[ntp:questions] Can't get Windows/2000 Client to synchronize with my NTP server on CentOS 4.2

Mark Hansen meh at NOSPAMwinfirst.com
Sat Jan 14 01:33:52 UTC 2006

I'm running Centos 4.2 a machine which acts as my 'network host', in that it
connects my internal home network to the Internet, provides a firewall (via
iptables), and other basic services, like NTP. I'm running ntp version 4.2.0.a

I was running RedHat 7.3 before this installation, and all was working fine.

I have tested this without the firewall, to make sure the firewall was not
the problem. The firewall should not restrict access to/from the network host
machine and the in-house network.

I have NTP configured to synchronize against three servers, a Statum 2 server
owned by a friend of mine, and two other public NTP servers.

When I run ntpq -p, I can see that my server is properly synchronizing against
these other servers (also, I can see by looking at syslog that things appear to
be working).

However, when synchronization software on my Windows/2000 workstation attempts
to synchronize with my NTP server, it doesn't get a response.

I ran tcpdump on the CentOS machine and watched the NTP UDP port, and saw the
request come in, but nothing was sent back. On the Windows/2000 machine, I've
run w32tm -test -v -once, and get back the following (this is just a snippet,
but I think it's the pertinent part):

> BEGIN:NTPTry -- try
>    BEGIN:ComputeInterval
>    END  Line 2479
>    Sending to server  48 bytes...
>    NTP: didn't receive datagram
>    Logging event 0x8000000B. 15 min until this event is allowed again.
>    0x8000000B reported to System Log in Event Viewer
> END  Line 1951
> Time source failed to produce usable timestamp.

When w32tm is making its attempts, I see the following messages
in the syslog (when running ntpd with debugging turned on):

> ntpd: input_handler: if=3 fd=7 length 48 from 0a0a9714
> ntpd: receive: at 16<- restrict 184
> ntpd: receive: at 16<- mode 3 code 2
> ntpd: addto_syslog: select(): nfound=-1, error: Interrupted system call

( is my CentOS server machine, running ntpd, and
is my Windows/2000 client machine).

In looking at the source for ntpd, I see that one of the bits represented
by the "184" is Authentication Required, so I tried running ntpd with the
-A flag, but this didn't help.

I've tried two additional clients on the Windows/2000 machine, one of
which allows me to select the protocol version to use, and I tried 4, 3 and
2 - with the same result.

I went through some debugging tips found after googling, but these all were
focused on getting the server to synchronize with its peers, and not about
problems connecting to the server from clients.

I'll include, below, my configuration file, and would be grateful if someone
could either spot the problem, or give me a pointer to how I can debug client
connection problems.


server my-friends-ntp-server prefer
server ntp1.Level3.net
server ntp2.Level3.net

driftfile /var/lib/ntp/drift

broadcastdelay 0.008

restrict my-friends-ntp-server-IP mask nomodify
restrict ntp1-IP mask nomodify
restrict ntp2-IP mask nomodify

# This is my internal network:
restrict mask nomodify notrust notrap

# The local address are unrestricted:

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