--- title: "Transparent Proxying" menu: howto: weight: 1 --- # Transparent Proxying When a transparent proxy is used, traffic is redirected into a proxy at the network layer, without any client configuration being required. This makes transparent proxying ideal for those situations where you can't change client behaviour - proxy-oblivious mobile applications being a common example. To set up transparent proxying, we need two new components. The first is a redirection mechanism that transparently reroutes a TCP connection destined for a server on the Internet to a listening proxy server. This usually takes the form of a firewall on the same host as the proxy server - [iptables](http://www.netfilter.org/) on Linux or [pf](https://en.wikipedia.org/wiki/PF_(firewall)) on OSX. When the proxy receives a redirected connection, it sees a vanilla HTTP request, without a host specification. This is where the second new component comes in - a host module that allows us to query the redirector for the original destination of the TCP connection. At the moment, mitmproxy supports transparent proxying on OSX Lion and above, and all current flavors of Linux. ## Linux On Linux, mitmproxy integrates with the iptables redirection mechanism to achieve transparent mode. ### 1. Enable IP forwarding. {{< highlight bash >}} sysctl -w net.ipv4.ip_forward=1 sysctl -w net.ipv6.conf.all.forwarding=1 {{< / highlight >}} This makes sure that your machine forwards packets instead of rejecting them. If you want to persist this across reboots, you need to adjust your `/etc/sysctl.conf` or a newly created `/etc/sysctl.d/mitmproxy.conf` (see [here](https://superuser.com/a/625852)). ### 2. Disable ICMP redirects. {{< highlight bash >}} sysctl -w net.ipv4.conf.all.send_redirects=0 {{< / highlight >}} If your test device is on the same physical network, your machine shouldn't inform the device that there's a shorter route available by skipping the proxy. If you want to persist this across reboots, see above. ### 3. Create an iptables ruleset that redirects the desired traffic to mitmproxy. Details will differ according to your setup, but the ruleset should look something like this: {{< highlight bash >}} iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8080 iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 8080 ip6tables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8080 ip6tables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 8080 {{< / highlight >}} If you want to persist this across reboots, you can use the `iptables-persistent` package (see [here](http://www.microhowto.info/howto/make_the_configuration_of_iptables_persistent_on_debian.html)). ### 4. Fire up mitmproxy. You probably want a command like this: {{< highlight bash >}} mitmproxy --mode transparent --showhost {{< / highlight >}} The `--mode transparent` option turns on transparent mode, and the `--showhost` argument tells mitmproxy to use the value of the Host header for URL display. ### 5. Finally, configure your test device. Set the test device up to use the host on which mitmproxy is running as the default gateway and [install the mitmproxy certificate authority on the test device]({{< relref "concepts-certificates" >}}). ### Work-around to redirect traffic originating from the machine itself Follow steps **1, 2** as above, but *instead* of the commands in step **3**, run the following Create a user to run the mitmproxy {{< highlight bash >}} sudo useradd --create-home mitmproxyuser sudo -u mitmproxyuser bash -c 'cd ~ && pip install --user mitmproxy' {{< / highlight >}} Then, configure the iptables rules to redirect all traffic from our local machine to mitmproxy. **Note**, as soon as you run these, you won't be able to perform successful network calls *until* you start mitmproxy. If you run into issues, `iptables -t nat -F` is a heavy handed way to flush (clear) *all* the rules from the iptables `nat` table (which includes any other rules you had configured). {{< highlight bash >}} iptables -t nat -A OUTPUT -p tcp -m owner ! --uid-owner mitmproxyuser --dport 80 -j REDIRECT --to-port 8080 iptables -t nat -A OUTPUT -p tcp -m owner ! --uid-owner mitmproxyuser --dport 443 -j REDIRECT --to-port 8080 ip6tables -t nat -A OUTPUT -p tcp -m owner ! --uid-owner mitmproxyuser --dport 80 -j REDIRECT --to-port 8080 ip6tables -t nat -A OUTPUT -p tcp -m owner ! --uid-owner mitmproxyuser --dport 443 -j REDIRECT --to-port 8080 {{< / highlight >}} This will redirect the packets from all users other than `mitmproxyuser` on the machine to mitmproxy. To avoid circularity, run mitmproxy as the user `mitmproxyuser`. Hence step **4** should look like: {{< highlight bash >}} sudo -u mitmproxyuser bash -c '$HOME/.local/bin/mitmproxy --mode transparent --showhost --set block_global=false' {{< / highlight >}} ## OpenBSD ### 1. Enable IP forwarding. {{< highlight bash >}} sudo sysctl -w net.inet.ip.forwarding=1 {{< / highlight >}} ### 2. Place the following two lines in **/etc/pf.conf**. {{< highlight none >}} mitm_if = "re2" pass in quick proto tcp from $mitm_if to port { 80, 443 } divert-to 127.0.0.1 port 8080 {{< / highlight >}} These rules tell pf to divert all traffic from `$mitm_if` destined for port 80 or 443 to the local mitmproxy instance running on port 8080. You should replace `$mitm_if` value with the interface on which your test device will appear. ### 3. Configure pf with the rules. {{< highlight bash >}} doas pfctl -f /etc/pf.conf {{< / highlight >}} ### 4. And now enable it. {{< highlight bash >}} doas pfctl -e {{< / highlight >}} ### 5. Fire up mitmproxy. You probably want a command like this: {{< highlight bash >}} mitmproxy --mode transparent --listen-host 127.0.0.1 --showhost {{< / highlight >}} The `--mode transparent` option turns on transparent mode, and the `--showhost` argument tells mitmproxy to use the value of the Host header for URL display. ### 6. Finally, configure your test device. Set the test device up to use the host on which mitmproxy is running as the default gateway and [install the mitmproxy certificate authority on the test device]({{< relref "concepts-certificates" >}}). {{% note %}} Note that the **divert-to** rules in the pf.conf given above only apply to inbound traffic. **This means that they will NOT redirect traffic coming from the box running pf itself.