ethtool is a Linux command-line utility for inspecting and changing Ethernet interface settings at the driver and NIC level. It is especially useful when a server appears connected but negotiates the wrong speed, reports half-duplex, drops link, or shows hardware-level errors that higher-level tools do not explain.
The safest first use is read-only: run sudo ethtool enp1s0 and replace enp1s0 with the actual interface name. The output can show supported link modes, advertised modes, negotiated speed, duplex, auto-negotiation state, port type, and whether the physical link is detected. The Linux ethtool manual documents these controls, while the Kali Linux package reference provides a current command summary.
Read the Link Before Changing It
A healthy wired server link will commonly report a valid speed such as 1000Mb/s, 10000Mb/s, or higher, Duplex: Full, and Link detected: yes. If a 1 Gb/s server unexpectedly negotiates at 100 Mb/s, investigate the cable, switch port, transceiver, driver, and advertised link modes before forcing a value. This complements the earlier BitcoinVersus.Tech lesson on Ethernet link negotiation.
Useful read-only commands include ethtool INTERFACE for link settings, ethtool -S INTERFACE for driver statistics, ethtool -i INTERFACE for driver and firmware information, and ethtool -k INTERFACE for offload features. Pair those results with ss when you need socket visibility and with the site’s server network validation workflow when troubleshooting an entire host.
Changing Settings Requires Caution
sudo ethtool -s INTERFACE speed 1000 duplex full autoneg on asks the NIC to use specific link settings. Do not use that command casually on a remote production server: a mismatch with the switch can degrade throughput or drop the connection entirely. Modern Ethernet normally works best with compatible auto-negotiation enabled on both ends.
ethtool can also change offload features with -K, ring parameters with -G, pause settings with -A, and other hardware-specific options. These are powerful troubleshooting tools, but changes may be temporary and can interact with NetworkManager, systemd-networkd, distribution configuration, virtualization, or driver defaults after a reboot.
Exercise
On a Linux system you control, identify the active wired interface with ip -br link. Run sudo ethtool INTERFACE, then record the negotiated speed, duplex mode, auto-negotiation state, and link-detected value. Next run sudo ethtool -i INTERFACE and identify the driver and firmware version. Do not change any settings during the exercise.
Knowledge Check
1. What does Link detected: yes confirm? 2. Why should you investigate before forcing speed and duplex? 3. Which option displays NIC driver statistics?
Answers
1. The NIC currently detects a physical Ethernet link. 2. Forced settings can create a mismatch with the switch and cause poor performance or loss of connectivity. 3. ethtool -S INTERFACE.
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