Meinberg Ntp Client __EXCLUSIVE_

A time client NLM can be loaded on additional file serverswithout own radio clock. The time client queries the reference timefrom a time server NLM running on another file server on the network.

These instructions describe how to prepare a LANTIME for NTP symmetric key authentication and explains how to configure the NTP client side, using the example of a Linux ntpd client. It's worth to mention that NTP clients which request the LANTIME without a key are still getting served with time, even if the LANTIME is prepared for symmetric key authentication. The feature is simply an additional security feature for those clients who want to use symmetric key authentication.

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In the example above the keys with ID 1, 6 and 19 were defined as trusted keys. The key ID can be found in the first column of the key file. If the LANTIME shall support more than 1 key, the IDs have to be separated by a blank in the Local Trusted Keys field (as in the example above). Keys which are not defined as trusted key cannot be used by a NTP client for symmetric key authentication.

1. The key has to be copied to the client's key file. On a Linux ntpd client, the file is usually stored in /etc/ntp.keys. For example, if the ntpd client has to be configured to use the generated key with ID 1, the entire key has to be copied from the LANTIME key file and appended to the /etc/ntp.keys file, for example:

So back in 2002 a workaround was implemented in ntpd where it doesn't mobilize a peer association unless the request comes from an authenticated peer, but anyway sends a reply, just to satisfy those w32time clients.

The version of ntpd where the workaround for w32time clients was unintentionally disabled was shipped with LANTIME firmware versions 6.24.013 and 6.24.014. So this should work properly with all older LANTIME firmware versions, and with LANTIME firmware version 6.24.015 or newer.

the following flags are supported: 0x1 SpecialInterval Wait the for the special interval instead of the standard interval before sending the next query, see Registry Settings 0x2 UseAsFallbackOnly Use the specified NTP server as fallback only 0x4 SymmatricActive Force sending symmetric active peer requests to the specified NTP server 0x8 Client Force sending client requests to the specified NTP server

As already mentioned above, some versions of w32time used to send symmetric active peer requests to NTP servers by default, but if the NTP server runs the standard NTP software (ntpd),the server may not reply to such unauthenticated peer requests at all. The normal behavior is to send client requests to a server, in which case the serversends a server reply.

The network packets normally exchanged between NTP clients and servers to synchronize time don't contain any secrets, just the current time. So the policy of the NTP project maintainers is not to encrypt the whole NTP packets, but just to append a cryptographic signature to each network packet, so the NTP client can be sure the NTP packet it receives really originates from the NTP server it is expected from, and has not been spoofed modified by a man in the middle.

If encryption is really required for an application then an encrypted tunnel (VPN) between the NTP client andserver can be set up, across which the unencrypted NTP packets can be exchanged. However, this alsoaffects the network delay and jitter, and thus reduces the accuracy that can be achieved.

Symmetric key authentication was already introduced in NTP v3, but is still supported in current NTP v4 versions of the NTP reference implementation. The drawback of symmetric keys is that a secret key has to be exchanged in a safe way between servers and clients, while with public key authentication schemes only a public key had to be copied to clients.

Assuming the NTP server and client should use the key f294fa0he from the example above, The key ID can be different on the server and client, but the configuration must be such that the same key type and string is used on both.

Assuming a time source provides accurate time, the accuray you can yield when synchronizing your system time on a specific client hardware depends mostly on the implementation of the software on the system to be synchronized. The only things an NTP server has to do when it receives an NTP request packet from a client is to:

So obviously it's the task of the client software to determine and compensate the packet delays on the network, eventually filter out outliers if the a request and/or reply packet has been delayed more than usually, and adjust its own system time more or less accurately. The server can't do this, it can just provide accurate time.

A good client implementation like ntpd evaluates the time stamps from a series of subsequent packet exchanges, tries to filter out network delays and jitter, and takes much care to adjust the system time as smoothly as possible, unnoticeably for applications.

A simple client implementation could just pick up the time from the server reply and just set the system time whenever it has queried the time from a server. This is what some SNTP (Simple NTP) implementations are doing, and this obviously only yields less accuracy than a more sophisticated implementation.

For any of the combinations above the computed time offset may be correct, or be more or less off due to some systematic errors. E.g., an asymmetric network connection (e.g. an ADSL line) can cause a real time offset of a few milliseconds which the client software can't determine, and thus such systematic offsets can't be compensated. In case of a 1 PPS hardware signal a time offset error up to a few 10s of microseconds can be caused by interrupt latency, depending on the system characteristics.

Ensuring that you have a very accurate time synchronization in your mount is essential if you wish to have accurate pointing and for some mounts, accurate tracking. Since the earth rotates at 15 arc seconds per second, being 1 sec out means that you can inadvertently introduce a 15 arc secs error into your telescope pointing. Whilst I've previously used the excellent free software NTP client Meinberg ( _stable) to synchronize my PC to my NEQ6 mount, I've never really explored what it was doing to my PC clock or how effective it was at keeping time synchronization. My motivation for exploring this a bit further was driven by my recent purchase of a 10 micron mount which requires a very precise time reference.

In my investigations I discovered a very useful installation guide written by Whitham D. Reeve ( Papers/Reeve_NTP-MeinMon_Install.pdf). Here, my major discovery was that to prevent conflicts, you need to disable Windows from performing an internet time synchronization. So you end up with the message this computer is not set to automatically synchronize with an internet time server, which seems misleading since the Meinberg client using the internet to synchronize to a time server !

My second discovery was to find out what the Meinberg client was actually doing my PC clock, for this you need to download a separate bit of free software known as the NTP Time Server Monitor ( -server-monitor.htm), a detailed guide of how to use and configure this is given here Papers/Reeve_MeinbergMonGuide.pdf One interesting option is to use the statistics function to plot how your PC clock is slowly changed via the Meinberg client to synchronize with the various NTP servers. Eventually, you end up with a plot like this:

This means that the NTP service (ntpd) on this machine must not adjust the system time on this computer since this is already done by the Meinberg time adjustment service. The only task of the ntpd instance on this machine is to make the disciplined time available on the network for other NTP clients.

Normally an NTP client sends a request packets to a server, and the server sends a reply packet back to the client. This allows the client to estimate and thus compensate the network delay for each individual packet exchange.See also Time Synchronization Accuracy With NTP.In broadcast mode the client doesn't know how long a packet has been traveling on the network before the packet was received, so the propagation delay can't be compensated reliably, and thus the resulting accuracy is usually worse than with a standard client/server setup.

Please note authentication is required by default for broadcast mode in order to prevent broadcast clients from accepting NTP broadcasts from unauthenticated sources on the network. Otherwise clients might accept broadcast packets from any device on the network which sends NTP broadcasts intentionally or unintentionally, and set their system time wrong.Usually symmetric key authentication is used with broadcast mode.

The configuration above is sufficient for 3rd party SNTP clients on the subnets which just expect to receive unauthenticated NTP broadcast packets,but ntpd in the role of a broadcast client wouldn't accept such unauthenticated packets by default. Instead, symmetric key authentication can be configured, and either the same key can be used for all subnets, or different keys can be used for different subnets as in the example below:

NTP cients which are to receive NTP broadcast packets also need to be explicitely configured as broadcast clients by adding the broadcastclient directive to the ntp.conf file.In addition, authentication has to be configured to match the server's authentication for the particular network. For a Windows client in the 192.168.1.255 broadcast domain running ntpd the ntp.conf file could contain the following lines: 17dc91bb1f