Check NIC link speed and driver with `ethtool`

Gigabit negotiating down to 100 Mbps, a flapping cable or a missing driver — `ethtool <iface>` shows the physical layer state that `ip link` cannot.

LinuxethernetDiagnosticsdriverethtoolLow risk

Run `ethtool eth0` to display the negotiated Speed, Duplex, Link detected flag and supported link modes of the NIC.

01Command

Command

$ ethtool eth0

02What it does

The plain `ethtool <device>` call reports what the PHY negotiated with the switch: if Speed shows 100Mb/s on a gigabit port, the problem is usually the cable or the other end. `ethtool -i eth0` adds the driver and firmware version, useful when diagnosing NIC driver bugs. It also enables forgotten fixes like forcing the speed with `ethtool -s eth0 speed 1000 duplex full autoneg on`.

03Risk

RiskLow risk

RequirementsRequires ethtool (apt install ethtool / dnf install ethtool); replace eth0 with your interface name (see `ip -br link`)

04Undo

✓No undo needed — this command does not change persistent state.

05Sources

ethtool(8) — query or control network driver and hardware settings
Ubuntu Manpages · technical · 2026-09-17
Evidence: Documents `ethtool DEVNAME` output: Speed, Duplex, Link detected, and the -i/-P driver-info flags

Last verified: 2026-09-17 · Category: Network

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