Date: Sat, 21 Jul 2018 19:29:02 +0000 From: Grzegorz Junka <list1@gjunka.com> To: Chad Jacob Milios <freebsd-list@nuos.org> Cc: freebsd-security@freebsd.org Subject: Re: Possible break-in attempt? Message-ID: <91123dcd-529a-1c92-16bf-f9060d3f1fa6@gjunka.com> In-Reply-To: <0DDFA4FB-4FAB-49F0-99E8-9958DB1D889F@nuos.org> References: <594ba84b-0691-8471-4bd4-076d0ae3da98@gjunka.com> <368EABCF-A10A-49E9-9473-7753F6BEAA50@patpro.net> <fd0ab13d-0dda-fa5d-a867-533720d9f47f@gjunka.com> <8EDDBDB2-77F5-4CF5-8744-41BEA187C08A@FreeBSD.org> <201807201905.w6KJ59hn079229@donotpassgo.dyslexicfish.net> <2E502F45-E6F6-44D7-AE9E-9B8B08C1CEBE@nuos.org> <d5f56af2-bd11-60d2-ba8d-06ed50872ef9@gjunka.com> <0DDFA4FB-4FAB-49F0-99E8-9958DB1D889F@nuos.org>
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On 21/07/2018 12:05, Chad Jacob Milios wrote: >> On Jul 21, 2018, at 7:57 AM, Grzegorz Junka <list1@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@catflap.org> wrote: >>>> >>>> Dimitry Andric <dim@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@openbsd.org <dtucker@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”.
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