6to4 relay check

How does it work?
=================

Monitoring server pings itself from its native IPv4 address to its native IPv6 address. Echo request messages are relayed from IPv4 network to IPv6 network via the 6to4 relay server. Similarly echo reply messages from IPv6 network are relayed to IPv4 network via the 6to4 relay server.

1. Monitoring server calculates its 6to4 prefix based on the IPv4 address.
2. Monitoring server sends ICMPv6 ECHO request from its 6to4 IPv6 address to its native IPv6 address encapsulated in IPv4.
3. Message gets routed to 6to4 relay server where IPv4 header gets dropped and packet is sent back to monitoring server native IPv6 address.
4. Monitoring host OS sends ICMPv6 echo reply packet from its native IPv6 address to its 6to4 IPv6 address.
5. Echo reply messge gets to the 6to4 relay server where additional IPv4 header is added. IPv4 from address is 6to4 relay server address to address is monitoring server IPv4 address.
6. Monitoring server receives the packet and measures RTT.

Requirements
============

* Python
* Scapy
* IPv4 and IPv6 connectivity
* Monitoring host must reply to ICMPv6 echo requests
* Check must be run with root privileges to be able to receive encapsulated echo reply packet.

Arguments
=========

-H address relay IP address (default: 192.88.99.1)
-w timeout warning threshold of 2RTA in milliseconds (default: 10 ms)
-p INTEGER number of ICMP ECHO packets to send (default: 5)
-t INTEGER seconds before connection times out (default: 3)

Misc
====

To run this check with root privileges sudo tool can be used.

Example sudoers config line:
nagios ALL = (ALL) NOPASSWD: /usr/lib/nagios/plugins/check_6to4

Example command definition:
define command {
command_name check_6to4
command_line sudo /usr/lib/nagios/plugins/check_6to4 -t 1 -p 5 -w 10
}