Fixing Docker Container DNS Resolution Issues on Ubuntu 14.04 Host

When running Docker containers on Ubuntu 14.04, you might encounter a frustrating situation where DNS resolution fails inside containers while basic network connectivity (like ping) works perfectly. The error typically appears when trying to run apt-get update or other network operations requiring DNS:

Err http://archive.ubuntu.com trusty InRelease
Cannot initiate the connection to archive.ubuntu.com:80 (2001:67c:1360:8c01::19). 
- connect (101: Network is unreachable) [IP: 2001:67c:1360:8c01::19 80]

Key symptoms include:

Containers can ping external IPs (8.8.8.8) and domains (google.com)
DNS-based operations fail (apt-get, curl domain names)
IPv6 errors appear despite IPv6 being disabled on host
Changing to Google DNS (8.8.8.8) doesn't resolve the issue

First, check the container's network interfaces:

docker run -it ubuntu /bin/bash
root@container-id:/# cat /etc/resolv.conf
root@container-id:/# apt-get update -o Debug::Acquire::http=1

You'll likely see Docker's default bridge network configuration with proper IP assignment but broken DNS resolution.

The issue stems from how Docker handles DNS on Ubuntu 14.04. Here's the definitive fix:

# Stop Docker service
sudo service docker stop

# Edit Docker configuration
sudo nano /etc/default/docker

# Add these lines (or modify existing DOCKER_OPTS):
DOCKER_OPTS="--dns 8.8.8.8 --dns 8.8.4.4 --dns-search=."

# Restart Docker
sudo service docker start

For newer Docker installations, use the daemon.json approach:

sudo mkdir -p /etc/docker
sudo tee /etc/docker/daemon.json <<EOF
{
  "dns": ["8.8.8.8", "8.8.4.4"],
  "dns-search": ["."]
}
EOF

sudo service docker restart

After applying the changes, test DNS resolution:

docker run --rm ubuntu bash -c "cat /etc/resolv.conf; apt-get update"

You should now see successful package updates without DNS errors.

The solution works because:

Forces Docker to use reliable public DNS servers
Prevents conflicts with host network configurations
Works around Ubuntu 14.04's specific networking quirks
Maintains compatibility with both IPv4 and IPv6 operations

If issues persist, try these steps:

# Flush Docker networks
docker network prune

# Check iptables rules
sudo iptables -L -n

# Verify DNS in container
docker run --rm busybox nslookup google.com

When working with Docker on Ubuntu 14.04, I encountered a particularly frustrating scenario where containers suddenly lost DNS resolution after initially working fine. The containers could ping external IP addresses but failed to resolve domain names, making package management impossible.

# Typical error output showing IPv6 connection failures
Err http://archive.ubuntu.com trusty InRelease
Cannot initiate the connection to archive.ubuntu.com:80 (2001:67c:1360:8c01::19). 
- connect (101: Network is unreachable) [IP: 2001:67c:1360:8c01::19 80]

Before diving into solutions, let's verify some basic connectivity from within a container:

docker run -it ubuntu:14.04 /bin/bash

# Inside container:
ping 8.8.8.8  # Should work
ping google.com  # Might work initially but fail later
apt-get update  # Will likely fail

The key observations from my troubleshooting:

Ping to IP addresses succeeds (both IPv4 and IPv6)
DNS resolution fails for package repositories
Forcing IPv4 doesn't solve the issue
Even after changing to Google's DNS (8.8.8.8), problems persist

After extensive testing, here are the effective approaches:

Solution 1: Configure Docker Daemon DNS Settings

Edit /etc/default/docker:

DOCKER_OPTS="--dns 8.8.8.8 --dns 8.8.4.4 --dns-search ."

Then restart Docker:

sudo service docker restart

Solution 2: Disable IPv6 System-Wide

Edit /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Apply changes:

sudo sysctl -p

Solution 3: Custom Container DNS Configuration

Run containers with explicit DNS settings:

docker run --dns=8.8.8.8 --dns=8.8.4.4 -it ubuntu:14.04

These commands help diagnose networking issues:

# Check container network interfaces
docker exec -it container_name cat /etc/resolv.conf

# Inspect Docker network configuration
docker network inspect bridge

# Check DNS resolution inside container
docker exec -it container_name nslookup google.com

For Docker builds failing due to DNS, add this to your Dockerfile:

RUN echo 'Acquire::ForceIPv4 "true";' > /etc/apt/apt.conf.d/99force-ipv4

After applying fixes, verify DNS works in new containers:

docker run --rm ubuntu:14.04 sh -c 'apt-get update && apt-get install -y dnsutils && nslookup google.com'