Date: Sat, 21 Jul 2018 08:05:36 -0400 From: Chad Jacob Milios <freebsd-list@nuos.org> To: Grzegorz Junka <list1@gjunka.com> Cc: freebsd-security@freebsd.org Subject: Re: Possible break-in attempt? Message-ID: <0DDFA4FB-4FAB-49F0-99E8-9958DB1D889F@nuos.org> In-Reply-To: <d5f56af2-bd11-60d2-ba8d-06ed50872ef9@gjunka.com> 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>
next in thread | previous in thread | raw e-mail | index | archive | help
> 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: >>>=20 >>> Dimitry Andric <dim@freebsd.org> wrote: >>>=20 >>>> 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!". >>>>=20 >>>> 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: >>>=20 >>> | 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 >>>=20 >>> cheers, Jamie >> adding: >>=20 >> UseDNS no >>=20 >> 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=E2=80=99ve stood up for a decade. >>=20 >> 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. >>=20 >> 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. >>=20 >=20 > 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. >=20 > GrzegorzJ In which man page or web page are you seeing this information?=
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?0DDFA4FB-4FAB-49F0-99E8-9958DB1D889F>