Ethtool Command in Linux

Introduction

The ethtool command is a powerful utility in Linux used for viewing and modifying the settings of Ethernet network interfaces. It provides system administrators with the ability to control hardware settings, diagnose network issues, and optimize network performance directly from the command line.

This utility is particularly valuable for server environments where network performance is critical, allowing administrators to fine-tune network interface parameters without having to recompile the kernel or modify hardware settings through BIOS.

Basic Syntax

The basic syntax of the ethtool command is:

ethtool [OPTIONS] DEVICE

Where:

• OPTIONS: Various flags that determine the operation to perform
• DEVICE: The network interface to operate on (e.g., eth0, ens33, enp0s3)

Key Capabilities

1. Viewing Network Interface Information

The most basic use of ethtool is to display the current settings of a network interface:

ethtool eth0

This command provides comprehensive information about the interface, including:

• Supported link modes (speed and duplex settings)
• Current speed and duplex settings
• Auto-negotiation status
• Port type
• Link status

Example output:

Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised pause frame use: No
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        MDI-X: on
        Supports Wake-on: pumbg
        Wake-on: d
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes

2. Changing Speed and Duplex Settings

One of the most common uses of ethtool is to modify the speed and duplex settings of a network interface:

ethtool -s eth0 speed 100 duplex full autoneg off

This command:

• Sets the speed to 100 Mbps
• Sets the duplex mode to full
• Disables auto-negotiation

It's important to note that changing these settings may temporarily disrupt network connectivity, and the interface may need to be restarted for changes to take effect.

3. Viewing Driver Information

To display information about the network interface driver:

ethtool -i eth0

Example output:

driver: e1000e
version: 3.2.6-k
firmware-version: 0.13-4
bus-info: 0000:00:19.0
supports-statistics: yes
supports-test: yes
supports-eeprom-access: yes
supports-register-dump: yes
supports-priv-flags: no

This information is particularly useful for troubleshooting driver-related issues or when checking compatibility with specific hardware.

4. Checking Network Statistics

To view detailed statistics for a network interface:

ethtool -S eth0

This command displays a comprehensive set of statistics, including:

• Packets received/transmitted
• Bytes received/transmitted
• Errors
• Dropped packets
• Collisions

These statistics are invaluable for diagnosing network performance issues and identifying potential hardware problems.

5. Testing Network Interface

The ethtool command can perform various tests on a network interface to check for hardware issues:

ethtool -t eth0

This runs a series of diagnostic tests on the interface and reports any failures, which can help identify hardware problems before they cause network outages.

Advanced Features

1. Managing Wake-on-LAN Settings

ethtool allows you to configure Wake-on-LAN (WoL) settings for supported network interfaces:

ethtool -s eth0 wol g

This command enables Wake-on-LAN for magic packet (g). Other options include:

• d: Disable (no wake)
• p: Wake on PHY activity
• u: Wake on unicast messages
• m: Wake on multicast messages
• b: Wake on broadcast messages
• a: Wake on ARP
• g: Wake on magic packet

2. Identifying Physical Ports

In systems with multiple network interfaces, it can be challenging to identify which physical port corresponds to which interface name. ethtool provides a solution:

ethtool -p eth0 5

This command causes the LED on the physical port associated with eth0 to blink for 5 seconds, making it easy to identify the correct port.

3. Adjusting Ring Buffer Size

For performance tuning, ethtool allows adjustment of the network interface's ring buffer size:

ethtool -g eth0      # View current ring buffer settings
ethtool -G eth0 rx 512 tx 512   # Set ring buffer sizes

Increasing the ring buffer size can help reduce packet loss in high-traffic environments, though it may increase latency slightly.

4. Managing Offload Features

Modern network cards support various offload features that can improve performance by handling certain tasks in hardware:

ethtool -k eth0      # View current offload settings
ethtool -K eth0 tso on gso on   # Enable TCP segmentation offload and generic segmentation offload

Enabling appropriate offload features can significantly improve network performance, especially for high-throughput applications.

Practical Examples

Example 1: Troubleshooting Link Issues

If a network interface is experiencing connectivity problems, you can use ethtool to check the link status:

ethtool eth0 | grep "Link detected"

If the output shows "Link detected: no", there may be a physical connection issue. You can then try:

ethtool -s eth0 autoneg off speed 100 duplex full

This disables auto-negotiation and sets a specific speed and duplex mode, which can sometimes resolve compatibility issues with older network equipment.

Example 2: Optimizing for High-Performance Networking

For servers handling high network traffic:

# Increase ring buffer sizes
ethtool -G eth0 rx 4096 tx 4096
 
# Enable hardware offloading features
ethtool -K eth0 tso on gso on gro on lro on
 
# Set interrupt coalescing parameters
ethtool -C eth0 rx-usecs 100 tx-usecs 100

These settings can significantly improve network throughput and reduce CPU usage for network processing.

Example 3: Making Settings Persistent

Changes made with ethtool are not persistent across reboots. To make them permanent, you need to add the commands to your system's network configuration.

For Ubuntu/Debian systems, add to /etc/network/interfaces:

auto eth0
iface eth0 inet dhcp
    post-up ethtool -s eth0 speed 1000 duplex full autoneg off

For RHEL/CentOS systems, create a script in /etc/sysconfig/network-scripts/:

# /etc/sysconfig/network-scripts/ifup-local
#!/bin/bash
if [ "$1" = "eth0" ]; then
    ethtool -s eth0 speed 1000 duplex full autoneg off
fi

Make the script executable with chmod +x /etc/sysconfig/network-scripts/ifup-local.

Common Options Reference

Troubleshooting with Ethtool

Issue: Link Not Detected

If ethtool eth0 shows "Link detected: no":

1. Check physical connections (cables, switch ports)
2. Try forcing speed and duplex settings:

   ethtool -s eth0 speed 100 duplex full autoneg off

3. Check for driver issues:

   ethtool -i eth0

Issue: Poor Network Performance

If experiencing slow network speeds:

1. Check current link speed:

   ethtool eth0 | grep Speed

2. Verify that the interface is operating at the expected speed
3. Check for errors in statistics:

   ethtool -S eth0 | grep -E 'error|drop|collision'

4. Optimize offload settings:

   ethtool -K eth0 tso on gso on gro on

Conclusion

The ethtool command is an essential tool for Linux system administrators managing network interfaces. It provides comprehensive capabilities for viewing and modifying Ethernet device settings, troubleshooting network issues, and optimizing network performance.

By mastering ethtool, administrators can ensure optimal network configuration, diagnose hardware problems, and fine-tune network performance for specific workloads. Whether you're managing a single server or a large data center, ethtool is an indispensable utility in your Linux networking toolkit.