TCP IP Illustrated

Htm 6 of 12 12092001 144700 chapter 12 broadcasting

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Unformatted text preview: as not designed to support broadcasting, such as TFTP? (We cover TFTP in more detail in Chapter 15.) bsdi % tftp tftp> connect 140.252.13.63 start the client specify the IP address of the server file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (6 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting tftp> get temp.foo tftp: sendto: Permission denied tftp> quit and try to fetch a file from the server terminate the client Here we get an error immediately, and nothing is sent on the cable. What's happening here is that the sockets API doesn't allow a process to send a UDP datagram to the broadcast address unless the process specifically states that it plans to broadcast. This is intended to prevent users from mistakenly specifying a broadcast address (as we did here) when the application was never intended to broadcast. With the sockets API the application must set the SO_BROADCAST socket option before sending a UDP datagram to a broadcast address. Not all systems enforce this restriction. Some implementations allow any process to broadcast UDP datagrams, without requiring the process to say so. Others are more restrictive and require a process to have superuser privileges to broadcast. The next question is whether directed broadcasts are forwarded or not. Some kernels and routers have an option to enable or disable this feature. (See Appendix E.) If we enable this feature on our router bsdi and run ping from the host slip, we can see if the subnet-directed broadcasts are forwarded by bsdi. Forwarding a directed broadcast means the router takes the incoming unicast datagram, determines that the destination address is the directed broadcast for one of its interfaces, and then forwards the datagram onto the appropriate network using a link-layer broadcast. slip % ping 140.252.13.63 PING 140.252.13.63 (140.252.13.63): 56 data bytes 64 bytes from 140.252.13.35: icmp_seq=0 ttl=255 time=190 ms 64 bytes from 140.252.13.33: i...
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