[RndTbl] MTS blocking NTP

Wyatt Zacharias wyatt at magitech.ca
Fri Jan 25 09:16:04 CST 2019

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

Wyatt Zacharias

On Fri, Jan 25, 2019 at 8:55 AM Colin Stanners <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> 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>
>> 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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