Amazon ELB for EC2 instances in private subnet in VPC

amazon-web-services amazon-ec2 amazon-elb amazon-vpc vpc

I'm using Amazon EC2, and I want to put an internet-facing ELB (load balancer) to 2 instances on a private subnet. I am using VPC with public and private subnets.

If I just add the private subnet to the ELB, it will not get any connections.

If I attach both subnets to the ELB then it can access the instances, but it often will get time-outs. (Refer Screenshot 1)

If I attach to only public subnet then my instance attached to ELB gets OutOfService because I do not have any instance in the Public Subnet, instance count shows 0. (Refer Screenshot 2)

https://i.stack.imgur.com/X0W63.png

https://i.stack.imgur.com/QEn2k.png

My question is actually an extension to this question. After following all 6 steps mentioned in the accepted answer, I am still getting struck, my instance attached to ELB gets OutOfService. I have even tried with allowing ports in the Security Groups for EC2 instances and ELB, but it did not help.

Please help, I am breaking my head with this.

Ben Whaley

The other SO question you referenced is spot on. Double/Triple check the following

You need to attach only public subnets to your ELB, making sure that the availability zones those subnets are aligned with the availability zones of the private subnets that your instances are in.

Make sure that the security group of your instances allows access from the security group of your load balancer

The load balancer security group should have an egress rule allowing the health check to reach the instance

Make sure that your health check is working locally on the instance. For example, if your health check in the ELB is HTTP:8080/health_check, on the instance you can curl x.x.x.x:8080/health_check (where x.x.x.x is the private IP of the instance) and get a 200 response code.

The public subnet routing table should route 0.0.0.0/0 to the internet gateway attached to your VPC.

The private subnet routing table should route 0.0.0.0/0 to a NAT instance or gateway in a public subnet

I notice in screenshot 1 that the healthy instance is in AZ ap-southeast-1b. Make sure that the public subnet for you ELB is in the same zone. E.g. public subnet for ELB is in ap-southeast-1b, and private subnet for instance is in ap-southeast-1b. If the zones do not match it will not work.

@BenWhaley is correct. To expand this just a bit: Your ELB should only be on public subnets, and only on subnets in the same availability zone(s) as your instance(s).

So doesn't the "CrossZone" setting on the ELB allow the ELB and the instances to be in different AZs? It doesn't appear to actually work for me. Not a major issue anyway, as I'll just ensure AZ-alignment.

Cross Zone ensures that a load balancer interface within one zone will send traffic to instances in other AZs. Docs

The reason for this requirement is that AWS puts an ELB "node" in each subnet associated with the ELB. The network interfaces of these nodes have public IPs and need to be able to talk to the Internet, and thus need to be in a subnet with an IGW route.

jouell

The other SO question helped me as well. For me I kept forgetting that I needed to install software to server (i.e Apache) and if you don't create a NAT GW or some other method to allow software installs it will fail. If you want to try an automated fashion w/o much hassle, you can try a script:

https://github.com/jouellnyc/AWS/tree/master/create_aws_vpc3

Note there's plenty of good docs and discussions already but:

HTTP codes I got from the ELB:

504 - Probably security groups - allow access to the port 80 of the instances

503 - Probably the wrong target group setup

502 - Probably Apache/Server not running b/c it’s not installed because there's no nat gw or method to install software.

Amazon ELB for EC2 instances in private subnet in VPC

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