mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-12-22 17:34:18 +03:00
b4ae5dbe4a
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
155 lines
6.4 KiB
ReStructuredText
155 lines
6.4 KiB
ReStructuredText
==================
|
|
Libvirt NSS module
|
|
==================
|
|
|
|
.. contents::
|
|
|
|
When it comes to managing guests and executing commands inside them, logging
|
|
into guest operating system and doing the job is convenient. Users are used to
|
|
ssh in this case. Ideally:
|
|
|
|
``ssh user@virtualMachine``
|
|
|
|
would be nice. But depending on virtual network configuration it might not be
|
|
always possible. For instance, when using libvirt NATed network it's dnsmasq
|
|
(spawned by libvirt) who assigns IP addresses to domains. But by default, the
|
|
dnsmasq process is then not consulted when it comes to host name translation.
|
|
Users work around this problem by configuring their libvirt network to assign
|
|
static IP addresses and maintaining ``/etc/hosts`` file in sync. But this puts
|
|
needless burden onto users. This is where NSS module comes handy.
|
|
|
|
Installation
|
|
------------
|
|
|
|
Installing the module is really easy:
|
|
|
|
::
|
|
|
|
# yum install libvirt-nss
|
|
|
|
Configuration
|
|
-------------
|
|
|
|
Enabling the module is really easy. Just add **libvirt** into
|
|
``/etc/nsswitch.conf`` file. For instance:
|
|
|
|
::
|
|
|
|
$ cat /etc/nsswitch.conf
|
|
# /etc/nsswitch.conf:
|
|
passwd: compat
|
|
shadow: compat
|
|
group: compat
|
|
hosts: files libvirt dns
|
|
# ...
|
|
|
|
So, in this specific case, whenever ssh program is looking up the host user is
|
|
trying to connect to, **files** module is consulted first (which boils down to
|
|
looking up the host name in ``/etc/hosts`` file), if not found **libvirt**
|
|
module is consulted then. The DNS is the last effort then, if none of the
|
|
previous modules matched the host in question. Therefore users should consider
|
|
the order in which they want the modules to lookup given host name.
|
|
|
|
Sources of information
|
|
----------------------
|
|
|
|
As of ``v3.0.0`` release, libvirt offers two NSS modules implementing two
|
|
different methods of hostname translation. The first and older method is
|
|
implemented by ``libvirt`` plugin and basically looks up the hostname to IP
|
|
address translation in DHCP server records. Therefore this is dependent on
|
|
hostname provided by guests. Thing is, not all the guests out there provide one
|
|
in DHCP transactions, or not every sysadmin out there believes all the guests.
|
|
Hence libvirt implements second method in ``libvirt_guest`` module which does
|
|
libvirt guest name to IP address translation (regardless of hostname set in the
|
|
guest).
|
|
|
|
To enable either of the modules put their name into the ``nsswitch.conf`` file.
|
|
For instance, to enable ``libvirt_guest`` module:
|
|
|
|
::
|
|
|
|
$ cat /etc/nsswitch.conf
|
|
# /etc/nsswitch.conf:
|
|
hosts: files libvirt_guest dns
|
|
# ...
|
|
|
|
Or users can enable both at the same time:
|
|
|
|
::
|
|
|
|
$ cat /etc/nsswitch.conf
|
|
# /etc/nsswitch.conf:
|
|
hosts: files libvirt libvirt_guest dns
|
|
# ...
|
|
|
|
This configuration will mean that if hostname is not found by the ``libvirt``
|
|
module (e.g. because a guest did not sent hostname during DHCP transaction), the
|
|
``libvirt_guest`` module is consulted (and if the hostname matches libvirt guest
|
|
name it will be resolved).
|
|
|
|
How does it work?
|
|
-----------------
|
|
|
|
Whenever an Unix process wants to do a host name translation
|
|
`gethostbyname() <https://linux.die.net/man/3/gethostbyname>`__ or some variant
|
|
of it is called. This is a glibc function that takes a string containing the
|
|
host name, crunch it and produces a list of IP addresses assigned to that host.
|
|
Now, glibc developers made a really good decision when implementing the
|
|
internals of the function when they decided to make the function pluggable.
|
|
Since there can be several sources for the records (e.g. ``/etc/hosts`` file,
|
|
DNS, LDAP, etc.) it would not make much sense to create one big implementation
|
|
containing all possible cases. What they have done instead is this pluggable
|
|
mechanism. Small plugins implementing nothing but specific technology for lookup
|
|
process are provided and the function then calls those plugins. There is just
|
|
one configuration file that instructs the lookup function in which order should
|
|
the plugins be called and which plugins should be loaded. For more info reading
|
|
`wiki page <https://en.wikipedia.org/wiki/Name_Service_Switch>`__ is
|
|
recommended.
|
|
|
|
And this is point where libvirt comes in. Libvirt provides plugin for the NSS
|
|
ecosystem. For some time now libvirt keeps a list of assigned IP addresses for
|
|
libvirt networks. The NSS plugin does no more than search the list trying to
|
|
find matching record for given host name. When found, matching IP address is
|
|
returned to the caller. If not found, translation process continues with the
|
|
next plugin configured. At this point it is important to stress the order in
|
|
which plugins are called. Users should be aware that a hostname might match in
|
|
multiple plugins and right after first match, translation process is terminated
|
|
and no other plugin is consulted. Therefore, if there are two different records
|
|
for the same host name users should carefully chose the lookup order.
|
|
|
|
Limitations
|
|
-----------
|
|
|
|
#. The ``libvirt`` NSS module matches only hostnames provided by guest. If the
|
|
libvirt name and one advertised by guest differs, the latter is matched.
|
|
However, as of ``v3.0.0`` there are two libvirt NSS modules translating both
|
|
hostnames provided by guest and libvirt guest names.
|
|
#. The module works only in that cases where IP addresses are assigned by
|
|
dnsmasq spawned by libvirt. Libvirt NATed networks are typical example.
|
|
|
|
*The following paragraph describes implementation limitation of the ``libvirt``
|
|
NSS module.* These limitation are result of libvirt's internal implementation.
|
|
While libvirt can report IP addresses regardless of their origin, a public API
|
|
must be used to obtain those. However, for the API a connection object is
|
|
required. Doing that for every name translation request would be too costly.
|
|
Fortunately, libvirt spawns dnsmasq for NATed networks. Not only that, it
|
|
provides small executable that on each IP address space change updates an
|
|
internal list of addresses thus keeping it in sync. The NSS module then merely
|
|
consults the list trying to find the match. Users can view the list themselves:
|
|
|
|
::
|
|
|
|
virsh net-dhcp-leases $network
|
|
|
|
where ``$network`` iterates through all running networks. So the module does
|
|
merely the same as
|
|
|
|
::
|
|
|
|
virsh domifaddr --source lease $domain
|
|
|
|
If there's no record for either of the aforementioned commands, it's very likely
|
|
that NSS module won't find anything and vice versa. As of ``v3.0.0`` libvirt
|
|
provides ``libvirt_guest`` NSS module that doesn't have this limitation.
|
|
However, the statement is still true for the ``libvirt`` NSS module.
|