[Qemu-devel] [Bug 1838228] Re: Slirp Broadcast traffic

[Chris Koch]

Thanks. Opened https://gitlab.freedesktop.org/slirp/libslirp/issues/9.

** Bug watch added: gitlab.freedesktop.org/slirp/libslirp/issues #9
   https://gitlab.freedesktop.org/slirp/libslirp/issues/9

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https://bugs.launchpad.net/bugs/1838228

Title:
  Slirp Broadcast traffic

Status in QEMU:
  New

Bug description:
  Hi all,

  Version: QEMU emulator version 3.1.0 (Debian 1:3.1+dfsg-7+build1)

  I'm running some DHCP traffic to a *custom* DHCP server with user-mode
  networking in QEMU. I'm using port 30067 for the server, so this does
  not conflict with the built-in DHCP server.

  DHCP broadcasts to and from the server, and I'm observing issues with
  both sending and receiving packets.

  Firstly, from the VM, a packet sent to IPv4 x.x.x.2:30067 (gateway)
  makes it to the server, but a packet sent to 255.255.255.255 does not.
  I'd suspect that Slirp has to support sending to the broadcast IP
  address? Or is this something I can turn on with a configuration
  option? (My QEMU version too old?)

  Secondly, the source address in a DHCP IPv4 packet must be 0.0.0.0 (by
  RFC). That means that any return packet will have 0.0.0.0 swapped in
  as its destination address. However, that packet doesn't make it into
  the VM at all. I know that if you deliver this packet to Linux, a raw
  socket will spit it back out. The packets' destination address should
  not prevent the packet from being delivered to the right VM, since
  Slirp (should?) know exactly which VM the session belongs to. (It's a
  proxy, not a router.)

  WDYT? Did I miss some configuration options or use too old a version?

  Thanks,
  Chris

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