[PATCH 1/1] ping: Add support for IPv4-mapped IPv6 address format

[Henrique de Moraes Holschuh]

On 20/10/2024 09:53, Lars Uffmann wrote:
> What I didn't know was whether there was any valid context in which an 
> ipv4-mapped ipv6 address would ever be able to get an ICMP response.

IPv4-mapped addresses (::ffff:0/96) are "namespaced away" from IPv6 by 
the current IPv6 standards.  It is IPv4, for all intents and purposes, 
you cannot have a valid IPv6 packet with source or destination addresses 
in the ::ffff:0/96 range.

You might want low-level network diagnostics like "ping" to generate 
such *invalid* IPv6 packets (that have source addresses or destination 
addresses in the ::ffff:0/96 range) to check if the network stack 
correctly drops them, though.  You might even need to use a packet 
sniffer to check if the packet went somewhere, as a reply is extremely 
unlikely even if it arrived at the destination: no L3 device is supposed 
to route or reply to such packets.

And obviously invalid IPv6 packets are *not* something ping should send 
by default without warnings and without explicit requests.  A suggestion 
would be to refuse to send to ::ffff:0/96 if neither -4 or -6 is given, 
send valid IPv4 if -4 is given (*this* could be the default and not 
check for -4 on the grounds that it is the expected behavior of 
::ffff:0/96, though).  On this suggestion, to send invalid IPv6 packets, 
either -6 or some other explicit CLI option would be required.

-- 
Henrique de Moraes Holschuh