9. The TC (truncated) header bit

The TC bit is set in a response when the response has been truncated. That is, when the response is too large to fit in a single UDP packet, the server sets the TC bit, and returns as much of the response as will fit. The client must then re-issue the query using TCP, which does not have the same size limits.

The TC bit must only be set in a response when the response has been truncated. It must not be set in a response that has not been truncated.

The TC bit must not be set in a query. A query with the TC bit set is invalid, and must be ignored by the server.

When a server sets the TC bit, it must include as much of the response as will fit in the UDP packet. It must not include any RRs that are not part of the response. In particular, it must not include RRs from the additional section that are not required to answer the query.

A server must not set the TC bit in a response that is sent using TCP. TCP has no size limit, and so a response sent using TCP can never be truncated.