** We can't distinguish between an outbound connection from a non-mitmproxy app, and an outbound connection from mitmproxy itself - if you want to intercept your traffic, you should use an external host to run mitmproxy. Nonetheless, pf is flexible to cater for a range of creative possibilities, like intercepting traffic emanating from VMs. See the **pf.conf** man page for more. {{% /note %}} ## macOS OSX Lion integrated the [pf](https://en.wikipedia.org/wiki/PF_(firewall)) packet filter from the OpenBSD project, which mitmproxy uses to implement transparent mode on OSX. Note that this means we don't support transparent mode for earlier versions of OSX. ### 1. Enable IP forwarding. {{< highlight bash >}} sudo sysctl -w net.inet.ip.forwarding=1 {{< / highlight >}} ### 2. Place the following line in a file called, say, **pf.conf**. {{< highlight none >}} rdr pass on en0 inet proto tcp to any port {80, 443} -> 127.0.0.1 port 8080 {{< / highlight >}} This rule tells pf to redirect all traffic destined for port 80 or 443 to the local mitmproxy instance running on port 8080. You should replace `en0` with the interface on which your test device will appear. ### 3. Configure pf with the rules. {{< highlight bash >}} sudo pfctl -f pf.conf {{< / highlight >}} ### 4. And now enable it. {{< highlight bash >}} sudo pfctl -e {{< / highlight >}} ### 5. Configure sudoers to allow mitmproxy to access pfctl. Edit the file **/etc/sudoers** on your system as root. Add the following line to the end of the file: {{< highlight none >}} ALL ALL=NOPASSWD: /sbin/pfctl -s state {{< / highlight >}} Note that this allows any user on the system to run the command `/sbin/pfctl -s state` as root without a password. This only allows inspection of the state table, so should not be an undue security risk. If you're special feel free to tighten the restriction up to the user running mitmproxy. ### 6. Fire up mitmproxy. You probably want a command like this: {{< highlight bash >}} mitmproxy --mode transparent --showhost {{< / highlight >}} The `--mode transparent` flag turns on transparent mode, and the `--showhost` argument tells mitmproxy to use the value of the Host header for URL display. ### 7. Finally, configure your test device. Set the test device up to use the host on which mitmproxy is running as the default gateway and [install the mitmproxy certificate authority on the test device]({{< relref "concepts-certificates" >}}). {{% note %}} Note that the **rdr** rules in the pf.conf given above only apply to inbound traffic. **This means that they will NOT redirect traffic coming from the box running pf itself.** We can't distinguish between an outbound connection from a non-mitmproxy app, and an outbound connection from mitmproxy itself. If you want to intercept your own macOS traffic, see the work-around below or use an external host to run mitmproxy. In fact, PF is flexible to cater for a range of creative possibilities, like intercepting traffic emanating from VMs. See the **pf.conf** man page for more. {{% /note %}} ### Work-around to redirect traffic originating from the machine itself Follow steps **1, 2** as above, but in step **2** change the contents of the file **pf.conf** to {{< highlight none >}} #The ports to redirect to proxy redir_ports = "{http, https}" #The address the transparent proxy is listening on tproxy = "127.0.0.1 port 8080" #The user the transparent proxy is running as tproxy_user = "nobody" #The users whose connection must be redirected. # #This cannot involve the user which runs the #transparent proxy as that would cause an infinite loop. # rdr pass proto tcp from any to any port $redir_ports -> $tproxy pass out route-to (lo0 127.0.0.1) proto tcp from any to any port $redir_ports user { != $tproxy_user } {{< / highlight >}} Follow steps **3-5** above. This will redirect the packets from all users other than `nobody` on the machine to mitmproxy. To avoid circularity, run mitmproxy as the user `nobody`. Hence step **6** should look like: {{< highlight bash >}} sudo -u nobody mitmproxy --mode transparent --showhost {{< / highlight >}} ## "Full" transparent mode on Linux By default mitmproxy will use its own local IP address for its server-side connections. In case this isn't desired, the --spoof-source-address argument can be used to use the client's IP address for server-side connections. The following config is required for this mode to work: {{< highlight bash >}} CLIENT_NET=192.168.1.0/24 TABLE_ID=100 MARK=1 echo "$TABLE_ID mitmproxy" >> /etc/iproute2/rt_tables iptables -t mangle -A PREROUTING -d $CLIENT_NET -j MARK --set-mark $MARK iptables -t nat \ -A PREROUTING -p tcp -s $CLIENT_NET \ --match multiport --dports 80,443 -j \ REDIRECT --to-port 8080 ip rule add fwmark $MARK lookup $TABLE_ID ip route add local $CLIENT_NET dev lo table $TABLE_ID {{< / highlight >}} This mode does require root privileges though. There's a wrapper in the examples directory called 'mitmproxy_shim.c', which will enable you to use this mode with dropped privileges. It can be used as follows: {{< highlight bash >}} gcc examples/complex/full_transparency_shim.c -o mitmproxy_shim -lcap sudo chown root:root mitmproxy_shim sudo chmod u+s mitmproxy_shim ./mitmproxy_shim $(which mitmproxy) --mode transparent --set spoof-source-address {{< / highlight >}}