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#  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 on Linux or pf 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.

sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.all.forwarding=1

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).

#  2. Disable ICMP redirects.

sysctl -w net.ipv4.conf.all.send_redirects=0

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:

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

If you want to persist this across reboots, you can use the iptables-persistent package (see here).

#  4. Fire up mitmproxy.

You probably want a command like this:

mitmproxy --mode transparent --showhost

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.

#  OpenBSD

#  1. Enable IP forwarding.

sudo sysctl -w net.inet.ip.forwarding=1

#  2. Place the following two lines in /etc/pf.conf.

mitm_if = "re2"
pass in quick proto tcp from $mitm_if to port { 80, 443 } divert-to 127.0.0.1 port 8080

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.

doas pfctl -f /etc/pf.conf

#  4. And now enable it.

doas pfctl -e

#  5. Fire up mitmproxy.

You probably want a command like this:

mitmproxy --mode transparent --showhost

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.

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.

#  macOS

OSX Lion integrated the pf 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.

sudo sysctl -w net.inet.ip.forwarding=1

#  2. Place the following two lines in a file called, say, pf.conf.

rdr on en0 inet proto tcp to any port {80, 443} -> 127.0.0.1 port 8080

These rules tell 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.

sudo pfctl -f pf.conf

#  4. And now enable it.

sudo pfctl -e

#  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:

ALL ALL=NOPASSWD: /sbin/pfctl -s state

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:

mitmproxy --mode transparent --showhost

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.

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.

#  Work-around to redirect traffic originating from the machine itself

Follow the steps 1, 2 as above. In step 3 change the contents of the file pf.conf to

#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.
#
#Here we redirect for all users which don't run transparent proxy.
redir_users = "{ !=" $tproxy_user "}"

#If you only wish to redirect traffic for particular users
#you may also do:
#redir_users = "{= john, = jane}"

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 $redir_users

Follow steps 4-6 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 7 should look like:

sudo -u nobody mitmproxy --mode transparent --showhost

#  “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:

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

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:

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