[Qemu-devel] [Bug 1078305] Re: source IP address of broadcast packets gets rewritten when using NAT

[Pallinger Péter]

Ok, I srewed up the iptables rules a little, so this is a working
workaround script:

IFACE=$(route -n | grep '^0.0.0.0 ' | sed 's/.* //g')
main_broadcast_addr=$(ip -4 addr show "$IFACE" | grep 'inet .* brd ' | sed 
's/.* brd //;s/\([0-9.]*\).*/\1/')

iptables -t nat -A OUTPUT -d 255.255.255.255 -p tcp -j DNAT --to-destination 
$main_broadcast_addr
iptables -t nat -A OUTPUT -d 255.255.255.255 -p udp -j DNAT --to-destination 
$main_broadcast_addr

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https://bugs.launchpad.net/bugs/1078305

Title:
  source IP address of broadcast packets gets rewritten when using NAT

Status in QEMU:
  New

Bug description:
  If I send a broadcast message like this to the limited broadcast
  address:

      echo a | nc -bu 255.255.255.255 5000

  then the resulting packet looks like this on the sender side:

      14:36:17.997662 02:00:c0:a8:7a:fb (oui Unknown) > Broadcast, ethertype 
IPv4 (0x0800), length 44: (tos 0x0, ttl 64, id 25278, offset 0, flags [DF], 
proto UDP (17), length 30)
      192.168.122.251.42141 > 255.255.255.255.5000: [bad udp cksum 476f!] UDP, 
length 2

  However, an other VM on the same host sees the following packet:

  14:36:19.247793 02:00:c0:a8:7a:fb (oui Unknown) > Broadcast, ethertype IPv4 
(0x0800), length 44: (tos 0x0, ttl 64, id 25278, offset 0, flags [DF], proto 
UDP (17), length 30)
      192.168.122.1.42141 > 255.255.255.255.5000: [bad udp cksum 3b71!] UDP, 
length 2

  So the source MAC address and other headers are untouched, but the
  source IP address is changed to the default gateway's!

  If I use a the subnet-specific broadcast, then the packets are left
  untouched, i.e.:

  echo a | nc -bu 192.168.122.255 5000

  14:41:33.313490 02:00:c0:a8:7a:fb (oui Unknown) > Broadcast, ethertype IPv4 
(0x0800), length 44: (tos 0x0, ttl 64, id 38571, offset 0, flags [DF], proto 
UDP (17), length 30)
      192.168.122.251.46821 > 192.168.122.255.5000: [bad udp cksum aee5!] UDP, 
length 2

  14:41:34.563615 02:00:c0:a8:7a:fb (oui Unknown) > Broadcast, ethertype IPv4 
(0x0800), length 44: (tos 0x0, ttl 64, id 38571, offset 0, flags [DF], proto 
UDP (17), length 30)
      192.168.122.251.46821 > 192.168.122.255.5000: [bad udp cksum aee5!] UDP, 
length 2

  Is this a configuration issue, or a more fundamental issue of the
  virtual switch itself?

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