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ntp
===

The ntp package is an implementation of a Simple NTP (SNTP) client based on
[RFC 5905](https://tools.ietf.org/html/rfc5905). It allows you to connect to
a remote NTP server and request information about the current time.


## Querying the current time

If all you care about is the current time according to a remote NTP server,
simply use the `Time` function:
```go
time, err := ntp.Time("0.beevik-ntp.pool.ntp.org")
```


## Querying time synchronization data

To obtain the current time as well as some additional synchronization data,
use the [`Query`](https://godoc.org/github.com/beevik/ntp#Query) function:
```go
response, err := ntp.Query("0.beevik-ntp.pool.ntp.org")
time := time.Now().Add(response.ClockOffset)
```

The [`Response`](https://godoc.org/github.com/beevik/ntp#Response) structure
returned by `Query` includes the following information:
* `ClockOffset`: The estimated offset of the local system clock relative to
  the server's clock. For a more accurate time reading, you may add this
  offset to any subsequent system clock reading.
* `Time`: The time the server transmitted its response, according to its own
  clock.
* `RTT`: An estimate of the round-trip-time delay between the client and the
  server.
* `Precision`: The precision of the server's clock reading.
* `Stratum`: The server's stratum, which indicates the number of hops from the
  server to the reference clock. A stratum 1 server is directly attached to
  the reference clock. If the stratum is zero, the server has responded with
  the "kiss of death" and you should examine the `KissCode`.
* `ReferenceID`: A unique identifier for the consulted reference clock.
* `ReferenceTime`: The time at which the server last updated its local clock setting.
* `RootDelay`: The server's aggregate round-trip-time delay to the stratum 1 server.
* `RootDispersion`: The server's estimated maximum measurement error relative
  to the reference clock.
* `RootDistance`: An estimate of the root synchronization distance between the
  client and the stratum 1 server.
* `Leap`: The leap second indicator, indicating whether a second should be
  added to or removed from the current month's last minute.
* `MinError`: A lower bound on the clock error between the client and the
  server.
* `KissCode`: A 4-character string describing the reason for a "kiss of death"
  response (stratum=0).
* `Poll`: The maximum polling interval between successive messages to the
  server.

The `Response` structure's [`Validate`](https://godoc.org/github.com/beevik/ntp#Response.Validate)
function performs additional sanity checks to determine whether the response
is suitable for time synchronization purposes.
```go
err := response.Validate()
if err == nil {
    // response data is suitable for synchronization purposes
}
```

If you wish to customize the behavior of the NTP query, use the
[`QueryWithOptions`](https://godoc.org/github.com/beevik/ntp#QueryWithOptions)
function:
```go
options := ntp.QueryOptions{ Timeout: 30*time.Second, TTL: 5 }
response, err := ntp.QueryWithOptions("0.beevik-ntp.pool.ntp.org", options)
time := time.Now().Add(response.ClockOffset)
```

Configurable [`QueryOptions`](https://godoc.org/github.com/beevik/ntp#QueryOptions)
include:
* `Timeout`: How long to wait before giving up on a response from the NTP
  server.
* `Version`: Which version of the NTP protocol to use (2, 3 or 4).
* `TTL`: The maximum number of IP hops before the request packet is discarded.
* `LocalAddress`: The local IP address to use when sending the query. Useful
  when the host has multiple network interfaces.
* `Auth`: The symmetric authentication key and algorithm used by the server to
  authenticate the query. The same information is used by the client to
  authenticate the server's response.
* `Extensions`: Extensions may be added to modify NTP queries before they are
	transmitted and to process NTP responses after they arrive.
* `GetSystemTime`: A custom function to obtain the current system time, used
  to override the default `time.Now` function.
* `Dialer`: A custom network connection "dialer" function used to override the
  default UDP dialer function.

## Using the NTP pool

The NTP pool is a shared resource provided by the [NTP Pool
Project](https://www.pool.ntp.org/en/) and used by people and services all
over the world. To prevent it from becoming overloaded, please avoid querying
the standard `pool.ntp.org` zone names in your applications. Instead, consider
requesting your own [vendor zone](http://www.pool.ntp.org/en/vendors.html) or
[joining the pool](http://www.pool.ntp.org/join.html).


## Network Time Security (NTS)

Network Time Security (NTS) is a recent enhancement of NTP, designed to add
better authentication and message integrity to the protocol. It is defined by
[RFC 8915](https://tools.ietf.org/html/rfc8915). If you wish to use NTS, see
the [nts package](https://github.com/beevik/nts). (The nts package is
implemented as an extension to this package.)
