Delete Default Gateway on Ubuntu 20.04
A default gateway is an IP address usually assigned automatically to a computer on a network that allows it to connect to other networks. It is the address of the router or other device that connects the computer to the wider network. In Ubuntu 20, multiple default gateways can be assigned to a computer, providing redundancy in case one gateway becomes unavailable. However, in some situations, it may be necessary to delete one of the default gateways.

Before deleting a default gateway, check if the remaining gateways are functioning correctly. To view the list of default gateways assigned to a computer, open the terminal and type the command “ip route”. The output will display a list of all routes, including the default gateways. If any of the gateways are marked as “down” or “unreachable,” they should be deleted. To delete a default gateway, use the command “ip route del default via ” where is the IP address of the gateway you want to delete. For example, to delete the default gateway with the IP address 192.168.1.1, you would type the command “ip route del default via 192.168.1.1”.

After deleting a default gateway, verify that the computer can still connect to the network. If the computer is unable to connect, it may be necessary to add a new default gateway. To add a new default gateway, use the command “ip route add default via ” where is the IP address of the new gateway. For example, to add a new default gateway with the IP address 192.168.1.2, you would type the command “ip route add default via 192.168.1.2”.

Understanding Default Gateways

A default gateway is a network device that connects a local area network (LAN) to other networks. It acts as the primary route for network traffic between the LAN and the rest of the Internet. Multiple default gateways can be configured on a system, but only one can be active at a time.

By default, Ubuntu assigns a default gateway to each network interface. However, you may encounter situations where you need to modify the default gateway configuration, such as when adding a new network route or removing an existing one.

Route Table

The kernel maintains a routing table that stores the list of default gateways and their associated network routes. You can view the routing table using the following command:

$ netstat -nr

The output of this command will display a table similar to this:

Destination	Gateway	Genmask	Flags	MSS	Window	irtt	Iface
0.0.0.0	192.168.1.1	0.0.0.0	UG	0	0	0	eth0
192.168.1.0	0.0.0.0	255.255.255.0	U	0	0	0	eth0

In this table, the first entry represents the default gateway for the system. It shows that the gateway IP address is 192.168.1.1 and the interface is eth0.

Identifying Default Gateways

To identify the default gateway on your Ubuntu 20 system, you can use the following steps:

1. Open a terminal window

To open a terminal window, press Ctrl+Alt+T on your keyboard.

2. Run the following command:

“`
route -n
“`

This command will display the routing table for your system. The default gateway will be listed as the gateway for the 0.0.0.0/0 network.

For example, if your default gateway is 192.168.1.1, the output of the command will look like this:

“`
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 100 0 0 eth0
“`

3. Find the default gateway

The default gateway is the IP address listed in the “Gateway” column for the 0.0.0.0/0 network.

In the example above, the default gateway is 192.168.1.1.

Removing Gateways with the Delete Option

If you have added multiple default gateways to your Ubuntu 20 system, you may want to delete one of them. You can do this using the ip command with the delete option. The syntax for this command is as follows:

“`
ip route del default via GATEWAY_ADDRESS dev INTERFACE
“`

Where:

* GATEWAY_ADDRESS is the IP address of the gateway you want to delete.
* INTERFACE is the name of the interface that the gateway is associated with.

For example, to delete the default gateway 192.168.1.1 on the interface enp0s3, you would run the following command:

“`
ip route del default via 192.168.1.1 dev enp0s3
“`

You can also use the nmcli command to delete a default gateway. The syntax for this command is as follows:

“`
nmcli connection modify CONNECTION_NAME ipv4.gateway “”
“`

Where:

* CONNECTION_NAME is the name of the network connection that you want to modify.

For example, to delete the default gateway for the network connection Wired connection 1, you would run the following command:

“`
nmcli connection modify “Wired connection 1” ipv4.gateway “”
“`

Once you have deleted the default gateway, your system will no longer use it to route traffic. You can verify that the gateway has been deleted by running the ip route command.

