Broadcast traffic & Routing

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