189c099f75
While we are at it add a brief preamble that explains some of the common concepts in QEMU's device emulation which will hopefully lead to less confusing about our dizzying command line options. Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Reviewed-by: Markus Armbruster <armbru@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Daniel P. Berrangé <berrange@redhat.com> Cc: Eduardo Habkost <ehabkost@redhat.com> Message-Id: <20210720232703.10650-3-alex.bennee@linaro.org>
101 lines
4.1 KiB
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101 lines
4.1 KiB
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.. _pcsys_005fnetwork:
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Network emulation
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-----------------
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QEMU can simulate several network cards (e.g. PCI or ISA cards on the PC
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target) and can connect them to a network backend on the host or an
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emulated hub. The various host network backends can either be used to
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connect the NIC of the guest to a real network (e.g. by using a TAP
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devices or the non-privileged user mode network stack), or to other
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guest instances running in another QEMU process (e.g. by using the
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socket host network backend).
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Using TAP network interfaces
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This is the standard way to connect QEMU to a real network. QEMU adds a
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virtual network device on your host (called ``tapN``), and you can then
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configure it as if it was a real ethernet card.
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Linux host
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^^^^^^^^^^
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As an example, you can download the ``linux-test-xxx.tar.gz`` archive
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and copy the script ``qemu-ifup`` in ``/etc`` and configure properly
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``sudo`` so that the command ``ifconfig`` contained in ``qemu-ifup`` can
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be executed as root. You must verify that your host kernel supports the
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TAP network interfaces: the device ``/dev/net/tun`` must be present.
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See :ref:`sec_005finvocation` to have examples of command
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lines using the TAP network interfaces.
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Windows host
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^^^^^^^^^^^^
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There is a virtual ethernet driver for Windows 2000/XP systems, called
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TAP-Win32. But it is not included in standard QEMU for Windows, so you
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will need to get it separately. It is part of OpenVPN package, so
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download OpenVPN from : https://openvpn.net/.
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Using the user mode network stack
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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By using the option ``-net user`` (default configuration if no ``-net``
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option is specified), QEMU uses a completely user mode network stack
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(you don't need root privilege to use the virtual network). The virtual
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network configuration is the following::
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guest (10.0.2.15) <------> Firewall/DHCP server <-----> Internet
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| (10.0.2.2)
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----> DNS server (10.0.2.3)
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----> SMB server (10.0.2.4)
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The QEMU VM behaves as if it was behind a firewall which blocks all
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incoming connections. You can use a DHCP client to automatically
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configure the network in the QEMU VM. The DHCP server assign addresses
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to the hosts starting from 10.0.2.15.
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In order to check that the user mode network is working, you can ping
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the address 10.0.2.2 and verify that you got an address in the range
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10.0.2.x from the QEMU virtual DHCP server.
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Note that ICMP traffic in general does not work with user mode
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networking. ``ping``, aka. ICMP echo, to the local router (10.0.2.2)
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shall work, however. If you're using QEMU on Linux >= 3.0, it can use
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unprivileged ICMP ping sockets to allow ``ping`` to the Internet. The
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host admin has to set the ping_group_range in order to grant access to
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those sockets. To allow ping for GID 100 (usually users group)::
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echo 100 100 > /proc/sys/net/ipv4/ping_group_range
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When using the built-in TFTP server, the router is also the TFTP server.
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When using the ``'-netdev user,hostfwd=...'`` option, TCP or UDP
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connections can be redirected from the host to the guest. It allows for
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example to redirect X11, telnet or SSH connections.
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Hubs
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~~~~
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QEMU can simulate several hubs. A hub can be thought of as a virtual
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connection between several network devices. These devices can be for
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example QEMU virtual ethernet cards or virtual Host ethernet devices
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(TAP devices). You can connect guest NICs or host network backends to
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such a hub using the ``-netdev
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hubport`` or ``-nic hubport`` options. The legacy ``-net`` option also
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connects the given device to the emulated hub with ID 0 (i.e. the
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default hub) unless you specify a netdev with ``-net nic,netdev=xxx``
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here.
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Connecting emulated networks between QEMU instances
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Using the ``-netdev socket`` (or ``-nic socket`` or ``-net socket``)
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option, it is possible to create emulated networks that span several
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QEMU instances. See the description of the ``-netdev socket`` option in
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:ref:`sec_005finvocation` to have a basic
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example.
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