Here is a table summarizing the commands you can use to delete a default gateway on Ubuntu 20:

Command	Description
`ip route del default via GATEWAY_ADDRESS dev INTERFACE`	Deletes the default gateway `GATEWAY_ADDRESS` on the interface `INTERFACE`.
`nmcli connection modify CONNECTION_NAME ipv4.gateway ""`	Deletes the default gateway for the network connection `CONNECTION_NAME`.

Modifying the Gateway Table

To modify the default gateway table, you’ll need to use the ‘route’ command. This command allows you to view and manipulate the kernel’s routing table, which contains a list of all the known routes to different networks and their corresponding gateways.

Here’s an example of how to use the ‘route’ command to view the current default gateway:

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eno1
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eno1

As you can see, the default gateway for this system is 192.168.1.1. To delete this gateway, you would use the following command:

$ sudo route del default gw 192.168.1.1

This command will remove the default gateway from the routing table. You can verify that the gateway has been deleted by running the ‘route -n’ command again.

Gateway	Action
192.168.1.1	Delete

Once you have deleted the default gateway, you will need to add a new one. To do this, you can use the ‘route add’ command. For example, to add a new default gateway of 192.168.2.1, you would use the following command:

$ sudo route add default gw 192.168.2.1

You can verify that the new gateway has been added by running the ‘route -n’ command again.

Permanent Gateway Removal

Step 1: Identify the Default Gateway

Before removing a default gateway, you need to identify it using the ‘route -n’ command:

“`Bash
route -n | grep “UG” | awk ‘{print $2}’
“`

Step 2: Open the Network Interfaces File

Use a text editor to open the network interfaces file:

“`Bash
sudo nano /etc/network/interfaces
“`

Step 3: Locate the Gateway Configuration

Locate the section for the network interface that uses the default gateway you want to remove. It usually starts with “auto eth0” or “iface eth0 inet static”.

Step 4: Remove the Gateway Line

Within the interface section, find the line that contains “gateway” and delete it.

Step 5: Save and Exit the File

Save the changes to the file and exit the editor.

Step 6: Restart Networking

Finally, restart the networking service using one of the following methods:

Method	Command
Systemd (Ubuntu 16.04 and later)	sudo systemctl restart networking
SysV (Ubuntu 14.04 and earlier)	sudo service networking restart

How to Delete One Default Gateway on Ubuntu 20

Prerequisites

Ensure you have superuser privileges (root) or have access to a user account configured with ‘sudo’ privileges.

Have a network interface configured with multiple default gateways.

Procedure

Identify the Default Gateway to be Removed
Execute the command:
```
route -n
```
to display the network routing table.
Delete the Default Gateway
Use the command:
```
route del default gw [Default Gateway Address]
```
where ‘[Default Gateway Address]’ is the IP address of the gateway to delete.
Verify the Gateway Removal
Run the command:
```
route -n
```
again to verify that the default gateway entry has been removed.

Common Gateway Removal Errors

Error: Destination Address Required

Possible Causes:

The ‘route del’ command was executed without specifying a destination address (default gateway address).

Solution:

Ensure to provide the IP address of the default gateway to delete in the ‘route del’ command.

Error: Network is Unreachable

Possible Causes:

After removing the default gateway, no other valid default gateway is available for the network interface.

Solution:

Configure an alternative default gateway for the network interface to ensure continued network connectivity.

Error: Sudo Privileges Required

Possible Causes:

The command was executed without elevated privileges.

Solution:

Execute the command with ‘sudo’ or switch to a user account with root privileges.

Troubleshooting Gateway Removal Issues

1. Verify Gateway Removal

Run the ‘route -n’ command to confirm the gateway has been removed. If it is still listed, repeat the deletion process.

2. Network Interface Status

Check the status of the network interface using ‘ifconfig’. Ensure it has an IP address and is configured properly.

3. Default Route Table

Use ‘ip route’ to check the default route table. If a default gateway is still present, remove it using ‘ip route del default via [gateway IP]’.

4. Gateway is Dynamic

If the gateway is assigned dynamically through DHCP, it may automatically re-add itself after removal. Disable DHCP or manually configure a static IP address.

5. Network Manager

Check the NetworkManager configuration. It may be overriding the gateway deletion. Disable or modify its settings accordingly.

