r3 - 2008-11-13 - 22:57:59 - HarlanStennYou are here: NTP >  Support Web > ConfiguringNTP > ConfiguringRefclocks > ConfiguringModemRefclocks
NTP users are strongly urged to take immediate action to ensure that their NTP daemons are not susceptible to being used in distributed denial-of-service (DDoS) attacks. Please also take this opportunity to defeat denial-of-service attacks by implementing Ingress and Egress filtering through BCP38.

ntp-4.2.8p10 was released on 21 March 2017. It addresses 6 medium- and 5 low-severity security issues, 4 informational security topics, 15 bugfixes, and contains other improvements over 4.2.8p9.

Please see the NTP Security Notice for vulnerability and mitigation details.

Are you using Autokey in production? If so, please contact Harlan - he's got some questions for you.
REFACTOR See ConfiguringModemRefclocksDev for discussion of this topic.

6.1.10. Configuring Modem Refclocks

ACTS

Dave Mills uses:

 server 127.127.18.1 minpoll 12 maxpoll 17 
 phone atdt913034944774 atdt917195676742 

This calls NIST in Boulder CO first, and if that fails USNO in Boulder is called.

The initial '9' is to obtain an outside line.

Dave also says: The phone string is copied verbatim to the modem as per Hayes command set. You can test by sending the same string to the modem using a terminal program. You can also test using the NTP filegen facility and flag 4 set on the fudge command. The modem protocol and command responses are written to the clockstats file.

PTB

Dave found the following:

I stumbled on http://www.heret.de/radioclock/ptb.htm#K5 . Apparently, PTB does something similar to NIST. It can measure and correct for the propagation delay. However, while the NIST correction is automatic, the PTB correction must be enabled by a special sequence, as quoted below.

quote

The communication parameters are: CCITT-V.22 modem, 1200 baud, 8 ASCII data bits, one stop bit, no parity. The change from CR (carriage return) to LF (line feed) indicates the beginning of each transmitted second. The information transmitted before this time marker (leading edge of the start bit of the LF) refers to the next following second.

In order to correct for the propagation delay time, the code transmission is stopped by the command "//" (two slashes), the incoming CR-LF time markers are echoed back, and the time code generator is set by the command "GDM" (do generator delay time measurements) into the GDM mode. In this mode, the time code generator measures 8 round-loop delays, calculates the mean value and the standard deviation and advances the time marker for half the mean value determined. If the GDM has been successful, the visible time marker will change from "*" to "#".

The result of the delay time determination is reported in the time protocols

\quote



Related Topics: ModemRefclockUsers
Edit | WYSIWYG | Attach | Printable | Raw View | Backlinks: Web, All Webs | History: r3 < r2 < r1 | More topic actions
 
SSL security by CAcert
Get the CAcert Root Certificate
This site is powered by the TWiki collaboration platform
IPv6 Ready
Copyright & 1999-2017 by the contributing authors. All material on this collaboration platform is the property of the contributing authors. Ideas, requests, problems regarding the site? Send feedback