Promiscuous ports under Linux

[Peter Pearson]

Cobbling together a minimalist ethernet-sniffing program, I was hoping
to use this simple mechanism for setting the socket to "promiscuous
mode" (to see all traffic going past, instead of just traffic addressed
to my machine):

    s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)

Unfortunately, it seems that that mechanism is available under Windows
but not under Linux.  Googling around for Linux equivalents, I found
only very contorted solutions, and concluded that maybe this lacuna
persists because Linux users find it convenient to invoke promiscuous
mode from the command line, instead:

    $ sudo ip link set eth0 promisc on
    $ netstat -i        # (Verify that the P flag is set.)

This somehow fails: my sniffer continues to see only broadcasts,
but if I run dumpcap at the same time, dumpcap captures lots of traffic.

So my question is now two questions:

 . Is it true that going permiscuous under Linux must be somewhat ugly?
   (It's OK if it is, I'll just copy the ugly code and get moving again.)

 . Why doesn't the command-line "promisc on" work?  (Granted, this is
   maybe a Linux question.)

Thanks.

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