Question

We’re using Amazon EC2, and we want to put an ELB (load balancer) to 2 instances on a private subnet. If we just add the private subnet to the ELB, it will not get any connections, if we attach both subnets to the ELB then it can access the instances, but it often will get time-outs. Has anyone successfully implemented an ELB within the private subnet of their VPC? If so, could you perhaps explain the procedure to me?

Thanks

Answer 1

My teammate and I just have implemented ELB in a VPC with 2 private subnets in different availability zones. The reason you get timeouts is that for each subnet you add to the load balancer, it gets one external IP address. (try ‘dig elb-dns-name-here’ and you will see several IP addresses). If one of these IP address maps a private subnet, it will timeout. The IP that maps into your public subnet will work. Because DNS may give you any one of the IP addresses, sometimes it works, sometimes it times out.

After some back and forth with amazon, we discovered that the ELB should only be placed in ‘public’ subnets, that is subnets that have a route out to the Internet Gateway. We wanted to keep our web servers in our private subnets but allow the ELB to talk to them. To solve this, we had to ensure that we had a corresponding public subnet for each availability zone in which we had private subnets. We then added to the ELB, the public subnets for each availability zone.

At first, this didn’t seem to work, but after trying everything, we recreated the ELB and everything worked as it should. I think this is a bug, or the ELB was just in an odd state from so many changes.

Here is more or less what we did:

1. WebServer-1 is running in PrivateSubnet-1 in availability zone us-east-1b with security group called web-server.
2. WebServer-2 is running in PrivateSubnet-2 in availability zone us-east-1c with security group called web-server.
3. Created a public subnet in zone us-east-1b, we’ll call it PublicSubnet-1. We ensured that we associated the routing table that includes the route to the Internet Gateway (ig-xxxxx) with this new subnet. (If you used the wizard to create a public/private VPC, this route already exists.)
4. Created a public subnet in zone us-east-1c, we’ll call it PublicSubnet-2. We ensured that we associated the routing table that includes the route to the Internet Gateway (ig-xxxxx) with this new subnet. (If you used the wizard to create a public/private VPC, this route already exists.)
5. Created a new ELB, adding to it PublicSubnet-1 and PublicSubnet-2 (not the PrivateSubnet-X). Also, picked the instances to run in the ELB, in this case WebServer-1 and WebServer-2. Made sure to assign a security group that allows incoming port 80 and 443. Lets call this group elb-group.
6. In the web-server group, allow traffic from port 80 and 443 from the elb-group.