Possible break-in attempt?

[Chad Jacob Milios]

> On Jul 21, 2018, at 7:57 AM, Grzegorz Junka <list1 at gjunka.com> wrote:
> On 21/07/2018 11:03, Chad Jacob Milios wrote:
>>> On Jul 20, 2018, at 3:05 PM, Jamie Landeg-Jones <jamie at catflap.org> wrote:
>>> 
>>> Dimitry Andric <dim at freebsd.org> wrote:
>>> 
>>>> For each incoming IP address, sshd does a reverse lookup, and if that
>>>> results in a hostname, it does another lookup of that hostname, to see
>>>> if *that* result matches the original incoming IP address.  If it does
>>>> not, you get this scary warning in syslog about a "possible break-in
>>>> attempt!".
>>>> 
>>>> In my opinion, this is fairly misleading, since almost always the actual
>>>> cause is badly configured DNS, a very common occurrence.  In addition,
>>>> matching forward and reverse DNS records is no guarantee at all that the
>>>> incoming IP address is in any way trustworthy.
>>> I'm not sure which version this made it into, but they actually removed this
>>> over 2 years ago. It's not in the openssh that ships with FreeBSD 11.2:
>>> 
>>> | commit e690fe85750e93fca1fb7c7c8587d4130a4f7aba
>>> | Author: dtucker at openbsd.org <dtucker at openbsd.org>
>>> | Date:   Wed Jun 15 00:40:40 2016 +0000
>>> |
>>> |     upstream commit
>>> |
>>> |     Remove "POSSIBLE BREAK-IN ATTEMPT!" from log message
>>> |     about forward and reverse DNS not matching.  We haven't supported IP-based
>>> |     auth methods for a very long time so it's now misleading.  part of bz#2585,
>>> |     ok markus@
>>> |
>>> |     Upstream-ID: 5565ef0ee0599b27f0bd1d3bb1f8a323d8274e29
>>> 
>>> cheers, Jamie
>> adding:
>> 
>> UseDNS no
>> 
>> has the added benefit of avoiding a grueling delay when YOU are the one behind an IP address with a misconfigured reverse DNS mapping (which is horribly common on consumer networks). It goes into /etc/ssh/sshd_config and has been among my initial configuration to every FreeBSD box i’ve stood up for a decade.
>> 
>> openssh-portable (in ports, produced by the paranoid fellows at OpenBSD) has actually switched to adopt this, UseDNS no, as their default configuration for, i think its been a couple years now. This is in addition to dropping the message from their log output if UseDNS yes.
>> 
>> There is no point to this foolishly alarming message. Be mindful of the OTHER ways you must surely have in place to keep your sshd hard against attack.
>> 
> 
> Good to know. But the documentation says setting to no prevents from using DNS in known_hosts. When I look into my known_hosts I see many dns-only names, e.g. github.com among others.
> 
> GrzegorzJ

In which man page or web page are you seeing this information?