ethtool — Ethernet Interface Settings and Diagnostics

What it is¶

ethtool reads and changes the settings of Ethernet interfaces: link state, negotiated speed and duplex, driver and firmware, offload features, and hardware counters.

Is the cable connected?¶

sudo ethtool eth0

Settings for eth0:
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supports auto-negotiation: Yes
        Speed: 1000Mb/s
        Duplex: Full
        Auto-negotiation: on
        Link detected: yes

Link detected: no at the bottom means nothing is plugged in, or the far end is down. A link negotiating at 100Mb/s on a gigabit port usually points at a bad cable or patch, since gigabit needs all four pairs. ip -br link shows the same state as UP/NO-CARRIER.

Driver and firmware¶

sudo ethtool -i eth0

driver: e1000e
version: 5.15.0
firmware-version: 0.9-4
bus-info: 0000:00:1f.6

Statistics¶

sudo ethtool -S eth0                    # NIC hardware counters (names vary by driver)
sudo ethtool -S eth0 | grep -Ei 'err|drop|crc|miss'

Rising CRC or error counters generally mean a physical problem. Rising rx_missed/fifo counters mean the host is not draining the ring buffer fast enough (see ethtool -g below).

Which physical port is eth0?¶

sudo ethtool -p eth0 30                 # blink the port LED for 30 seconds

Hardware self-test¶

sudo ethtool -t eth0 online             # non-disruptive tests
sudo ethtool -t eth0 offline            # more thorough, but takes the link down

Changing speed and duplex¶

sudo ethtool -s eth0 speed 1000 duplex full autoneg on

Forcing speed with auto-negotiation off is a common way to break a link: both ends must agree, and gigabit requires auto-negotiation by specification. Do this from a console, not over the interface you are changing. Settings made this way do not persist across reboots; put them in NetworkManager (802-3-ethernet.speed/.duplex) or a udev/systemd .link file.

Offloads and ring buffers¶

ethtool -k eth0                         # offload features (tso, gso, gro, tc-offload ...)
sudo ethtool -K eth0 gro off            # toggle one
ethtool -g eth0                         # ring buffer sizes (current vs maximum)
sudo ethtool -G eth0 rx 4096            # enlarge the receive ring

Turn offloads off when debugging with tcpdump on the sending host, or checksums will look wrong.

Wake-on-LAN¶

sudo ethtool eth0 | grep Wake-on        # 'g' = wake on magic packet, 'd' = disabled
sudo ethtool -s eth0 wol g