When implementing Docker with userns-remap for enhanced security, many developers encounter the frustrating "ping: permission denied (are you root?)" error. This occurs because user namespace remapping affects how Linux capabilities are handled within containers.

The ping command requires two specific capabilities to function:

CAP_NET_RAW (for raw socket creation)
CAP_NET_ADMIN (for network interface operations)

With user namespace remapping enabled, these capabilities aren't properly mapped to the container's user context. Even if you're running as root inside the container, the host system sees a non-privileged user.

Here are three approaches to resolve this:

1. Add Required Capabilities

Modify your container run command or compose file:

docker run --cap-add=NET_RAW --cap-add=NET_ADMIN your_image

Or in docker-compose:

services:
  your_service:
    image: your_image
    cap_add:
      - NET_RAW
      - NET_ADMIN

2. Use Setuid Binary Workaround

For Alpine-based containers, you can reinstall ping with setuid:

FROM alpine:latest
RUN apk add --no-cache busybox-extras
RUN chmod u+s /bin/ping
USER containeruser

3. Network Namespace Sharing

For containers needing to ping each other:

docker run --network container:other_container your_image

While adding capabilities solves the immediate problem, consider:

• Using dedicated network debugging containers instead of enabling ping everywhere
• Implementing network policy controls
• Monitoring capability usage in production

To diagnose capability issues:

# Check effective capabilities inside container
getpcaps 1

# Verify user namespace mapping
cat /proc/self/uid_map

---

When working with Docker's userns-remap feature for enhanced security, many developers encounter the frustrating "permission denied" error when trying to use basic network utilities like ping. This occurs even between containers in the same network.

The fundamental problem stems from how Linux capabilities are handled in user namespaces. The ping command requires CAP_NET_RAW capability, which gets filtered out when using user namespace remapping. Even if you explicitly add capabilities via --cap-add, the kernel still enforces restrictions in user namespaces.

# Current ineffective approach many try
docker run --cap-add=NET_RAW -it alpine ping 172.16.0.3
# Still results in "ping: permission denied"

Option 1: Setcap Binary Modification

The most reliable solution involves modifying the ping binary's capabilities:

# In your Dockerfile
RUN apk add --no-cache libcap && \
    setcap cap_net_raw+p /bin/busybox

# Or alternatively for full ping package
RUN apk add --no-cache iputils && \
    setcap cap_net_raw+p /bin/ping

Option 2: Kernel Parameter Adjustment

For development environments, you can modify kernel parameters:

# Add to /etc/sysctl.conf
kernel.capabilities=1

# Or temporarily
echo 1 > /proc/sys/kernel/capabilities

Option 3: SUID Bit Alternative

A less secure but functional workaround:

RUN chmod u+s /bin/ping
# Or for busybox
RUN chmod u+s /bin/busybox

While these solutions work, they each have security implications:

• setcap approach is the most granular but requires proper filesystem capabilities support
• Kernel parameter changes affect the entire system
• SUID bits create potential privilege escalation vectors

For production environments, consider implementing network health checks through alternative methods:

# Using curl for HTTP checks
HEALTHCHECK --interval=30s --timeout=3s \
  curl -f http://localhost/ || exit 1

# Or using nc for TCP checks
HEALTHCHECK --interval=30s --timeout=3s \
  nc -zv 172.16.0.3 80 || exit 1

After implementing any solution, verify with:

docker run --rm -it your_image sh -c "getcap /bin/ping && ping -c 1 172.16.0.3"