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12047PATCH: Enhance simulation networking in Linux

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  • Steve
    May 20, 2016
    Hiya, folks.

    I've been playing with NuttX for about a week now, and so far I'm pretty happy
    with it.  I did find an issue with the simluation's network code, however, so
    I set about making it work for my environment (which turned into more
    extensive work than I intended, but oh well).  Please find information on the
    attached patch below.  Greg, please feel free to merge if you like it.

    - Steve

       This patch enhances networking support for the simulation under Linux.
       Includes updated support for Linux TUN/TAP, and the addition of support for
       Linux bridge devices.

       o Check to see if the d_txavail callback is present before calling it in
         the arp send code.  This prevents a segfault when simulating the telnetd
         daemon with arp send enabled.

       o Adjust the simulation's netdriver_loop() so it will detect and respond to
         ARP requests.

       o Do not attempt to take the tap device's hardware address for use by the
         simulation.  That hardware address belongs to the host end of the link,
         not the simulation end.  Generate a randomized MAC address instead.

       o Do not assign an IP address to the interface on the host side of the TAP

       + Provide two modes: "host route" and "bridge".

       + In host route mode, maintain a host route that points any traffic for the
         simulation's IP address to the tap device.  In this mode, so long as the
         simulation's IP is a free address in the same subnet as the host, no
         additional configuration will be required to talk to it from the host.
         Note that address changes are handled automatically if they follow the
         rule of if-down/set-address/if-up, which everything seems to.

       + In bridge mode, add the tap device to the specified bridge instance.  See
         configs/sim/NETWORK-LINUX.txt for information and usage examples.  This
         enables much more flexible configurations (with fewer headaches), such as
         running multiple simulations on a single host, all of which can access
         the network the host is connected to.

       o Refresh configurations in configs/sim where CONFIG_NET=y.  They default
         to "host route" mode.

       o Add configs/sim/NETWORK-LINUX.txt

       - The MAC address generation code is extremely simplistic, and does not
         check for potential conflicts on the network.  Probably not an issue, but
         something to be aware of.

       - I was careful to leave it in a state where Cygwin/pcap should still work,
         but I don't have a Windows environment to test in.  This should be

       - I don't know if this was ever intended to work with OS X.  I didn't even
         try to test it there.

       - Was able to get telnetd working and simulate nsh over telnet, but only so
         long as listen backlogs were disabled.

         There appears to be a bug in the backlog code where sockets are being
         returned in SYN_RCVD state instead of waiting until they're ESTABLISHED;
         if you perform an immediate send after accepting the connection, it will
         confuse the stack and the send will hang; additionally, the connection
         will never reach ESTABLISHED state.

         Can be worked around by adding a sleep(1) after the accept in telnetd.  I
         don't have the necessary knowledge of the IP stack to know what the
         correct fix is.

       - If you would prefer this in the form of a merge request, let me know.

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