Ubuntu Ethernet Login Page Not Loading

Symptoms

When connecting to campus ethernet, a login page (e.g. gateway.iitj.ac.in) normally opens for authentication.

On Ubuntu the interface shows Connected, but:

No websites load (even google.com)
Login page never appears
Browser shows ERR_ADDRESS_UNREACHABLE
Pinging the gateway returns Destination Host Unreachable

Linux always prefers local routes over default routes. If Docker/VPN creates a bridge using the same subnet as your campus gateway, the OS thinks the gateway is local and sends packets into that virtual bridge instead of the real network.

Result: No packets reach the gateway → captive portal never loads.

Diagnosis

First, check where the gateway IP is being routed:

getent hosts gateway.iitj.ac.in
ip route get <gateway-ip>

If the route shows something like:

dev docker0
dev br-xxxx
dev tailscale0

instead of your ethernet interface (e.g., enpXsY), then traffic is being captured by a local virtual network instead of the real gateway.

Common Causes

Docker default bridge (172.17.0.0/16)
Docker Compose networks
Old/stale Docker bridges
VPNs (Tailscale, etc.) adding routing rules
Any local subnet overlapping the campus network

Example of a broken route:

172.17.0.3 dev docker0 src 172.17.0.1

Packets never leave your laptop.

Solution: Move Docker Away from Common Private Ranges

Step 1: Edit Docker daemon config

sudo nano /etc/docker/daemon.json

Use a safe, uncommon subnet:

{
  "default-address-pools": [
    {
      "base": "10.200.0.0/16",
      "size": 24
    }
  ]
}

Step 2: Restart Docker

sudo systemctl restart docker

Step 3: Recreate Compose networks (important)

Old networks keep old subnets. Remove them:

docker compose down
docker network prune -f

Then start again:

docker compose up -d

Step 4: Delete any stale bridges (if still present)

ip a | grep br-
sudo ip link delete br-xxxx

Verification

Check that the route now points to your ethernet interface:

ip route get <gateway-ip>

It should now show:

via <ethernet-gateway> dev enpXsY