[RndTbl] MTS blocking NTP

Gilles Detillieux grdetil at scrc.umanitoba.ca
Fri Jan 25 13:27:50 CST 2019

I had an issue with NTP port 123 being blocked when switching from MTS's 
phone-line based ADSL service to their fibre based "VDSL" service 
several years ago (well before the Bell takeover). Colin is right: they 
wanted to block NTP amplification attacks and went way overboard. Even 
after the threat had pretty much passed, they refused to relax that 
restriction on their home fibre Internet service. Apparently the only 
solution is to use MTS's own NTP server. I think it's ntp.mts.net, but 
I'm not at home now so I can neither check my NTP settings, nor verify 
that it's still working correctly. It's a pain for devices where you 
can't change the NTP server setting, because there's really no other 
recourse but to live with inaccurate clocks. At least my phone can sync 
off of LTE.

On 1/25/19 9:16 AM, Wyatt Zacharias wrote:
> Our "high-end" fibre business internet from MTS at work hasn't blocked 
> any NTP traffic.
> Our internal NTP servers sync out on port 123, and it's all been 
> allowed so far.
> --
> Wyatt Zacharias
> On Fri, Jan 25, 2019 at 8:55 AM Colin Stanners <cstanners at gmail.com 
> <mailto:cstanners at gmail.com>> wrote:
>     Likely they're trying to block NTP amplification attacks,
>     https://www.incapsula.com/ddos/attack-glossary/ntp-amplification.html
>     , and they went overboard on the firewall rule.
>     On Fri, Jan 25, 2019 at 8:51 AM John Lange <john at johnlange.ca
>     <mailto:john at johnlange.ca>> wrote:
>         One of my favorite sayings is "never suspect a conspiracy,
>         that which can be explained by incompetence". This is likely
>         an accidental side effect of something else they've done
>         because unless they are now trying to sell you some kind of
>         Bell branded time-sync service, I can't think of any business
>         reason why they would do this intentionally.
>         Although, one thing I can think of is, perhaps there is a
>         whole lot of unsecured NTP on their network being actively
>         exploited?
>         Might be worth going through the pain of opening a ticket to
>         see if you can get an official answer. I believe the CRTC
>         regulations prevent them from arbitrarily manipulating,
>         blocking, or shaping the network traffic without disclosing
>         what they are doing.
>         John
>         On Fri, Jan 25, 2019 at 4:55 AM Trevor Cordes
>         <trevor at tecnopolis.ca <mailto:trevor at tecnopolis.ca>> wrote:
>             On 2019-01-25 Trevor Cordes wrote:
>             > Looks like chrony (and others) lets you specify src
>             port, but I'm
>             > loathe to uproot the system I know because Bell is
>             braindead.  (MTS
>             > didn't use to block it, and block-happy Shaw does not
>             block it.)
>             Epiphany moment: iptables can probably solve this.  20
>             minutes later:
>             iptables -t mangle -A OUTPUT -o $iext -p udp --sport 123
>             --dport 123 -j MARK --set-mark 30
>             iptables -t nat -A POSTROUTING -p udp -m mark --mark 30 -j
>             SNAT --to-source :60000-61000
>             Works perfectly!  ntpd now syncs with peers.  ntpdate
>             doesn't need -u.
>             I don't need to switch to chrony.  And I don't need to
>             wait for ntpd to
>             add this feature*.  Go take a hike Bell!!!
>             *http://bugs.ntp.org/show_bug.cgi?id=1109 ... looks like never
>             Note, it could be just 1 rule, but I used 2 to make sure
>             that I only
>             SNAT packets originating from within the actual
>             firewall/router itself,
>             and not packets being forwarded from within the internal
>             LAN (PC's).  I
>             can't figure out a way to specify "really originated
>             locally" other
>             than with mark, but I'm open to ideas.  It's not as easy
>             as it sounds
>             with multiple interfaces on the box, unless there's a
>             trick I'm missing.
>             If I wanted internal LAN PCs to also have their traffic go
>             through, I'd
>             need to use a -j MASQUERADE (it's a dynamic IP) in an
>             extra rule and
>             change the syntax slightly.  Since all internal PCs should
>             be set to
>             use the firewall as ntp server, this shouldn't be a
>             problem (in fact
>             could help me id broken PC ntp setups).
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>         -- 
>         John Lange
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Gilles R. Detillieux              E-mail: <grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre       WWW:    http://www.scrc.umanitoba.ca/
Dept. of Physiology and Pathophysiology, Rady Faculty of Health Sciences,
Univ. of Manitoba  Winnipeg, MB  R3E 0J9  (Canada)

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