> 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?