header
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... Use of the TC (truncated) header bit ...
... must cause the reply to be sent with the source address field in the
IP header set to the address that was in the destination address
field of the IP header ...
... IP header set to the address that was in the destination address
field of the IP header of the packet containing the query causing the
response. If this would cause the response to be sent from an IP
address ...
... The TC (truncated) header bit ...