Into.the.Void.

A few years ago I created a set of scripts to start applications inside a linux namespace and automatically “Tor-ify” their network traffic. The main reason behind this effort was to provide some isolation and Tor support for applications that don’t have socks5 support, for example claws-mail. While this worked it was hard to keep adding sandboxing features like the ones firejail already provided. So I decided to take a look at how I could automatically send/receive traffic from a firejail-ed application through Tor.

This blog post is NOT meant to be used as copy/paste commands but to explain why each step is needed and how to overcome the problems found in the path.
If you have reasons to proxy all your traffic through Tor as securely as possible use Tails on a different machine, this guide is NOT for you.

A dedicated bridge
First of all create a Linux bridge and assign an IP address to it. Use this bridge to attach the veth interfaces that firejail creates when using the ‘net’ option. This option creates a new network namespace for each sandboxed application.

# brctl addbr tornet
# ip link set dev tornet up
# ip addr add 10.100.100.1/24 dev tornet

NAT
Then enable NAT from/to your “external” interface (eno1 in my case) for tcp connections and udp port 53 (DNS) and enable IP(v4) forwarding, if you don’t already use it. Some rules about sane default policy for FORWARD chain are added here well, modify to your needs.

# sysctl -w net.ipv4.conf.all.forwarding=1
# iptables -P FORWARD DROP
# iptables -A INPUT -m state --state INVALID -j DROP
# iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
# iptables -A FORWARD -i tornet -o eno1 -p tcp -j ACCEPT
# iptables -A FORWARD -i tornet -o eno1 -p udp --dport=53 -j ACCEPT
# iptables -t nat -A POSTROUTING -s 10.100.100.0/24 -o eno1 -j MASQUERADE

This configuration is enough to start a sandboxed application that will have it’s traffic NAT-ed from the Linux host.

$ firejail --net=tornet /bin/bash
Parent pid 26730, child pid 26731

Interface        MAC                IP               Mask             Status
lo                                  127.0.0.1        255.0.0.0        UP    
eth0             72:cc:f6:d8:6a:09  10.100.100.29    255.255.255.0    UP    
Default gateway 10.100.100.1

$ host www.debian.org
www.debian.org has address 5.153.231.4
Host www.debian.org not found: 3(NXDOMAIN)
Host www.debian.org not found: 4(NOTIMP)
$ host whoami.akamai.net
whoami.akamai.net has address 83.235.72.202
$ curl wtfismyip.com/text
3.4.5.6

(where 3.4.5.6 is your real IP and 83.235.72.202 should be the IP address of the final DNS recursive resolver requesting information from whois.akamai.net)

So NAT works and the shell is sandboxed.

“Tor-ify” traffic
Edit /etc/tor/torrc and enable TransPort and VirtualAddrNetwork Tor features to transparently proxy to the Tor network connections landing on Tor daemon’s port 9040. DNSPort is used to resolve DNS queries through the Tor network. You don’t have to use IsolateDestAddr for your setup, but I like it.

TransPort 9040
VirtualAddrNetwork 172.30.0.0/16
DNSPort 5353 IsolateDestAddr

Then use iptables to redirect traffic from tornet bridge to TransPort and DNSPort specified in torrc. You also need to ACCEPT that traffic in your INPUT chain if your policy is DROP (it is right ?)

# iptables -t nat -A PREROUTING -i tornet -p udp -m udp --dport 53 -j DNAT --to-destination 127.0.0.1:5353
# iptables -t nat -A PREROUTING -i tornet -p tcp -j DNAT --to-destination 127.0.0.1:9040
# iptables -A INPUT -i tornet -p tcp --dport 9040 -j ACCEPT
# iptables -A INPUT -i tornet -p udp --dport 5353 -j ACCEPT

Run your sandbox again and try to access the same website:

$ firejail --net=tornet /bin/bash
$ curl wtfismyip.com/text
curl: (7) Failed to connect to wtfismyip.com port 80: Connection timed out

aaaand nothing happens. The problem is that you have tried to route traffic from a “normal” interface to loopback which is considered a “martian” and is not allowed by default by the Linux kernel.

sysctl magic
To enable loopback to be used for routing the route_localnet sysctl setting must be set.
# sysctl -w net.ipv4.conf.tornet.route_localnet=1

Try again:

$ firejail --net=tornet /bin/bash
$ host whoami.akamai.net
whoami.akamai.net has address 74.125.181.10
Host whoami.akamai.net not found: 3(NXDOMAIN)
Host whoami.akamai.net not found: 4(NOTIMP)
$ curl wtfismyip.com/text
176.10.104.243
$ host 176.10.104.243
243.104.10.176.in-addr.arpa domain name pointer tor2e1.digitale-gesellschaft.ch.

it works!

You can actually run any program you want like that:
$ firejail --net=tornet google-chrome

Accessing onion services
There’s one problem left though, accessing onion services.
If you try and access www.debian.org onion service from your firejail+tor setup you will get an error.

$ firejail --net=tornet /bin/bash
$ curl http://sejnfjrq6szgca7v.onion/
curl: (6) Could not resolve host: sejnfjrq6szgca7v.onion

To fix that you need to modify /etc/tor/torrc again and add AutomapHostsOnResolve option.
AutomapHostsOnResolve 1

$ firejail --net=tornet /bin/bash
$ curl -I http://sejnfjrq6szgca7v.onion/
HTTP/1.1 200 OK
Date: Fri, 09 Dec 2016 12:05:56 GMT
Server: Apache
Content-Location: index.en.html
Vary: negotiate,accept-language,Accept-Encoding
TCN: choice
Last-Modified: Thu, 08 Dec 2016 15:42:34 GMT
ETag: "3a40-543277c74dd5b"
Accept-Ranges: bytes
Content-Length: 14912
Cache-Control: max-age=86400
Expires: Sat, 10 Dec 2016 12:05:56 GMT
X-Clacks-Overhead: GNU Terry Pratchett
Content-Type: text/html
Content-Language: en

Accessing onion services works as well now.

Applications supporting socks5
If you already have some of your applications proxying connections to tor using 127.0.0.1:9050 then you need to add another iptables rule to redirect the socks traffic from inside firejail’s namespace to Tor SocksPort.
# iptables -t nat -A PREROUTING -i tornet -p tcp -m tcp --dport 9050 -j DNAT --to-destination 127.0.0.1:9050

Filed by kargig at 18:36 under Internet,Linux,Networking,Privacy
Tags: debian, firejail, iptables, Linux, Networking, sandbox, tor
4 Comments | 38,840 views