtool is to alter the speed and duplex of a network interface. Imagine the scenario where your 100Mbps NIC is causing all sorts of reliability issues and you desperately need it to continue working, even if that means working more slowly. One thing to try is to switch off auto-negotiation, but you can also consider forcing the link speed to much slower by dropping it all the way down to 10Mbps:

# ethtool -s eth0 speed 10 autoneg off

Or, throw duplex preference into the mix:

# ethtool -s eth0 speed 10 duplex half autoneg off

This command slows the link down to its minimum and also disables auto-negotiation, giving a better chance for some form of connectivity, even if it is much slower.

If you can access the machine out-of-band (dial-up through "mgetty," for instance, is perfectly suitable for this kind of test), try a few settings and revert them quickly if they prove to be unsuitable. This type of troubleshooting is often the fastest approach if you are debugging the NIC for a server located many miles away in a data center.

Persistence

The examples I've described so far are not persistent changes and will disappear following a reboot.

On RHEL and its associative flavors, you can add a line to the end of your networking config file (which is /etc/sysconfig/network-scripts/ifcfg-eth0 for the eth0 interface):

ETHTOOL_OPTS="speed 10 duplex half autoneg off"

You could then (carefully, to avoid being locked out) stop and start your NIC (preferably by using /etc/init.d/network restart or service network restart in the first instance) with a natty little command such as:

# ifdown eth0 && ifup eth0

You can achieve a similar result on Debian-based systems, although it's a pretty nasty hack that's about as graceful as an elephant high-diving into a swimming pool full of custard.

Append the following lines into the long-serving Unix-like hidden file /etc/rc.local , and your NIC settings should survive a reboot. Obviously, you can adjust them to your needs.

ethtool -s eth0 autoneg off
ethtool -s eth0 speed 10
ethtool -s eth0 duplex half

With different versions, your mileage may well vary, so be prepared for some potential head-scratching.

Another approach is to write a custom init script that will run at boot time (and then adjust it as needed when you upgrade to a systemd-based version). Also, a more suitable way might be to insert the config into the file /etc/network/interfaces.

Once you've found the section where the pertinent NIC is mentioned (I'll stick with the trusty eth0), you can append the pre-up line, as shown below, adjusting it as needed:

auto eth0
iface eth0 inet static
pre-up /sbin/ethtool -s eth0 speed 10 duplex half autoneg off

In the worst case, you could always use the nasty /etc/rc.local fix, which has been a bone of contention since time immemorial. If there were version headaches, I would cobble together a simple init script instead, which is only one rung higher up on the laziness ladder.

More Options

I've barely scratched the surface of the useful reporting that the excellent ethtool can offer. My favorite part about ethtool is you can easily combine the output into a script in order to display the current status of a network card. I can imagine some of these configurable settings also being easily changeable within a web interface, along the lines of GUIs, such as Webmin, for example. Listing 3 shows ethtool with the -i switch, which reveals driver details for the NIC.

Listing 3

Viewing NIC Driver Details

 

You can use ethtool for lots of other configuration changes. The Fedora documentation provides a useful summary of some of the most important options [2].

The Fedora page offers a useful setting for how to quickly change your MAC address; ethtool refers to this type of address as a physical address and uses the following syntax:

--phyad HEX-VALUE devname

Simply replace HEX-VALUE with a hexadecimal MAC address like 00:20:12:1a:9f:de and replace devname with eth0.

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