How to run an HTTP filtering proxy in a FirwallVM

Introduction

By default Qubes uses a special firewall VM that sits between the networking VM and each AppVM. This VM controls the traffic for AppVMs and can be used to restrict what AppVMs can send or receive. The traffic rules can be setup using filtering rules GUI in Qubes VM manager. The manager translates user-defined setup into iptables rules for the kernel of firewall VM.

The primary goal of the filtering rule setup in firewall VM is to allow for the user to protect either from own mistakes (like accessing an arbitrary website from a browser running in a banking VM) or from mistakes of websites (like a banking website that loads JS code from a social network operator when the user logs in into the bank).

As the rules in the firewall are IP-based, it has drawbacks. First the rules cannot be used if one has to use a HTTP proxy to connect for websites (a common setup on corporate networks). Second the Qubes resolves DNS names from the firewall rules when the AppVM loads. This prevents websites that use DNS-based load balancers from working unless the user reloads the firewall rules (which re-resolve the DNS names) whenever the balancer transfer her session to another IP. Third the initial setup of the rules is complicated as the firewall drops the connection silently. As a workaround on can use browser’s network console to see what is blocked, but this is time-consuming and one can trivially miss some important cases like including in the firewall white list sites for OCSP SSL certificate verification.

These drawbacks can be mitigated if one replaces iptable-based rules with a filtering HTTP proxy. The following describes how to setup tinyproxy-based proxy in the firewall VM to archive such filtering.

Warning

Running a HTTP proxy in your firewall VM increases the attack surface against that VM from a compromised AppVM. tinyproxy has a relatively simple code and a reasonable track record to allow to certain level of trust. But one cannot exclude bugs especially in the case of a hostile proxy clients as this is less tested scenario. So it is not advisable to use the proxy in a shared firewall VM against untrusted AppVM to black-list some unwanted connection like advertisement sites.

Less problematic setup is to white-list possible connections for several trusted and semi-trusted AppVMs within one firewall VM. Still for maximum safety one should consider running a separated firewall VM per each important AppVMs and run the proxy there.

As a counterweight to this warning it is important to point out that HTTP proxy decreases attack surface against AppVM. For example, with a proxy the AppVM does not need DNS connections so a bug in the kernel or in the browser in that area would not affect the AppVM. Also browsers typically avoid many latest and greatest HTTP features when connection through proxies minimizing exposure of new and unproven networking code.

Setup

  1. Copy this archive with the proxy control script, default tinyproxy config and a sample firewall filtering file into the firewall VM and unpack it in /rw/config folder there as root:

     cd /rw/config
     sudo tar xzf .../proxy.tar.xz
    
  2. If necessary adjust /rw/config/tinyproxy/config according to the man page for tinyproxy.conf. The included config file refuses the connection unless the host is white-listed in the filtering file, so this can be altered if one wants rather to black-list connection. One may also specify upstream proxies there. The file is a template file and the control script will replace {name} constructs in the file with actual parameters. In general lines with {} should be preserved as is.

  3. For each AppVM that one wants to run through the proxy create an the corresponding filtering file in the /rw/config/tinyproxy directory. With the default config the filtering file should contain regular expressions to match white-listed hosts with one regular expression per line, see the man page for tinyproxy.conf for details. The file should be named:

     name.ip-address-of-app-vm
    

    The name part before the dot can be arbitrary. For convenience one can use AppVm name here, but this is not required. It is important to get ip address part right as this is what the control script uses to determine for which AppVM to apply the proxy rules. One can check the IP address of AppVM in Qubes VM manager in the VM settings dialog, see the Networking session under the Basic tab.

    The attached archive includes tinyproxy/social.10.137.2.13 file with a rules for a AppVM allowing connection to google, facebook, linkedin, livejournal, youtube and few other other sites. One can use it as an example after changing the the IP address accordingly.

    When editing the rules remember to include $ at the end of the host name and to prefix each dot in the host name with the backslash. This way the pattern matches the whole host and not just some prefix and the dot is not interpreted as an instruction to match an arbitrary character according to regular expression syntax.

  4. Check that proxyctl.py script can properly recognize the rule files. For that run:

     sudo /rw/config/tinyproxy/proxyctl.py show
    

    For each rule file it should print the name, ip address, network interface of the running AppVM if AppVM runs and the id of the tinyproxy process that proxies that AppVM. The first time each pid should be --.

  5. Now run some AppVM with proxy and then run:

     sudo /rw/config/tinyproxy/proxyctl.py update
    

    The update command starts proxy processes and adjusts the iptable rules to allow for proxy traffic for each running AppVM from the filtering files list. For each stopped AppVM the proxy is killed.

    Check that proxy is started so the pid field of the show command is a number:

     sudo /rw/config/tinyproxy/proxyctl.py show
    
  6. Run the browser in the started AppVM and configure it to use the proxy on the port 8100 running at the IP address of the firewall VM gateway interface. In Qubes VM manager the address is given after the Gateway label in the Setting dialog for the firewall VM.

    In Firefox go to the Preferences dialog, select Advanced->Network, click Settings for the Connection section. In the Connection Settings dialog select Manual proxy configuration. For HTTP Proxy field use the IP address of the firewall gateway interface. Enter 8100 as the port and the select the checkbox “Use this proxy server for all protocols”.

    Go to some site. The browser should either load it if it was white-listed in the filtering file or show a page generated by tinyproxy that the page was filtered out.

    In the firewall VM see /run/tinyproxy/name/log file. For each filtered out website it contains an entry and one can adjust the filtering file to include the corresponding host. After changing the file run either:

     sudo /rw/config/tinyproxy/proxyctl.py restart name
    

    to restart proxy with the updated rules file only for the given VM or

     sudo /rw/config/tinyproxy/proxyctl.py kill-all-and-restart
    

    to restart all proxy processes.

  7. To make sure that the proxy is started automatically when the AppVM starts change /rw/config/qubes-firewall-user-script to include the following line:

     /rw/config/tinyproxy/proxyctl.py update
    

    If the file does not exist, create it so it looks like this:

     #!/usr/bin/bash
     /rw/config/tinyproxy/proxyctl.py update
    

    Make sure that the script is owned by root and executable:

     sudo chown root:root /rw/config/qubes-firewall-user-script
     sudo chmod 755 /rw/config/qubes-firewall-user-script
    
  8. In Qubes VM manager adjust Firewall rules for each AppVM with a proxy. In a typical case when only HTTP proxy should be used for outside connections, simply select “Deny network access except…,” make sure that the address list is empty and then unselect “Allow ICMP,” “DNS” and “Update proxy” checkboxes.

    There is no need to add any special entries for the proxy in the GUI as proxyctl.py adds rules for the proxy traffic itself.


This guide was initially written by Igor Bukanov in a message to the qubes-devel mailing list.