Security Idea/test: firewall rule for non AMPRGW routes - 44net

25 Jan 2022

All,

I thought on some security matters of AMPRNet; and one that could not be mitigated easily
was:

*  even if you firewall IPENCAP, how to block others except AMPRGW from sending routes to
port 520/udp

Someone mentioned previously they saw a MAC that was a hash that contained the outer SRC
IP; but after years, I cannot find an OS Kernel that produces it. 

I believe have a solution, it required some hexadecimal calculations (see:
http://www.stearns.org/doc/iptables-u32.current.html). Using the u32 iptables module for
those who can (and a compatible module on on other systems), we need to merely find in the
bit/byte positions in the packet:

Outer Header:

* the IP of AMPRGW (0xa9e42254)* Protocol type IPENCAP (IP Protocol No. 4) (0x04)

Inner Header (add 20 bytes to the offset of the first header):

* SRC IP 44.0.0.1 (0x2c000001)
* Protocol UDP (0x06)* UDP - port 520 (0x0208)

I believe this should read as an iptables rule like:

iptables <foo> -m u32 --u32 "12&0xFFFFFFFF=0xA9E42254 &&
6&0xFF=0x04  && 32&0xFFFFFFFF=0x2c000001 && 26&0xFF=0x06
&& 212&FFFF=0x0208" -j ACCEPT

My question: has anyone already calculated these positions and values or tested before?

I didn't want to have to "reinvent the wheel"...but you can just check my
math now before attempting to implement.

73,

Lynwood

KB3VWG

Member W3PGC/K3ERA -  APRS Coordinator / Co-SkyWarn Coordinator