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(gawkinet.info)File /inet/raw

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`/inet/raw'
...........

   This is an IP-level protocol. Only `root' is allowed to access this
special file. It is meant to be the basis for implementing and
experimenting with transport level protocols.(1) In the most general
case, the sender has to supply the encapsulating header bytes in front
of the packet and the receiver has to strip the additional bytes from
the message.

   RAW receivers cannot receive packets sent with TCP or UDP because the
operating system does not deliver the packets to a RAW receiver. The
operating system knows about some of the protocols on top of IP and
decides on its own which packet to deliver to which process.  (d.c.)
Therefore, the UDP receiver must be used for receiving UDP datagrams
sent with the RAW sender. This is a dark corner, not only of `gawk',
but also of TCP/IP.

   For extended experimentation with protocols, look into the approach
implemented in a tool called SPAK.  This tool reflects the hierarchical
layering of protocols (encapsulation) in the way data streams are piped
out of one program into the next one.  It shows which protocol is based
on which other (lower-level) protocol by looking at the command-line
ordering of the program calls.  Cleverly thought out, SPAK is much
better than `gawk''s `/inet' for learning the meaning of each and every
bit in the protocol headers.

   The next example uses the RAW protocol to emulate the behavior of
UDP. The sender program is the same as above, but with some additional
bytes that fill the places of the UDP fields:

     BEGIN {
       Message = "Hello world\n"
       SourcePort = 0
       DestinationPort = 8888
       MessageLength = length(Message)+8
       RawService = "/inet/raw/0/localhost/0"
       printf("%c%c%c%c%c%c%c%c%s",
           SourcePort/256, SourcePort%256,
           DestinationPort/256, DestinationPort%256,
           MessageLength/256, MessageLength%256,
           0, 0, Message) |& RawService
       fflush(RawService)
       close(RawService)
     }

   Since this program tries to emulate the behavior of UDP, it checks if
the RAW sender is understood by the UDP receiver but not if the RAW
receiver can understand the UDP sender. In a real network, the RAW
receiver is hardly of any use because it gets every IP packet that
comes across the network. There are usually so many packets that `gawk'
would be too slow for processing them.  Only on a network with little
traffic can the IP-level receiver program be tested. Programs for
analyzing IP traffic on modem or ISDN channels should be possible.

   Port numbers do not have a meaning when using `/inet/raw'. Their
fields have to be `0'. Only TCP and UDP use ports. Receiving data from
`/inet/raw' is difficult, not only because of processing speed but also
because data is usually binary and not restricted to ASCII. This
implies that line separation with `RS' does not work as usual.

   ---------- Footnotes ----------

   (1) This special file is reserved, but not otherwise currently
implemented.