How do I use iptables to test a Gateway Load Balancer that has an Amazon EC2 instance as a target?
I want to use iptables to test my Gateway Load Balancer that has an Amazon Elastic Compute Cloud (Amazon EC2) instance as a target. The instance runs Amazon Linux 2023.
Short description
Configure iptables rules on an Amazon EC2 instance that's running Amazon Linux 2023 to test how the Gateway Load Balancer works within your environment. Iptables rules can also help you determine connectivity issues with the firewall. If the Gateway Load Balancer successfully routes traffic through the instance, then the connectivity issues are related to the firewall.
Use iptables only to test your Gateway Load Balancer.
Resolution
Note the Gateway Load Balancer IP addresses
To configure the variables in the iptables rules, you need the IP addresses and their Availability Zones for the Gateway Load Balancer's elastic network interface.
To find the IP addresses, complete the following steps:
- Open the Amazon EC2 console.
- In the navigation pane, choose Network Interfaces.
- Choose Search, and then choose Description from the dropdown list.
- Enter your Gateway Load Balancer's name in the Description = field, and then select your Gateway Load Balancer.
You receive a list of network interfaces for each activated subnet in your Gateway Load Balancer. Each network interface has an IP address and Availability Zone associated with it.
Configure the iptables rules
Use SSH to access the EC2 instance that you're going to use as a target or firewall behind the Gateway Load Balancer. In the following example commands, Availability Zone A is the source zone, and Availability Zones B and C are the zones that the source zone sends traffic to.
Set the IP addresses for the Gateway Load Balancer and EC2 instance
Enter the IP addresses for the Gateway Load Balancer and the instance:
export GWLB_IP_A=<GLWB ENI IP from Availability Zone A> export INSTANCE_IP=<EC2 instance's IP>
Note: Replace GLWB IP from Availability Zone A and EC2 instance's IP with your values.
(Optional) Configure additional tables for cross-zone traffic
If you activated cross-zone traffic, then run the following commands:
export GWLB_IP_B=<GLWB ENI IP from Availability Zone B> export GWLB_IP_C=<GLWB ENI IP from Availability Zone C>
Note: Replace GLWB ENI IP from Availability Zone B and GLWB ENI IP from Availability Zone C with your values. You must have an iptables rule for the Gateway Load Balancer's network interface for every Availability Zone on every target EC2 instance.
Set up the iptables
To install and activate the iptables, run the following commands:
sudo sysctl -w net.ipv4.ip_forward=1 sudo yum install -y iptables-services sudo systemctl enable iptables sudo systemctl start iptables
Set the default policies to ACCEPT for each built-in chain
To set the default policies, run the following commands:
sudo iptables -P INPUT ACCEPT sudo iptables -P FORWARD ACCEPT sudo iptables -P OUTPUT ACCEPT
Flush the NAT and mangle tables, chains (-F) and delete settings
To flush the NAT and mangle tables, and chains and delete settings, run the following commands:
sudo iptables -t nat -F sudo iptables -t mangle -F sudo iptables -F sudo iptables -X
Configure the NAT table to reroute traffic back to the Gateway Load Balancer
To reroute the traffic, run the following commands:
sudo iptables -t nat -A PREROUTING -p udp -s $GWLB_IP_A -d $INSTANCE_IP -i enX0 -j DNAT --to-destination $GWLB_IP_A:6081 sudo iptables -t nat -A POSTROUTING -p udp --dport 6081 -s $GWLB_IP_A -d $GWLB_IP_A -o enX0 -j MASQUERADE,/code>
(Optional) Add NAT rules to manage cross-zone traffic
To add NAT rules for cross-zone traffic, run the following commands:
sudo iptables -t nat -A PREROUTING -p udp -s $GWLB_IP_B -d $INSTANCE_IP -i enX0 -j DNAT --to-destination $GWLB_IP_B:6081 sudo iptables -t nat -A POSTROUTING -p udp --dport 6081 -s $GWLB_IP_B -d $GWLB_IP_B -o enX0 -j MASQUERADE sudo iptables -t nat -A PREROUTING -p udp -s $GWLB_IP_C -d $INSTANCE_IP -i enX0 -j DNAT --to-destination $GWLB_IP_C:6081 sudo iptables -t nat -A POSTROUTING -p udp --dport 6081 -s $GWLB_IP_C -d $GWLB_IP_C -o enX0 -j MASQUERADE
Save the iptables
To save the iptables, run the following command:
sudo service iptables save
Check the status
To verify that the iptables rules are correctly configured, run the following command:
sudo service iptables status
Run a health check for the Gateway Load Balancer
To check the Gateway Load Balancer's health, run the following commands:
sudo su yum install -y httpd service httpd start chkconfig httpd on echo "Health check page" >>/var/www/html/index.html exit
Make sure to double check the interface id. The example above has enX0, but normally you might want to use eth0 for some instance types
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