freebsd security - Jul 2018 - Possible break-in attempt?

> 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?

On 21/07/2018 12:05, Chad Jacob Milios wrote:
>> 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?
 > man sshd_config

 ???? UseDNS? Specifies whether sshd(8) should look up the remote host name,
 ???????????? and to check that the resolved host name for the remote IP
 ???????????? address maps back to the very same IP address.

 ???????????? If this option is set to ?no?, then only addresses and not 
host
 ???????????? names may be used in ~/.ssh/known_hosts from and sshd_config
 ???????????? Match Host directives.? The default is ?yes?.