machine_virtual: bind multicast to localhost #3717
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When using socket-based network devices, with the `mcast` option (but
without an explicitly specified local address), qemu attempts to find a
suitable interface to use by scanning the available "up" interfaces for
ones that have the MULTICAST flag enabled (as per `ip link`).
The loopback interface doesn't normally have this flag enabled, so if
it's the only one that's up, the search will fail, causing the following
message:
libvirt: QEMU Driver error : internal error: qemu unexpectedly closed the monitor: 2022-08-09T15:52:35.668412Z qemu-system-x86_64: -netdev socket,mcast=230.0.0.1:5600,id=mcast0: can't add socket to multicast group 230.0.0.1: No such device
If we force qemu to use the loopback interface, however, it will
succeed — even without the multicast flag being set. Since we're not
actually interested in communicating between machines, force
127.0.0.1 as the local address.
This allows `image-customize`, `vm-run`, and testing in general to work
without the presence of any network connection, and without hacks such
as adding a dummy bridge or veth device.
Fixes cockpit-project/cockpit#12782
|
Manually testing on my laptop with no active network interfaces shows that runs successfully and passes. Before this change, it would fail immediately with the "No such device" message. I can also confirm that posting comments to GitHub doesn't work without an active network interface 🤣 |
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When using socket-based network devices, with the
mcastoption (butwithout an explicitly specified local address), qemu attempts to find a
suitable interface to use by scanning the available "up" interfaces for
ones that have the MULTICAST flag enabled (as per
ip link).The loopback interface doesn't normally have this flag enabled, so if
it's the only one that's up, the search will fail, causing the following
message:
If we force qemu to use the loopback interface, however, it will
succeed — even without the multicast flag being set. Since we're not
actually interested in communicating between machines, force
127.0.0.1 as the local address.
This allows
image-customize,vm-run, and testing in general to workwithout the presence of any network connection, and without hacks such
as adding a dummy bridge or veth device.
Fixes cockpit-project/cockpit#12782