Date: Sat, 21 Jul 2018 11:57:07 +0000 From: Grzegorz Junka <list1@gjunka.com> To: freebsd-security@freebsd.org Subject: Re: Possible break-in attempt? Message-ID: <d5f56af2-bd11-60d2-ba8d-06ed50872ef9@gjunka.com> In-Reply-To: <2E502F45-E6F6-44D7-AE9E-9B8B08C1CEBE@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>
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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
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