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[Qemu-devel] [Bug 1078305] Re: source IP address of broadcast packets ge
From: |
Pallinger Péter |
Subject: |
[Qemu-devel] [Bug 1078305] Re: source IP address of broadcast packets gets rewritten when using NAT |
Date: |
Wed, 14 Nov 2012 09:26:17 -0000 |
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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