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configuration/http-filtering-proxy.md
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@ -10,160 +10,115 @@ How to run an HTTP filtering proxy in a FirewallVM
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.
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 the filtering rules GUI in Qubes VM manager.
The manager translates user-defined setup into iptables rules for the firewall VM's kernel.
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).
The primary goal of the filtering rule setup in the firewall VM is to allow for the user to protect either from his own mistakes (like accessing an arbitrary website from a browser running in a banking VM) or from the mistakes of websites (like a banking website that loads JS code from a social network operator when the user logs 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 one 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.
As the rules in the firewall are IP-based, it has drawbacks.
First, the rules cannot be used if one has to use an HTTP proxy to connect to websites (a common setup on corporate networks).
Second, 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 transfers her session to another IP.
Third, the initial setup of the rules is complicated as the firewall drops the connection silently.
As a workaround, one can use a browser's network console to see what is blocked, but this is time-consuming and one can easily miss some important cases like including sites for OCSP SSL certificate verification in the firewall white-list.
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.
These drawbacks can be mitigated if one replaces iptable-based rules with a filtering HTTP proxy.
The following describes how to setup a tinyproxy-based proxy in the firewall VM to achieve 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.
Running an HTTP proxy in your firewall VM increases the attack surface against that VM from a compromised AppVM.
Tinyproxy has 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 a 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 connections such as 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.
A 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 separate firewall VM / proxy per each important AppVMs.
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.
As a counterweight to this warning it is important to point out that an HTTP proxy decreases the attack surface of an AppVM.
For example, with a proxy, the AppVM does not need to make direct 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 of the latest and greatest HTTP features when connecting 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:
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.
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 prefers to black-list connections.
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.
3. For each AppVM that one wants to run through the proxy, create a 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 name part before the dot is arbitrary.
For convenience, one can use an AppVM name here, but this is not required.
It is important to get the ip address part right as this is what the control script uses to determine on which AppVM it will 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.
The attached archive includes a `tinyproxy/social.10.137.2.13` file with rules for an AppVM allowing connections to Google, Facebook, Linkedin, Livejournal, Youtube, and few other other sites.
One can use it as an example after changing the IP addresses accordingly.
When editing the rules remember to include `$` at the end of the host
name and to prefix each dot in the host name with the backslash. This
way the pattern matches the whole host and not just some prefix and
the dot is not interpreted as an instruction to match an arbitrary
character according to regular expression syntax.
When editing the rules, remember to include a `$` at the end of the host name, and to prefix each dot in the host name with a backslash (like `\.`).
This way, the pattern matches the whole host and not just a 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:
4. Check that the `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 `--`.
For each rule file it should print the name, ip address, and network interface of the running AppVMs.
It will also display 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:
5. Now run some AppVM with a 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.
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:
Check that proxy is started and 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.
6. Run the browser in the active AppVM and configure it to use the proxy on port 8100 of the firewall VM gateway interface's IP address.
In Qubes VM manager, the IP address is displayed in the Gateway field in the Settings 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".
In Firefox, go to the Preferences dialog, select Advanced->Network, and click Settings for the Connection section.
In the Connection Settings dialog, select Manual proxy configuration. For the HTTP Proxy field use the IP address of the firewall gateway interface.
Enter 8100 as the port, and 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
Go to some web 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:
In the firewall VM see the `/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
to restart the proxy with an 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:
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
@ -177,19 +132,14 @@ Setup
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.
8. In Qubes VM manager, adjust the Firewall rules for each AppVM with a proxy.
In a typical case, when only an HTTP proxy should be permitted for outside connections, simply select "Deny network access except...", make sure that the address list is empty, and then unselect the "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.
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].
This guide was initially written by Igor Bukanov in a [message] to the `qubes-devel` [mailing list].
[archive]: https://groups.google.com/group/qubes-devel/attach/39c95d63fccca12b/proxy.tar.gz?part=0.1
[message]: https://groups.google.com/d/msg/qubes-devel/UlK8P27UtD4/K6HM_GNdyTkJ