如何阻止Netplan为每个网络接口绑定默认网关？

I ran into this exact same headache when switching to Netplan on Ubuntu Server, so I totally get how frustrating it is to have both interfaces grabbing a default gateway when you only want one. Let's break down how to fix this:

Step 1: Locate Your Netplan Config File #

First, head over to the Netplan configuration directory. You'll find a YAML file here—usually named something like 01-netcfg.yaml or 50-cloud-init.yaml if you used cloud-init during setup:

cd /etc/netplan/
ls

Step 2: Edit the Config to Explicitly Define Interface Behavior #

Open the file with your preferred editor (I use nano for simplicity) and structure it so only your external DHCP interface gets a default gateway. Replace the interface names (enp0s3/enp0s8) with your actual NIC names (find them with ip link show):

network:
  version: 2
  renderer: networkd  # Server edition uses networkd; use NetworkManager for desktop setups
  ethernets:
    enp0s3:  # Trusted internal LAN interface
      addresses:
        - 192.168.1.10/24  # Your static internal IP address
      nameservers:
        addresses: [192.168.1.1]  # Internal DNS server (if you have one)
      # Critical: Do NOT set gateway4 or any default routes here
    enp0s8:  # Untrusted external WAN interface
      dhcp4: true
      # Optional: If DHCP doesn't set the default gateway correctly, force it explicitly
      routes:
        - to: 0.0.0.0/0
          via: 203.0.113.1  # Replace with your ISP's actual gateway IP
          metric: 100  # Lower metric = higher priority, ensures this is the default route

YAML Syntax Tips to Avoid Errors: #

• Indentation is non-negotiable: Use 2 spaces per level (never tabs—YAML will throw errors).
• The internal interface has no gateway or default route defined, so Netplan won't assign one automatically.
• The metric value guarantees your external gateway takes precedence if any conflicting routes pop up.

Step 3: Apply the Configuration #

Save the file, then load the new settings without rebooting:

sudo netplan generate && sudo netplan apply

generate validates your config first, and apply pushes the changes live.

Step 4: Verify the Routing Table #

Check that only your external interface has a default gateway with:

ip route show

You should see output like this (only one default entry):

default via 203.0.113.1 dev enp0s8 proto dhcp src 203.0.113.10 metric 100
192.168.1.0/24 dev enp0s3 proto kernel scope link src 192.168.1.10
203.0.113.0/24 dev enp0s8 proto kernel scope link src 203.0.113.10

If you still see a default gateway for the internal interface, double-check your config for accidental gateway4 entries or extra routes—Netplan will always respect explicit settings you add.

内容的提问来源于stack exchange，提问作者Aaron Murray