Method and apparatus for controlling transmission of data packets in a packet-switched network. When a first end-host (A) sends an address query to a DNS system (300) for a second end-host, the DNS system responds by providing a sender key created from a destination key registered for the second end-host, if the first end-host is authorised to send packets to the second end-host. Thereby, the first end- host, if authorised, is able to get across data packets to the second end-host by attaching a sender tag (TAG) generated from the sender key, as ingress tag to each transmitted data packet. A router (302) in the network matches an ingress tag in a received packet with entries in a forwarding table and sends out the packet on an output port (X) according to a matching entry. Otherwise, the router discards the packet if no matching entry is found in the table.