Setting the SO_REUSEADDR property on a socket allows binding to a port
number that is in the TIMED_WAIT state. This is usually done on listener
sockets, to enable a server to restart itself without having to wait for
the completion of TIMED_WAIT on the port.
It is also possible, but highly unusual, to set it on client sockets. It
is rare to explicitly bind() a client socket, since it is almost always
fine to allow the kernel to auto-bind a client socket to a random free
port. Most systems will have many 10's of 1000's of free ports that
client sockets will be bound to.
eg on Linux
$ sysctl -a | grep local_port
net.ipv4.ip_local_port_range = 32768 60999
eg on OpenBSD
$ sysctl -a | grep net.inet.ip.port
net.inet.ip.portfirst=1024
net.inet.ip.portlast=49151
net.inet.ip.porthifirst=49152
net.inet.ip.porthilast=65535
A connected socket must have a unique set of value for
(protocol, localip, localport, remoteip, remoteport)
otherwise it is liable to get EADDRINUSE.
A client connection should trivially avoid EADDRINUSE if letting the
kernel auto-assign the 'localport' value, which QEMU always does.
When QEMU sets SO_REUSEADDR on a client socket on OpenBSD, however, it
upsets this situation.
The OpenBSD kernel appears to happily pick a 'localport' that is in the
TIMED_WAIT state, even if there are many other available local ports
available for use that are not in the TIMED_WAIT state.
A test program that just loops opening client sockets will start seeing
EADDRINUSE on OpenBSD when as few as 2000 ports are in TIMED_WAIT,
despite 10's of 1000's ports still being unused. This contrasts with
Linux which appears to avoid picking local ports in TIMED_WAIT state.
This problem on OpenBSD exhibits itself periodically with the migration
test failing with a message like[1]:
qemu-system-ppc64: Failed to connect to '127.0.0.1:24109': Address already in use
While I have not been able to reproduce the OpenBSD failure in my own
testing, given the scope of what QEMU tests do, it is entirely possible
that there could be a lot of ports in TIMED_WAIT state when the
migration test runs.
Removing SO_REUSEADDR from the client sockets should not affect normal
QEMU usage, and should improve reliability on OpenBSD.
This use of SO_REUSEADDR on client sockets is highly unusual, and
appears to have been present since the very start of the QEMU socket
helpers in 2008. The orignal commit has no comment about the use of
SO_REUSEADDR on the client, so is most likely just an 16 year old
copy+paste bug.
[1] https://lists.nongnu.org/archive/html/qemu-devel/2024-10/msg03427.htmlhttps://lists.nongnu.org/archive/html/qemu-devel/2024-02/msg01572.html
Fixes: d247d25f18
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
===========
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Documentation
=============
Documentation can be found hosted online at
`<https://www.qemu.org/documentation/>`_. The documentation for the
current development version that is available at
`<https://www.qemu.org/docs/master/>`_ is generated from the ``docs/``
folder in the source tree, and is built by `Sphinx
<https://www.sphinx-doc.org/en/master/>`_.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
.. code-block:: shell
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
* `<https://wiki.qemu.org/Hosts/Linux>`_
* `<https://wiki.qemu.org/Hosts/Mac>`_
* `<https://wiki.qemu.org/Hosts/W32>`_
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
.. code-block:: shell
git clone https://gitlab.com/qemu-project/qemu.git
When submitting patches, one common approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the `style section
<https://www.qemu.org/docs/master/devel/style.html>`_ of
the Developers Guide.
Additional information on submitting patches can be found online via
the QEMU website:
* `<https://wiki.qemu.org/Contribute/SubmitAPatch>`_
* `<https://wiki.qemu.org/Contribute/TrivialPatches>`_
The QEMU website is also maintained under source control.
.. code-block:: shell
git clone https://gitlab.com/qemu-project/qemu-web.git
* `<https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/>`_
A 'git-publish' utility was created to make above process less
cumbersome, and is highly recommended for making regular contributions,
or even just for sending consecutive patch series revisions. It also
requires a working 'git send-email' setup, and by default doesn't
automate everything, so you may want to go through the above steps
manually for once.
For installation instructions, please go to:
* `<https://github.com/stefanha/git-publish>`_
The workflow with 'git-publish' is:
.. code-block:: shell
$ git checkout master -b my-feature
$ # work on new commits, add your 'Signed-off-by' lines to each
$ git publish
Your patch series will be sent and tagged as my-feature-v1 if you need to refer
back to it in the future.
Sending v2:
.. code-block:: shell
$ git checkout my-feature # same topic branch
$ # making changes to the commits (using 'git rebase', for example)
$ git publish
Your patch series will be sent with 'v2' tag in the subject and the git tip
will be tagged as my-feature-v2.
Bug reporting
=============
The QEMU project uses GitLab issues to track bugs. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
* `<https://gitlab.com/qemu-project/qemu/-/issues>`_
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via GitLab.
For additional information on bug reporting consult:
* `<https://wiki.qemu.org/Contribute/ReportABug>`_
ChangeLog
=========
For version history and release notes, please visit
`<https://wiki.qemu.org/ChangeLog/>`_ or look at the git history for
more detailed information.
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC:
* `<mailto:qemu-devel@nongnu.org>`_
* `<https://lists.nongnu.org/mailman/listinfo/qemu-devel>`_
* #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
* `<https://wiki.qemu.org/Contribute/StartHere>`_
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