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DockerConflict.md

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Resolving Docker Network Conflict with College or VPN Network

Problem Overview

When Docker is installed, it creates a default network bridge (docker0) with the subnet 172.17.0.0/16. If this subnet overlaps with your college network or VPN, it can cause routing conflicts, making it impossible to access the internet or sign in to network-dependent services.

This document explains how to identify the issue, diagnose the conflict, and resolve it by changing Docker’s default bridge network.

Step 1: Understanding the Problem

When Docker starts, it automatically configures a virtual network for containers. This network, by default, uses the 172.17.0.0/16 subnet. If your college’s network or VPN uses an overlapping subnet, network traffic may be misrouted.

For example:

  • Your college network uses 10.9.0.0/21.
  • Docker creates a route for 172.17.0.0/16.

The conflicting route can prevent proper internet access.

Step 2: Diagnosing the Conflict

1. Check Current Routes

Run the following command to display your system’s routing table:

ip route

Example output:

default via 10.9.0.1 dev eth0 proto dhcp metric 20102 
10.9.0.0/21 dev eth0 proto kernel scope link src 10.9.1.178 metric 102 
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown

Here, the route 172.17.0.0/16 corresponds to Docker’s docker0 interface. If this overlaps with your network’s range, it will cause a problem.

2. Check Docker’s docker0 Subnet

To confirm Docker’s default subnet, run:

ip addr show docker0

Example output:

inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0

This shows that Docker is using 172.17.0.0/16.

Step 3: Temporary Fix

Delete the Conflicting Route

If you need immediate internet access, you can temporarily delete the Docker route:

sudo ip route del 172.17.0.0/16

Check the routing table again to confirm:

ip route

Example output:

default via 10.9.0.1 dev eth0 proto dhcp metric 20102 
10.9.0.0/21 dev eth0 proto kernel scope link src 10.9.1.178 metric 102

Now, you should be able to access the internet. However, this fix is temporary, as Docker will recreate the route on restart.

Step 4: Permanent Solution

To permanently resolve the issue, reconfigure Docker to use a non-conflicting subnet.

1. Edit Docker Configuration

Docker’s default network settings are controlled by the file /etc/docker/daemon.json. If the file does not exist, you can create it.

Open the file for editing:

sudo nano /etc/docker/daemon.json

Add the following configuration:

{
    "bip": "192.168.100.1/24"
}
  • bip: Specifies the Bridge IP for Docker’s docker0 network.
  • 192.168.100.1/24: A private subnet unlikely to conflict with other networks.

2. Restart Docker

Apply the changes by restarting Docker:

sudo systemctl restart docker

3. Verify the New Subnet

Check the docker0 interface to ensure it uses the new subnet:

ip addr show docker0

Example output:

inet 192.168.100.1/24 brd 192.168.100.255 scope global docker0

4. Confirm Routing Table

Verify that the routing table no longer includes the conflicting subnet:

ip route

Example output:

default via 10.9.0.1 dev eth0 proto dhcp metric 20102 
10.9.0.0/21 dev eth0 proto kernel scope link src 10.9.1.178 metric 102
192.168.100.0/24 dev docker0 proto kernel scope link src 192.168.100.1

Step 5: Test Docker and Internet Access

Test Internet Access

Ensure you can access the internet:

ping google.com

Example output:

PING google.com (142.250.183.78) 56(84) bytes of data.
64 bytes from bom07s27-in-f14.1e100.net (142.250.183.78): icmp_seq=1 ttl=118 time=5.02 ms

Test Docker Functionality

Run a simple Docker container to ensure everything works:

docker run hello-world

Example output:

Hello from Docker!
This message shows that your installation appears to be working correctly.

Troubleshooting Tips

  • Invalid JSON Configuration: If Docker fails to start after editing /etc/docker/daemon.json, check for syntax errors using: 
    sudo systemctl status docker
  • Still Unable to Access Internet: Verify there are no other conflicting routes in the output of ip route.
  • VPN Conflicts: If using a VPN, ensure its subnet does not overlap with Docker’s new subnet.

Conclusion

By reconfiguring Docker’s default network bridge to use a non-conflicting subnet, you can resolve routing conflicts with your college or VPN network. This ensures seamless internet access and Docker functionality.

If you encounter further issues, feel free to revisit the diagnostic steps or modify the Docker configuration as needed.