[ntp:questions] ntpd not responding to sntp from industrial controller

Dave Hart hart at ntp.org
Wed Jul 20 23:10:05 UTC 2011

On Mon, Jul 18, 2011 at 10:39, Danny Thomas <d.thomas at its.uq.edu.au> wrote:
> We've got a RedLion industrial controller which uses SNTP but
> doesn't seem to receive a response from either an AD domain
> controller or from ntpd 4.2.7p136 on linux.
> tcpdump shows the packets arriving but no response is sent.
> I'm not sure of the best way to debug this so tried
>  root at kolanut# /opt/ntp/bin/ntpd -g -D 100 2>&1 | grep
>  read_network_packet: fd=23 length 52 from
>  receive: at 1<- flags 19 restrict 190
>  receive: at 1<- mode 3 keyid 00000000 len 52 auth 3
> Nearly all the fields are ignored, so what's causing ntpd
> not to respond ?  Key ID ?, stratum of 0 as KOD ?

After reviewing the ntp_proto.c spots that can generate the output
ending in "keyid 00000000 len 52 auth 3", it appears the problem is
the request is authenticated with a 4-byte MAC (len 52 rather than the
typical 48) which ntpd can't authenticate (meaning the key ID is
unrecognized, or recognized but doesn't generate the same MAC value as
given in the request).

I would look at disabling authenticated NTP in the RedLion
configuration, or configuring it correctly in RedLion and ntpd so the
MAC authenticates correctly.

Dave Hart

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