13ec827052
When libvirt first implemented a stable and configurable MAC address for the bridges created for libvirt virtual networks (commit 5754dbd56d, in libvirt v0.8.8) most distro stable releases didn't support explicitly setting the MAC address of a bridge; the bridge just always assumed the lowest numbered MAC of all attached interfaces. Because of this, we stabilized the bridge MAC address by creating a "dummy" tap interface with a MAC address guaranteed to be lower than any of the guest tap devices' MACs (which all started with 0xFE, so it's not difficult to do) and attached it to the bridge - this was the inception of the "virbr0-nic" device that has confused so many people over the years. Even though the linux kernel had recently gained support for explicitly setting a bridge MAC, we deemed it unnecessary to set the MAC that way, because the other (indirect) method worked everywhere. But recently there have been reports that the bridge MAC address was not following the setting in the network config, and mismatched the MAC of the dummy tap device (which was still correct). It turns out that this is due to a change in systemd-242 that persists whatever MAC address is set for a bridge when it's initially started. According to the systemd NEWS file entry for version 242 (https://github.com/systemd/systemd/blob/master/NEWS): "if a bridge interface is created without any slaves, and gains a slave later, then now the bridge does not inherit slave's MAC." This change was the result of: https://github.com/systemd/systemd/issues/3374 (apparently if there is no MAC saved for a bridge by the name of a bridge being created, the random MAC generated during creation is saved, and then that same MAC is used to explicitly set the MAC each time it is created). Once a bridge has an explicitly set MAC, the "use the lowest numbered MAC of attached devices" rule is ignored, so our dummy tap device is like the goggles - it does nothing! (well, almost). We could whine about changes in default behavior, etc. etc., but because the change was in response to actual user problems, that seems likely a fruitless task. Fortunately, time has marched on, and even distro releases that are old enough that they are no longer supported by upstream libvirt (e.g. RHEL6) have support for explicitly setting a bridge device MAC address, either during creation or with a separate ioctl after creation, so we can now do that. To enable explicitly setting the mac during bridge creation, we add a mac arg to virNetDevBridgeCreate(). In the case of platforms where the bridge is created with a netlink RTM_NEWLINK message, we just add that mac to the message. For platforms that still use an ioctl (either SIOCBRADDBR or SIOCIFCREATE2), we make a separate call to virNetDevSetMAC() after creating the bridge. (NB: I was unable to test the calling of virNetDevSetMAC() from the SIOCIFCREATE2 (BSD) version of virNetDevBridgeCreate(); even though I managed to get a FreeBSD system setup and libvirt built there, when I tried to start the default network the SIOCIFCREATE2 ioctl itself failed, so it never even got to the virNetDevSetMAC(). That leaves the FreeBSD implementation untested.) This makes the dummy tap pointless for purposes of setting the MAC address, but it is still useful for IPv6 DAD initialization (which apparently requires at least one interface to be attached to the bridge and online), as well as for setting an initial MTU for the bridge, so it hasn't been removed. (NB: we can safely *always* call virNetDevBridgeCreate() with &def->mac from the network driver because, in spite of the existence of a "mac_specified" bool in the config suggesting that it may not always be present, in reality a mac address will always be added to any network that doesn't have one - this is guaranteed in all cases by commit a47ae7c004) https://bugzilla.redhat.com/show_bug.cgi?id=1760851 Signed-off-by: Laine Stump <laine@redhat.com> Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Libvirt API for virtualization
Libvirt provides a portable, long term stable C API for managing the virtualization technologies provided by many operating systems. It includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER Hypervisor.
For some of these hypervisors, it provides a stateful management daemon which runs on the virtualization host allowing access to the API both by non-privileged local users and remote users.
Layered packages provide bindings of the libvirt C API into other languages including Python, Perl, PHP, Go, Java, OCaml, as well as mappings into object systems such as GObject, CIM and SNMP.
Further information about the libvirt project can be found on the website:
License
The libvirt C API is distributed under the terms of GNU Lesser General
Public License, version 2.1 (or later). Some parts of the code that are
not part of the C library may have the more restrictive GNU General
Public License, version 2.0 (or later). See the files COPYING.LESSER
and COPYING for full license terms & conditions.
Installation
Libvirt uses the GNU Autotools build system, so in general can be built and installed with the usual commands. For example, to build in a manner that is suitable for installing as root, use:
$ ./configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var
$ make
$ sudo make install
While to build & install as an unprivileged user
$ ./configure --prefix=$HOME/usr
$ make
$ make install
The libvirt code relies on a large number of 3rd party libraries. These will
be detected during execution of the configure script and a summary printed
which lists any missing (optional) dependencies.
Contributing
The libvirt project welcomes contributions in many ways. For most components the best way to contribute is to send patches to the primary development mailing list. Further guidance on this can be found on the website:
https://libvirt.org/contribute.html
Contact
The libvirt project has two primary mailing lists:
- libvirt-users@redhat.com (for user discussions)
- libvir-list@redhat.com (for development only)
Further details on contacting the project are available on the website: