Frequently Asked Questions

Why should I use htpdate?

You shouldn't, use NTP if you can. NTP is much more accurate.... however NTP doesn't work in combination with proxy servers. Also NTP network traffic (UDP port 123) is often blocked by firewalls and doesn't always work if you are behind a NAT device. If you have access to websites, htpdate will work.

How accurate is htpdate?

It depends on the accuracy of the time stamps provided by the choosen web servers. With multiple servers the accuracy increases. In general htpdate will keep your time close to ±0.5 seconds. Take a look at here to get an impression.

Which web servers can I use for time synchronization?

All web servers are required to return a time stamp in their response. The quality/accuracy isn't always, what you might expect. Use htpdate with verbose/debug output ("-d" switch), to find some good web servers close to you.

How many server should I use with htpdate?

"A man with one watch knows what time it is. A man with two watches is never sure.". Three servers is the recommended minimum. Just in case one server is way off, I would recommend to use 4 servers. A maximum number of 16 web servers is supported by htpdate.

Which algorithms does htpdate use?

Read the comments in the source...

How much bandwidth does htpdate use?

Short answer : Very, very little (too little to worry about). Long answer : One poll of htpdate will require about 10 packets (about 800 bytes, depending on the HTTP header size of the web server) of TCP traffic. If a DNS lookup is required, some UDP packets may be needed. With the default installation htpdate will poll, in worst case, every 17 minutes (2^10 seconds) per web server. In general HTP will require less bandwidth than NTP.

Does htpdate support leap seconds?

No. Htpdate will adjust the second, as if a second offset was detected.