Meziantou.Framework

Response validation

edit

What the client checks

The socket of the client is connected to the server, so the operating system drops datagrams from any other address. With ValidateResponse set to true, the default, the client also requires that a reply:

  • echoes the transmit timestamp of the request in its originate timestamp, byte for byte (RFC 5905 TEST2), which ties the reply to this request;
  • has version 3 or 4, and the server mode;
  • is not a Kiss-o'-Death packet (stratum 0);
  • does not report an alarm condition (LeapIndicator.AlarmCondition), which means the clock of the server is not synchronized.

A reply that fails is ignored, and the client keeps waiting for a valid one until Timeout: a single stray or forged datagram cannot make the query fail. Even with ValidateResponse set to false, the client ignores packets that are too short, and replies without receive and transmit timestamps.

Kiss-o'-Death replies

A server that refuses to answer sends a Kiss-o'-Death packet: stratum 0, and a four-letter code in the reference identifier, such as RATE (the client queries too often), DENY or RSTR (the client is not allowed). With validation on, the client ignores it and the query ends with TimeoutException.

To see the code, set ValidateResponse to false, then check IsKissOfDeath and KissCode on the response. The timestamps of such a reply carry no time: do not use its offset. When a server sends RATE, query it less often.

This only works with servers whose refusal carries timestamps: the RATE reply of NtpServer has none, so the client ignores it even with validation off. See Rate limiting.

What the checks do not protect against

The checks tie a reply to the request; they do not authenticate the server. The library implements neither NTS (RFC 8915) nor symmetric key authentication, so an attacker who can read and inject packets on the path to the server can still choose the time the client reports.

Do not use the result alone for a security decision, such as the validity period of a certificate or a token. To limit the risk, query several servers and compare their answers, and check RootDispersion and Stratum.