README: add documentation

README.md

@@ -1,3 +1,45 @@
+```
+|\        /|
+| \______/ |
+| ,,,,,,,, |
+| |||||||| |
+| |||||||| |
+| ¯¯¯¯¯¯¯¯ |
+|__________|
+  \-    -/
+```
+
 # EtherCat
 
-a tool to interface tap interfaces with stdin and stdout
+A simple tool written in C that connects a Linux tap interface to stdin and stdout
+
+## How to build
+
+- Prerequisites: Linux, C compiler
+  - Might be easily portable to other systems that support tap interfaces, Idk.
+- Run `./build.sh`
+- Install by copying to `/usr/local/bin` or just leave it where it is
+
+## How to use
+
+Run as root:
+```
+ip tuntap add mode tap user USERNAME group GROUPNAME name tapN
+ip link set tapN up
+```
+Run as user:
+`ethercat tapN`
+
+## Simple network tunnel:
+
+Be warned: This will behave as if you had both sides connected with a (slow and less reliable) cable. This is more of a bridge than a regular VPN.
+If you bridge both sides to a real NIC and you have DHCP servers on both networks, you will end up with two DHCP servers on the same network.
+
+- Set up a bridge between a physical NIC and the tap interface on one or both sides
+- Connect a port between the hosts, for example using SSH port forwarding
+- Make two FIFOs on both hosts (mkfifo)
+- Use netcat to connect the FIFOs between the hosts through the forwarded port
+  - `nc -l 10000 < fifo1 > fifo2`
+  - `nc localhost 10000 < fifo1 > fifo2`
+- Start an instance of EtherCat with the other ends of the FIFOs on both hosts
+  - `./ethercat tapN < fifo2 > fifo1`