4 ms·

In Search of a Secure Time Source

3 points by dankohn1 9y ago

CaliforniaKarl 9y ago
First off, an annoyance: I'm annoyed that there is no DHCP option to specify "This is the current UTC time". That would let the DHCP server tell the client what the current time is, with enough accuracy for most things. I mean, if you're already trusting in the info already provided by DHCP, you might as well trust this, too. DHCP Options 152-155 (from http://www.iana.org/go/rfc6926 http://www.iana.org/go/rfc6926) might work, but that's part of a different standard. Anyway, the main thing I wanted to note is that your ISP likely already has NTP servers that you can use. You might even be getting them, via DHCP Options 4 and/or 42 (though 42 is more appropriate). At least that way, your NTP queries don't go out to the wider Internet. If you have your own network, then you should have an NTP server. The bigger the network, the more NTP servers you should have. Each NTP server should be getting time from 10 different upstream servers (stratum 1 or 2). Ten is the maximum number of NTP servers which NTPd will check at once. Querying more NTP servers will allow NTPd to identify and exclude "falsetickers", which are serving bad time. Ideally, your NTP servers should have a local source. "Local source" means you are getting time from a device which queries something like GPS, GLONASS, WWVB, etc.. That way, when the local source is working, your NTP server becomes stratum-1; when not working, your NTP server becomes stratum-2 or -3, but can still serve time within your network. If you have multiple NTP servers, each one should have a different local source: One checking GPS, one checking WWVB, one checking GLONASS, etc.. You can tell NTPd that each of your servers should peer with the others, to increase accuracy. You could even have multiple local sources on each server. All of the is still unencrypted, though, but it is mitigated by keeping the NTP traffic within your own network. And also by having so many different time sources, it's possible to identify bad ones.

3 points by dankohn1 9y ago

First off, an annoyance: I'm annoyed that there is no DHCP option to specify "This is the current UTC time". That would let the DHCP server tell the client what the current time is, with enough accuracy for most things. I mean, if you're already trusting in the info already provided by DHCP, you might as well trust this, too. DHCP Options 152-155 (from http://www.iana.org/go/rfc6926 http://www.iana.org/go/rfc6926) might work, but that's part of a different standard. Anyway, the main thing I wanted to note is that your ISP likely already has NTP servers that you can use. You might even be getting them, via DHCP Options 4 and/or 42 (though 42 is more appropriate). At least that way, your NTP queries don't go out to the wider Internet. If you have your own network, then you should have an NTP server. The bigger the network, the more NTP servers you should have. Each NTP server should be getting time from 10 different upstream servers (stratum 1 or 2). Ten is the maximum number of NTP servers which NTPd will check at once. Querying more NTP servers will allow NTPd to identify and exclude "falsetickers", which are serving bad time. Ideally, your NTP servers should have a local source. "Local source" means you are getting time from a device which queries something like GPS, GLONASS, WWVB, etc.. That way, when the local source is working, your NTP server becomes stratum-1; when not working, your NTP server becomes stratum-2 or -3, but can still serve time within your network. If you have multiple NTP servers, each one should have a different local source: One checking GPS, one checking WWVB, one checking GLONASS, etc.. You can tell NTPd that each of your servers should peer with the others, to increase accuracy. You could even have multiple local sources on each server. All of the is still unencrypted, though, but it is mitigated by keeping the NTP traffic within your own network. And also by having so many different time sources, it's possible to identify bad ones.