Application Load Balancer Redirects

Question

I have an EC2 instance which hosts a web site running in IIS. I have also set up a AWS Application Load Balancer with a HTTPS listener into which I have added my web site's SSL certificate (held in AWS Certificate Manager). The HTTPS listener redirects traffic to my target group which in turn points to my EC2 instance. The EC2 instance uses a security group to only allow traffic from the Load Balancer.

Firstly, am I correct in thinking that the Application Load Balancer decrypts the SSL traffic and as a result, this is forwarded to my target group using HTTP (in other words, my EC2 instance's security group only needs to accept HTTP traffic from the Load Balancer and I can remove my SSL certificate from IIS)?

Secondly, the main problem that I am experiencing at the moment is that the site has relative paths to some of the resources (CSS, JavaScript, etc). As a result I am getting mixed content errors in my FireFox console when I browse the site. I set up a HTTP listener in my Load Balancer to redirect traffic to HTTPS but this doesn't seem to be working for relative paths.

I don't particularly want to edit the site to change relative paths to fully qualified HTTPS URLs as it is a very complex content management system. Also, before using the Load Balancer the site was serving content perfectly (the DNS entry pointed to my EC2 instance's public IP address and there was a rewrite rule in IIS to handle non HTTPS traffic).

Any advice would be greatly appreciated.

Answer 1

For your first query, I think Yes. You can remove my SSL certificate from IIS and point EC2 instance's security group to accept HTTP traffic from the Load Balancer.

For your concern on relative path's problem, please go through this link: After introducing ALB, Mixed Content Error happened

I think it will be helpful.

Answer 2

Open your browser and click on the Network tab and check these call should be going somewhere else which is using HTTP, these are an outgoing call from your application and redirect will not fix these issue. see the screenshot

Seems like same error in your case, you serving images from some other side, not from the EC2, It's mean those request which EC2 server is supposed to answer will work fine, but those which your code try to pull JS file or other resources these are outgoing call and this is nothing to do with LB redirect rule.

Better to update your application and all the URLs to https.

When a user visits a page served over HTTPS, their connection with the web server is encrypted with TLS and is therefore safeguarded from most sniffers and man-in-the-middle attacks. An HTTPS page that includes content fetched using cleartext HTTP is called a mixed content page. Pages like this are only partially encrypted, leaving the unencrypted content accessible to sniffers and man-in-the-middle attackers. That leaves the pages unsafe.

Answer 3

Thanks for the replies. I managed to get to the bottom of the second issue. The site's web.config had a rewrite rule which was forcing URLs to lowercase (something to do with SEO). This was causing the problems when serving the CSS and JS files. I've made the necessary updates to this rule and it now appears to work (see IIS URL rewrite module url's to lowercase).

For the first issue, the SSL certificates do not need to be installed on the IIS web server - they are only required in AWS Certificate Manager and then to be assigned to the Load Balancer which in turn has 2 listeners - one to direct HTTPS traffic to my Target Group and one to redirect HTTP traffic to HTTPS.

My IIS site now only has HTTP bindings and only accepts HTTP traffic from the Load Balancer.

All seems to work!