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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?
** Affects: libvirt (Ubuntu)
Importance: Undecided
Status: New
** Tags: broadcast nat network
--
source IP address of broadcast packets gets rewritten when using NAT
https://bugs.launchpad.net/bugs/1078305
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