Multiple Magento 2 Sites Varnish Configuration

Question

I have a dedicated server centos 6 Apache I have different Magento 2 sites on the server with there own ip and domain. I have installed and configured Varnish for one site / ip running Magento 2 site and used the Varnish configuration file generated in the admin and uploaded to [Home]/etc/varnish and renamed the file as default.vcl below is the settings section.

My problem is if I try to add any more sites/ip the varnish will not restart shows failed. please can someone tell me exactly how to add more sites to the configuration file and I can fill in my ip accordingly.

Secondly I want to ask when I have different Magento 2 sites and 1 varnish when I clear cache in any site will it also clear / purge for all sites - or will they work independently

backend default {
    .host = "88.151.196.121";
    .port = "8080";
    .first_byte_timeout = 600s;
    .probe = {
        .url = "/pub/health_check.php";
        .timeout = 2s;
        .interval = 5s;
        .window = 10;
        .threshold = 5;
   }
}

acl purge {
    "localhost";
}

sub vcl_recv {
    if (req.method == "PURGE") {
        if (client.ip !~ purge) {
            return (synth(405, "Method not allowed"));
        }

Answer 1

Based on your comment, I can conclude that the 2 Magento sites are served on 2 different IP addresses.

These sites will be defined as 2 separate backends, because their IP address differs.

Let's take these backend definitions as an example:

backend backend1 {
    .host = "88.151.196.121";
    .port = "8080";
    .first_byte_timeout = 600s;
    .probe = {
        .url = "/pub/health_check.php";
        .timeout = 2s;
        .interval = 5s;
        .window = 10;
        .threshold = 5;
   }
}
backend backend2 {
    .host = "88.151.196.123";
    .port = "8080";
    .first_byte_timeout = 600s;
    .probe = {
        .url = "/pub/health_check.php";
        .timeout = 2s;
        .interval = 5s;
        .window = 10;
        .threshold = 5;
   }
}

Varnish has now way of knowing which backend to pick. If both sites were hosted on the same IP address, Varnish would use the value of the Host header to determine the various sites.

In this case, we'll need to create a backend hint to tell Varnish when to pick which backend. We could use hostname matching for this.

This means you'll have to add the following VCL code:

sub vcl_recv {
    if(req.http.host ~ "(www\.)?example.com") {
        set req.backend_hint = backend1;
    } else {
        set req.backend_hint = backend2;
    }
}

Because this is VCL, you can tailor these routing rules to your exact needs.