Varnish: Route requests to subdomains based on URL

mask_subdomains

backend insights {
  .host = "insights.example.com"
  .port = "3000";
}

backend help {
  .host = "help.example.com"
  .port = "80";
}

backend blog {
  .host = "blog.example.com"
  .port = "8080";
}

sub vcl_recv {
  if (req.url ~ "insights") {
    set req.backend = insights;
  }
  if (req.url ~ "help") {
    set req.backend = help;
  }
  if (req.url ~ "blog") {
    set req.backend = blog;
  }
}