How to Send Outbound Traffic Over a Reserved IP

You can configure your Droplet’s network settings to send outbound traffic over a reserved IP address assigned to the Droplet. This causes traffic to originate from the reserved IP address instead of the Droplet’s original IPv4 address.

Prerequisites

To configure a Droplet to send its outbound traffic over a reserved IP address, you must first assign the reserved IP address to the Droplet and then obtain the Droplet’s gateway anchor IP address.

Most Droplets already have an anchor IP, but Droplets created before October 2015 and Droplets created using custom images don’t have anchor IPs assigned by default.

On Droplets without an anchor IP, first manually assign an anchor IP to the Droplet, then continue following this guide.

On Droplets with an anchor IP, get the gateway address by querying its metadata using a curl request. The -s flag mutes any progress meters or error messages and returns only the output.

curl -s http://169.254.169.254/metadata/v1/interfaces/public/0/anchor_ipv4/gateway

The command returns the anchor IP’s gateway address. You need to use this address to update your server’s default IPv4 gateway, enabling outbound traffic from your reserved IP.

Enable Outbound Reserved IP Traffic Immediately

To immediately update your network configuration, use the ip route command to add this address as the gateway for the default route. The following command removes the default route from your Droplet’s public network interface and replaces it with a route that uses the anchor’s gateway IP address. Replace <anchor-gateway-IP-address> with the IP address you retrieved in the previous step:

sudo sh -c "ip route del 0/0; ip route add default via <anchor-gateway-IP-address> dev eth0"

The command may take a moment to complete and prints no output.

Verify that the Droplet’s traffic is being routed through the reserved IP address by sending a curl request to icanhazip.com, a website that returns the request’s originating public IP. The -4 flag instructs curl to use the Droplet’s IPv4 address only:

curl -4 https://icanhazip.com/

Changes made with the ip route command are lost when you restart your Droplet. To make the setting persist after reboot, you need to modify the Droplet’s network configuration files. How you do this depends on which operating system you’re using.

Persist Outbound Reserved IP Traffic After Reboot

First, disable cloud-init’s automatic network configuration, otherwise your settings could be overwritten:

echo "network: {config: disabled}" | sudo tee /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg

sudo nano /etc/netplan/50-cloud-init.yaml

network:
    version: 2
    ethernets:
        eth0:
            addresses:
            - 203.0.113.216/20
            - 10.17.0.5/16
            match:
                macaddress: da:f8:7a:69:ce:ea
            mtu: 1500
            nameservers:
                addresses:
                - 67.207.67.2
                - 67.207.67.3
                search: []
            routes:
            -   to: 0.0.0.0/0
                via: <anchor-gateway-IP-address>
            set-name: eth0
        eth1:
            addresses:
            - 10.132.0.5/16
            match:
                macaddress: a6:08:53:fb:fb:7d
            mtu: 1500
            nameservers:
                addresses:
                - 67.207.67.2
                - 67.207.67.3
                search: []
            set-name: eth1

sudo netplan apply

sudo reboot

sudo nano /etc/network/interfaces

auto lo
iface lo inet loopback
        dns-nameservers  67.207.67.2 67.207.67.3

auto eth0
iface eth0 inet static
        hwaddress 5e:6c:28:98:28:ce
        address   203.0.113.216
        netmask   255.255.240.0
        gateway   <anchor-gateway-IP-address>
        post-up ifup eth0:1

sudo systemctl restart networking

sudo reboot

nano /etc/NetworkManager/system-connections/cloud-init-eth0.nmconnection

[connection]
id=cloud-init eth0
uuid=1dd9a779-d327-56e1-8454-c65e2556c12c
type=ethernet

[user]
org.freedesktop.NetworkManager.origin=cloud-init

[ethernet]
mtu=1500
mac-address=E2:67:39:7C:55:85

[ipv4]
method=manual
may-fail=false
address1=143.110.211.104/20
route1=0.0.0.0/0,<anchor-gateway-IP-address>    
address2=10.20.0.7/16

sudo reboot

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0

BOOTPROTO=none
DEFROUTE=yes
DEVICE=eth0
GATEWAY=<anchor-gateway-IP-address>
HWADDR=36:7d:f2:8d:72:15
IPADDR=203.0.113.216
IPADDR1=10.17.0.5
MTU=1500
NETMASK=255.255.240.0
NETMASK1=255.255.0.0
ONBOOT=yes
TYPE=Ethernet
USERCTL=no

sudo reboot

Once the Droplet has rebooted, log back in to the Droplet and verify that its traffic is being routed through the reserved IP address by sending another curl request to icanhazip.com:

curl -4 https://icanhazip.com/

Disable Outbound Reserved IP Traffic

To reverse this change, first retrieve the gateway address of the Droplet’s public interface:

curl -s http://169.254.169.254/metadata/v1/interfaces/public/0/ipv4/gateway

The command returns the interface’s gateway address.

Use the ip route command to remove the reserved IP gateway and add back the original gateway address of the Droplet. Replace with the IP address you retrieved in the previous step:

sudo sh -c "ip route del 0/0; ip route add default via <gateway-IP-address> dev eth0"

The command may take a moment to complete and prints no output.

Verify that the Droplet’s traffic is being routed through its assigned IP address by sending a curl request to icanhazip.com, a website that returns the request’s originating public IP. The -4 flag instructs curl to use the Droplet’s IPv4 address only:

curl -4 https://icanhazip.com/

Changes made with the ip route command are lost when you restart your Droplet. If you followed the Persist Outbound Reserved IP Traffic After Reboot section above, you need to revert those changes, as well. Follow the those directions again using the gateway address of the Droplet’s public interface that you retrieved at the beginning of this section.