This article explains how to configure an Additional IP Address on AlmaLinux 9 using NetworkManager Policy Routing, allowing the additional IP address to route traffic through its own gateway correctly.

How to Configure an Additional IP Address on AlmaLinux 9 with NetworkManager

Before you begin, make sure you have the following information available:

• Primary IP Address
• Additional IP Address
• Gateway for the Additional IP Address
• SSH access to the server
• Root privileges or a user account with sudo permission

This guide is intended for AlmaLinux 9 servers that manage networking using NetworkManager (nmcli).

Step 1: Backup the Network Configuration

Create a backup of the current network configuration before making any changes.

mkdir -p /root/network-backup-$(date +%F-%H%M)
cp -a /etc/NetworkManager/system-connections/* /root/network-backup-$(date +%F-%H%M)/ 2>/dev/null

Step 2: Configure IPv4 for the Interface

Assign the Additional IP Address to the network interface and prevent it from becoming the default route.

nmcli con modify "Wired connection 1" \
ipv4.method manual \
ipv4.addresses "118.27.152.57/23" \
ipv4.never-default yes

If the same interface contains multiple Additional IP Addresses, you can configure them together, for example:

ipv4.addresses "118.27.152.57/23,118.27.152.58/23,118.27.152.59/23"

All IP addresses can share the same gateway.

Step 3: Add Static Routes

Create a dedicated routing table for the Additional IP Address.

nmcli con modify "Wired connection 1" \
+ipv4.routes "118.27.152.0/23 table=102" \
+ipv4.routes "0.0.0.0/0 118.27.152.1 table=102"

Step 4: Add Routing Rules

Create a routing rule so that traffic originating from the Additional IP Address uses routing table 102.

nmcli con modify "Wired connection 1" \
+ipv4.routing-rules "priority 102 from 118.27.152.57/32 table 102"

If multiple Additional IP Addresses are assigned to the same interface and use the same gateway, they can share the same routing table (for example, table 102).

Create one routing rule for each source IP and assign a unique priority value.

Example:

nmcli con modify "Wired connection 1" \
+ipv4.routing-rules "priority 102 from 118.27.152.57/32 table 102" \
+ipv4.routing-rules "priority 103 from 118.27.152.58/32 table 102" \
+ipv4.routing-rules "priority 104 from 118.27.152.59/32 table 102"

Step 5: Apply the Configuration

Verify the connection name before running the commands.

nmcli connection show

Restart the connection to apply the new configuration.

nmcli con down "Wired connection 1"
nmcli con up "Wired connection 1"

Connection names vary between systems, such as Wired connection 1, System eth1, or another custom name.

Always verify the correct connection name using:

nmcli connection show

Step 6: Verify the Additional IP Address

Confirm that the Additional IP Address can access the Internet successfully.

curl --interface 118.27.152.57 https://ifconfig.me

The expected output should be:

118.27.152.57

Verify the Routing Configuration

ip a show eth1
ip route
ip rule show
ip route show table 102

After the configuration is complete:

• The primary interface remains the system's default route.

• The Additional IP Address routes traffic through its own gateway.

• Multiple Additional IP Addresses using the same gateway can share the same routing table by creating separate routing rules with unique priority values.

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