* cleanup ipv6 iputil helpers
With my refactoring in this PR I accidentally had some duplicate logic,
this PR cleans it up:
- https://github.com/slackhq/nebula/pull/1766
* skip ICMP reject for ICMP error packets and fragments
Per RFC 1122, ICMP error messages must not be generated in response to
other ICMP error messages to prevent infinite error loops. This applies
to both IPv4 (types 3, 4, 5, 11, 12) and IPv6 (types 1-4).
Do not generate reject packets for IPv4 or IPv6 fragments. For IPv4,
check MF flag and fragment offset. For IPv6, add isFragment return to
ipv6FindUpperProtocol so a single traversal handles both protocol
lookup and fragment detection.
* do send rejects for the initial fragment
RFC says "non-initial fragment"s
* fix fragment checks
The function previously only handled IPv4 ICMP Echo Request packets.
This adds handling for IPv6 ICMPv6 Echo Request (type 128) by generating
a proper Echo Reply (type 129) with correct pseudo-header checksum.
* add IPv6 reject packet generation (ICMPv6 Destination Unreachable and TCP RST)
* use ICMPv6 code 1 (administratively prohibited) and cap body at 1000 bytes
* cleanup, use ICMP error code 13 for ipv4
* better docs
* cleanup
Wade Simmons