Varnish 4.x Configuration

Backends

In the main section of your VCL:

import directors;

backend sitea {
  .host = "xxx.xx.xxx.103";
  .port = "8080";
}
backend siteb {
  .host = "xxx.xx.xxx.104";
  .port = "8080";
}

# Define the director that determines how to distribute incoming requests.
sub vcl_init {
  new bar = directors.fallback();
  bar.add_backend(sitea);
  bar.add_backend(siteb);
}

See also How to configure polling.

And in sub vcl_recv:

  set req.backend_hint = bar.backend();

  # Do not cache these paths.
  if (req.url ~ "^/status\.php$" ||
      req.url ~ "^/update\.php" ||
      req.url ~ "^/install\.php" ||
      req.url ~ "^/apc\.php$" ||
      req.url ~ "^/admin" ||
      req.url ~ "^/admin/.*$" ||
      req.url ~ "^/user" ||
      req.url ~ "^/user/.*$" ||
      req.url ~ "^/users/.*$" ||
      req.url ~ "^/info/.*$" ||
      req.url ~ "^/flag/.*$" ||
      req.url ~ "^.*/ajax/.*$" ||
      req.url ~ "^.*/ahah/.*$" ||
      req.url ~ "^/system/files/.*$") {

    return (pass);
  }

  # Always cache the following file types for all users. This list of extensions
  # appears twice, once here and again in vcl_backend_response so make sure you edit both
  # and keep them equal.
  if (req.url ~ "(?i)\.(pdf|asc|dat|txt|doc|xls|ppt|tgz|csv|png|gif|jpeg|jpg|ico|swf|css|js)(\?.*)?$") {
    unset req.http.Cookie;
  }

  # Remove all cookies that Drupal doesn't need to know about. We explicitly
  # list the ones that Drupal does need, the SESS and NO_CACHE. If, after
  # running this code we find that either of these two cookies remains, we
  # will pass as the page cannot be cached.
  if (req.http.Cookie) {
    # 1. Append a semi-colon to the front of the cookie string.
    # 2. Remove all spaces that appear after semi-colons.
    # 3. Match the cookies we want to keep, adding the space we removed
    #    previously back. (\1) is first matching group in the regsuball.
    # 4. Remove all other cookies, identifying them by the fact that they have
    #    no space after the preceding semi-colon.
    # 5. Remove all spaces and semi-colons from the beginning and end of the
    #    cookie string.
    set req.http.Cookie = ";" + req.http.Cookie;
    set req.http.Cookie = regsuball(req.http.Cookie, "; +", ";");
    set req.http.Cookie = regsuball(req.http.Cookie, ";(SESS[a-z0-9]+|SSESS[a-z0-9]+|NO_CACHE)=", "; \1=");
    set req.http.Cookie = regsuball(req.http.Cookie, ";[^ ][^;]*", "");
    set req.http.Cookie = regsuball(req.http.Cookie, "^[; ]+|[; ]+$", "");

    if (req.http.Cookie == "") {
      # If there are no remaining cookies, remove the cookie header. If there
      # aren't any cookie headers, Varnish's default behavior will be to cache
      # the page.
      unset req.http.Cookie;
    }
    else {
      # If there is any cookies left (a session or NO_CACHE cookie), do not
      # cache the page. Pass it on to Apache directly.
      return (pass);
    }
  }

Purge

near top, right after backend section:

acl purge {
  "localhost";
  "127.0.0.1";
}

in vcl_recv section:

  # Check the incoming request type is "PURGE", not "GET" or "POST".
  if (req.method == "PURGE") {
    # Check if the IP is allowed.
    if (!client.ip ~ purge) {
      # Return error code 405 (Forbidden) when not.
      return (synth(405, "Not allowed."));
    }
    return (purge);
  }