ethtool

What it does

ethtool queries and modifies low-level settings of a network interface's driver and hardware — link speed, duplex mode, autonegotiation, offload features (checksum offloading, TSO/GSO/GRO), ring buffer sizes, and Wake-on-LAN settings. It also surfaces hardware-reported statistics and error counters that tools like ifconfig/ip don't show. It's a Linux-only tool (built against the kernel's ethtool ioctl interface), so it isn't available on macOS.

Beginner examples

• ethtool eth0 — show link status, speed, duplex, and autonegotiation state
• ethtool -i eth0 — show driver name, version, and firmware version
• ethtool -S eth0 — show hardware statistics (RX/TX packets, errors, drops)
• ethtool -k eth0 — show which offload features are enabled/disabled

ethtool eth0

Advanced examples

• Diagnose whether a "slow network" complaint is actually a duplex mismatch: ethtool eth0 | grep -E "Speed|Duplex|Link detected"
• Force a specific speed/duplex when autonegotiation with an old switch is unreliable: sudo ethtool -s eth0 speed 100 duplex full autoneg off
• Disable a problematic offload feature (e.g., a buggy TSO implementation causing checksum errors) while debugging: sudo ethtool -K eth0 tso off
• Increase RX ring buffer size to reduce packet drops on a high-throughput interface: sudo ethtool -G eth0 rx 4096

ethtool -S eth0 | grep -i drop

Common mistakes

• Forgetting sudo when trying to change settings with -s/-K/-G, and getting a permission-denied error.
• Force-setting speed/duplex on one end of a link without matching it on the switch — mismatched autonegotiation settings can cause a link to work but perform terribly (classic duplex mismatch symptom: works but extremely slow with high error counts).
• Assuming ethtool statistics counter names are standardized — they're driver-specific, so -S output varies significantly between NIC vendors/drivers.
• Disabling offload features (-K) for a "quick fix" without understanding the performance tradeoff — disabling GRO/TSO/GSO can resolve certain bugs but increases CPU usage significantly under load.

Tips

• Always check ethtool -i first when debugging a NIC issue — knowing the driver and firmware version quickly rules in/out known driver bugs.
• Use ethtool -S interface | grep -i err or drop to zero in on error/drop counters without scrolling through the full statistics dump.
• Leave autonegotiation on (the default) unless you have a specific, well-understood reason to force speed/duplex — modern equipment negotiates correctly almost always.

Best practices

• Prefer leaving speed/duplex/autoneg on "auto" in production; only force settings when you've confirmed a specific hardware/driver incompatibility, and document why.
• When troubleshooting throughput issues, check ethtool -S error/drop counters before assuming the problem is purely application-layer.
• Changes made with ethtool -s/-K/-G are typically not persistent across reboots — bake them into a systemd unit, udev rule, or NetworkManager dispatcher script if they need to survive a restart.

Real-world use cases

• Diagnosing "the network feels slow" complaints by confirming actual negotiated link speed and duplex rather than trusting switch port labels.
• Investigating packet loss on a high-throughput server by inspecting hardware RX/TX error and drop counters that aren't visible via ifconfig/ip.
• Tuning NIC offload features and ring buffer sizes on a database or storage server to reduce CPU overhead or packet drops under heavy load.

Common interview questions

• What layer does ethtool operate at compared to ifconfig/ip? ethtool works at the driver/hardware level (link speed, duplex, offload features, hardware stats), while ifconfig/ip operate at the IP/link-configuration layer (addresses, routes, up/down).
• What's a classic symptom of a duplex mismatch, and how would you check for it? A link that appears "up" but has poor throughput and high error/collision counts; check with ethtool eth0 (Speed/Duplex fields) and ethtool -S eth0 (error counters) on both ends of the link.
• How would you check what driver and firmware version a NIC is using? ethtool -i eth0.