NTP control messages

I have had an opportunity to rework some code to query NTP servers directly in python rather than running ntpq -pn and scraping the result.  The over-the-wire protocol format is reasonably straight forward and my little module is now passing all of its nosetests which is wonderful.

While its only a single table, ntpq actually does multiple requests to get its results.  NTP control messages are all mode 6 which tells the client and server its not time sort of things, but more about queries. A control packet has a sequence number which the server responds with so you can line up your requests with responses.  For large responses, the more, offset and count fields work together to knit up a large block of data.  The first packet of the response will have the offset field of 0. The count is the size of the data payload in bytes.  If the response is too big, the more bit will be set to 1 to say there is more coming.  The second packet will have an offset that is the same value as the previous packets count value, so you can stitch them together.

The first query is a READSTAT which uses opcode 1.  This returns a list of 4 byte responses with the association ID taking the first two bytes and the peer status taking the second.  If you look at the output of ntpq you will notice a character in the first column which is the peer selection.  The important one for us is asterix, which equates to 110 in the lower 3 bits of the first status byte.  This is the system peer which is the main one. There may be others, up to 2, (usually symbol +) which are compared to the system peer.  ntpq doesn’t really care about the different peers and just displays the status but I want to know what peer the server is synchronised to.

Next we need more details about the system peer (or all peers in ntpq).  To do this a READVAR or opcode 2 is sent to the server. The association ID is set to the association or peer we are interested in.  The response for my setup is sent over two packets which need to be reassembled and it is all in plain ascii which is a comma-separated list with a key=value pair.  I use the following python line to split it up.

self.assoc_data.update(dict([tuple(ad.strip().split("=")) for ad in data[header_len:].split(",") if ad.find("=") > -1]))

I’m sure people that know python better than I will say I am doing it wrong (it looks a bit klunky to me too) but it does work. From that I can query the various values that are returned by the server about its peers, including the source of its clock.

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