[ipxe-devel] Broadcast UDP is rejected during pxebs

[Daniel Pieczko (dpieczko)]

I am testing PXE Boot Server Discovery using undionly.kpxe (from ipxe
commit f3c2da7) and I have noticed the broadcast replies are being rejected.

I PXE boot into undionly.kpxe and then run 'dhcp net0' to get an IP
address for my interface.

Then I run 'pxebs net0 65535' and during that PXEBS request the response
packet is rejected.

Here is a packet capture during pxebs:

[1] 00:0f:53:08:0a:10 > Broadcast, ethertype IPv4 (0x0800), length 447:
192.168.0.100.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from
00:0f:53:08:0a:10, length 405

[2] 00:0f:53:07:bf:04 > Broadcast, ethertype IPv4 (0x0800), length 312:
192.168.0.97.bootps > 255.255.255.255.bootpc: BOOTP/DHCP, Reply, length 270

[3] 00:0f:53:08:0a:10 > Broadcast, ethertype IPv4 (0x0800), length 447:
192.168.0.100.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from
00:0f:53:08:0a:10, length 405


Packet #2 is the reply from one of the servers and it is to a broadcast
IP address of 255.255.255.255 and broadcast MAC ff:ff:ff:ff:ff:ff.

Building undionly.kpxe with 'DEBUG=dhcp,ipv4,udp' I get the following
debug messages during pxebs:
=========================
DHCP 0x1f654 attempting 255.255.255.255
UDP 0x1f714 allocated
UDP 0x1f714 bound to port 68
DHCP 0x1f654 entering PXEBS state
DHCP 0x1f654 PXEBS REQUEST to 255.255.255.255:67 for type 65535
No UDP connection listening on port 68
IPv4 received packet rejected by stack: The socket is not connected
... more PXEBS requests
UDP 0x1f714 closed
Connection timed out
Could not discover boot server on net0: Connection timed out
=========================

The packet is rejected because it doesn't match any of the udp_conns in
udp_demux() - this is because we have added the local IP address to the
socket at the beginning of start_pxebs().

My understanding is the we should be operating in promiscuous mode and
we specifically set the BOOTP flag for BROADCAST since we have an IP
address at this point, so I believe the reply from the boot server is
correct.

Does what I have described make sense (is this a bug?) or do I have an
error in my setup?

Another way of looking at this by ignoring the boot server setup:  if I
start pxebs and then inject any UDP broadcast (MAC and IP) packet back
to the client I see it dropped in the same way.  I think that is a bug
but I'd appreciate other opinions in case I'm misunderstanding this.


Daniel

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