Qemu does not close its filedescriptors (or setting the FD_CLOEXEC) when
invoking the /etc/qemu-ifup script.
Hence any background process spawned from there (such as a dhcpd) will
also inherit the open filedescriptor, preventing the relevant decide
(/dev/net/tup) to be reused later on:
If for example you chose a fixed network name using the -net ...ifname=
option and have an /etc/init.d/dhcp start in your if-up qemu (in order
to instruct dhcpd to handle the new interface), the newly spawned dhcp
will inherit the filedescriptor controlling the tun device, a continue
hogging it even after the qemu is stopped, and is started again.
Workaround:
Putting the following into the script:
exec 3>/dev/null
exec 4>/dev/null
exec 5>/dev/null
exec 6>/dev/null
But a cleaner solution would be if qemu itself was careful not to leak
those descriptors in the first place.
Thanks,
Alain