6. Systemd Network Configuration

In systemd-based systems, network configuration may be managed through systemd services. Check for any gateway definitions in ‘/etc/systemd/network’ and modify them if necessary.

7. Persistent Network Scripts

In Ubuntu 20, persistent network scripts may still be used. Check ‘/etc/network/interfaces’ for any gateway configurations and remove them manually.

8. In-Depth Troubleshooting

For more detailed troubleshooting, consider the following steps:

Command	Description
‘route -nee’	Displays all routing entries, including the gateway
‘grep default /etc/resolv.conf’	Checks DNS settings for gateway information
‘traceroute [destination IP]’	Traces the network path to the destination, revealing any gateway hops
‘sudo iptables -L’	Lists active firewall rules that may be affecting gateway removal
‘sudo tcpdump -i [interface] -n’	Captures network traffic to observe gateway interactions

Best Practices for Gateway Management

To ensure the reliability and efficiency of network connectivity, it is essential to adhere to best practices for gateway management:

Determine Critical Gateways: Identify and prioritize gateways that are crucial for business operations and should be closely monitored.
Monitor Gateway Health: Regularly monitor gateway performance metrics like uptime, latency, and bandwidth utilization to detect potential issues early on.

Monitoring Parameter	Significance
Uptime	Ensures gateway availability and continuity of network services.
Latency	Measures the time taken for packets to traverse the gateway, impacting network responsiveness.
Bandwidth Utilization	Indicates the extent to which the gateway is being used, helping identify potential bottlenecks.

Redundancy and Load Balancing: Implement gateway redundancy and load balancing to minimize the impact of single gateway failures and distribute traffic efficiently.
Firmware Updates: Regularly apply firmware updates to keep gateways secure and address any performance issues or vulnerabilities.

9. Gateway Removal and Reconfiguration

To remove an unnecessary or redundant gateway:

Identify Gateway to Remove: Determine which gateway should be removed based on network requirements and redundancy levels.
Update Routing Tables: Modify routing tables to remove the gateway’s IP address as a default gateway.
Reconfigure Gateway: Disable the gateway and set it to a non-default status to prevent conflicts.
Verify Operation: Test network connectivity to ensure no disruption occurred due to gateway removal.

Additional Considerations

1. Multiple Gateways

If your system has multiple gateways, you may need to specify the gateway you want to delete using its numerical interface, e.g., “sudo route del default gw .”

2. Network Connectivity

Deleting the default gateway without configuring a new one can disrupt network connectivity. Ensure you have an alternative gateway in place before proceeding.

3. Persistent Changes (Optional)

By default, changes made to the routing table are temporary and will be lost upon system reboot. To make the deletion persistent, use:
“`
sudo nano /etc/network/interfaces
“`
Find the section for your network interface, add the following line:

“`
gateway
“`
Save and close the file.

4. Network Manager

If you’re using Network Manager, it may automatically add a default gateway. To prevent this, disable the “Manage Ethernet with Network Manager” option or use:

“`
sudo nmcli connection modify ipv4.method manual
“`

5. IPv6 Gateways

If you’re working with IPv6, the commands and syntax will be similar, but the “default gw” will be replaced with “default ::.”

6. Route Table Verification

Run “sudo netstat -r” to verify that the default gateway has been removed.

7. DNS Resolution

Deleting the default gateway may affect DNS resolution. Configure an alternate DNS server or check your system’s settings to ensure proper resolution.

8. Static IP Address

If your system is using a static IP address, deleting the default gateway will not automatically remove the static IP configuration. Disable or modify the static configuration as needed.

9. Troubleshooting

If you encounter errors or problems, check the following:
– Network cable connections and hardware
– Firewall settings
– DHCP configuration (if applicable)
– Other network services (e.g., DNS, NTP)

10. Routing Table Optimization

Consider optimizing your routing table by removing unused or redundant routes. This can improve network performance and reduce latency:
– Run “sudo route -n | grep UG” to find unused gateways
– Delete unused gateways using “sudo route del gw ”
– Run “sudo route -n | grep U” to find unreachable routes
– Delete unreachable routes using “sudo route del ”