[ntp:questions] Problem syncing NTP behind NAT

Dave Hart hart at ntp.org
Sun Apr 8 03:46:22 UTC 2012

On Sat, Apr 7, 2012 at 21:36, Dave Hart <hart at ntp.org> wrote:
> I notice the successful clients were querying using ports > 123, and
> the failing ones < 123.  I dimly recall seeing an inappropriate
> less-than-123 source port comparison in ntpd long ago, in fact I'd
> have guessed it had been removed before 4.2.6p3.

4.2.6p3 does suffer from the low-port bug:

 * Monitor the packet and get restrictions. Note that the packet
 * length for control and private mode packets must be checked
 * by the service routines. Some restrictions have to be handled
 * later in order to generate a kiss-o'-death packet.
 * Bogus port check is before anything, since it probably
 * reveals a clogging attack.
if (SRCPORT(&rbufp->recv_srcadr) < NTP_PORT) {
        return;                         /* bogus port */

In 4.2.7 that code rejects port 0 alone.

Dave Hart

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