When running dmesg inside a Docker container, you'll often encounter the frustrating "Permission denied" error. This happens because by default, Docker containers run with restricted capabilities that don't include access to kernel logs.

docker run -it ubuntu dmesg
# Output: dmesg: read kernel buffer failed: Permission denied

Many users first try running with sudo inside the container, but this doesn't work because:

docker run -it ubuntu sudo dmesg
# Same error occurs because the container itself lacks privileges

The issue isn't about user permissions inside the container, but rather the container's access to the host's kernel ring buffer.

1. Running with Elevated Privileges

The most straightforward solution is to run the container with the --privileged flag:

docker run --privileged -it ubuntu dmesg

However, this grants the container extensive privileges, which may not be ideal for security.

2. Specific Capability Grant

A more secure approach is to grant just the necessary capability:

docker run --cap-add=SYSLOG -it ubuntu dmesg

This specifically allows access to kernel logs without giving full privileged access.

3. Host Machine Alternative

If you only need occasional access to dmesg logs, consider running it on the host machine:

docker run -v /dev/kmsg:/dev/kmsg -it ubuntu dmesg

This mounts the kernel message device into the container.

For production environments, consider these best practices:

• Use dedicated logging solutions (Fluentd, Logstash) instead of direct dmesg access
• Implement proper log rotation for kernel messages
• Monitor kernel logs at the host level rather than container level

If you can't modify container privileges, consider these alternatives:

# View recent logs (may not show all messages)
docker run -it ubuntu cat /var/log/kern.log

# Alternative system log access
docker run -it ubuntu journalctl -k

Remember that different Linux distributions may store kernel logs in different locations.

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When running dmesg inside Docker containers, you might encounter the frustrating error:

dmesg: read kernel buffer failed: Permission denied

This occurs because modern Linux systems restrict access to kernel logs through /dev/kmsg by default. The error persists even when using sudo inside the container because the container itself lacks the necessary privileges.

The dmesg command requires the CAP_SYSLOG capability to read kernel messages. In Docker's default security configuration, containers run without this capability for security reasons.

Method 1: Running with Elevated Privileges

The simplest solution is to run the container with the --privileged flag:

docker run --privileged -it your_image dmesg

Note: This gives the container extensive system access, so use cautiously in production.

Method 2: Adding Specific Capabilities

A more secure approach is to grant only the required capability:

docker run --cap-add SYSLOG -it your_image dmesg

Alternatively, you can use:

docker run --cap-add CAP_SYSLOG -it your_image dmesg

Method 3: Adjusting Kernel Parameters

For systems where you control the host:

# Make kernel logs readable by all users
echo kernel.dmesg_restrict=0 | sudo tee -a /etc/sysctl.conf
sudo sysctl -p

While these solutions work, consider the security implications:

• Granting CAP_SYSLOG allows reading all kernel messages
• The --privileged flag gives near-host-level access
• Changing dmesg_restrict affects the entire system

If you're debugging systemd issues (as referenced in the original question), consider these alternatives:

# Check journal logs (if journald is running)
docker run -it your_image journalctl

# Or mount the host's logs
docker run -v /dev/log:/dev/log -it your_image

For production environments, prefer:

1. Using specific capabilities (--cap-add) over full privileges
2. Mounting only necessary host resources instead of granting broad access
3. Implementing proper logging from applications rather than relying on system logs