April 25, 2020

Best practices for Cloud access using proxies and Agent Forwarding

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Cloud providers will get angry with you if you deal with network access by popping huge holes in the firewall. If you don't get the angry emails, your account administrator is getting them. At any rate, the practice of simply popping holes in the firewall for web app access is not secure. This guide shows how to make use of a SOCKS proxy for all your web-based access to Cloud VMs.

Step-by-step guide to setting up and using a SOCKS proxy

For worry-free, fire-and-forget proxy usage create and/or steal a proxy.pac file, which will contain rules for using the proxy for various cloud providers. I use the following file for AWS, Google Cloud Platform and Microsoft Azure. My example is semi-fancy because it assumes two different proxy ports - this is to allow one proxy to handle a cluster on AWS or Azure, and another concurrent proxy to handle a cluster on GCP

Note that my proxy.pac does not attempt to match all the possible Class C addresses that will be used - that's a losing battle, and they will overlap with each other as well as our own internal ranges. So always use the fully qualified host name of the instances used by the cloud providers instead.

  // Allow for two ports; prefer 8157 for EC2, 8158 for Google
  function FindProxyForURL(url, host) {
      // match AWS external and internal DNS
      if (shExpMatch(host, "*ec2.amazonaws.com") || shExpMatch(host,
                                                               "ip-*.internal")) {
          // return "SOCKS5 localhost:8157;SOCKS5 localhost:8158";
          return "SOCKS5 localhost:8157";
      // match Google Cloud internal DNS (they do not provide external
      DNS by default (?) )
  if (shExpMatch(host, "*.internal")) {
      // return "SOCKS5 localhost:8158; SOCKS5 localhost:8157";
      return "SOCKS5 localhost:8158";
  // // match AWS and Google Cloud URLs
  // if (shExpMatch(host, "ec2-master") || shExpMatch(host, "*ec2*.
  amazonaws.com") || shExpMatch(host, "*.internal")) {
   // return "SOCKS5 localhost:8157";
   // }
   // match Azure compute URLs
   if (shExpMatch(host, "*.cloudapp.azure.com")) {
   return "SOCKS5 localhost:8157; DIRECT";
   // // Class C address in 10.* range
   // if (shExpMatch(host, "10.*")) {
   // return "SOCKS5 localhost:8157; DIRECT";
   // }
   // All other requests go direct, not through the proxy
   return "DIRECT";

Set the .pac file to be used automatically in your OS.

You can set up a simple web server like mongoose, or use a file URL



  • System Preferences -> interface (e.g., Wifi) -> Advanced -> Proxies; Select Auto Proxy Discovery and Automatic Proxy

  • Configuration with a file URL as for Windows


  • set environment variable auto_proxy

Choose one of your hosts that has an external IP as a bastion host

For example, the NAT gateway host

Use openssh from your workstation to that host

The command below initiates a SOCKS proxy and forwards ssh-agent

  ssh -A -D 8157 {user}@{external_ip}

The -A option forwards ssh-agent and starts a SOCKS proxy on port 8157 of your workstation. I'm not showing the -i option pointing to my key because I always use ssh-agent instead

Unlike most references on the net, I'm not specifying the -N option because I like having an interactive shell and don't mind sharing it with proxy traffic.

Access your web interfaces using the proxy

Now point your browser to the fully qualified internal host name of the host you want, and enjoy the benefits of proper proxy use with zero changes to the firewall/security group of your cluster instances



Related articles

I made a detailed video about this on YouTube - if you want to skip to any specific part, use the Table of Contents that you will find in the video's description.

© Greg Grubbs 2008-2020

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