[cge]

I recently switched my SSH servers to only listen to IPv6. They're still on port 22, and still pointed to by DNS, yet, while they received bot connection attempts every few seconds when listening for IPv4, they haven't received a single bot connection attempt in the last week since I made the switch.

Bots apparently simply don't bother with IPv6.

[bombcar]

Enumeration of all IPv4 is easy. Enumeration of all DNS entries is hard to impossible. Enumeration of all IPv6 is impossible.

[throw0101a]

> Enumeration of all IPv6 is impossible.

Though do take note of RFC 7707, "Network Reconnaissance in IPv6 Networks":

   IPv6 offers a much larger address space than that of its IPv4
   counterpart.  An IPv6 subnet of size /64 can (in theory) accommodate
   approximately 1.844 * 10^19 hosts, thus resulting in a much lower
   host density (#hosts/#addresses) than is typical in IPv4 networks,
   where a site typically has 65,000 or fewer unique addresses.  As a
   result, it is widely assumed that it would take a tremendous effort
   to perform address-scanning attacks against IPv6 networks; therefore,
   IPv6 address-scanning attacks have been considered unfeasible.  This
   document formally obsoletes RFC 5157, which first discussed this
   assumption, by providing further analysis on how traditional address-
   scanning techniques apply to IPv6 networks and exploring some
   additional techniques that can be employed for IPv6 network
   reconnaissance.

* https://datatracker.ietf.org/doc/html/rfc7707

[bombcar]

If we move to ipv6 majorly I suspect reflex scanning will become more of a thing (see a connection from X, scan X).

[duskwuff]

IPv4 is trivial to enumerate. IPv6 is not.

Guess how most SSH-scanning bots find targets?