mirror of
https://gitlab.com/libvirt/libvirt.git
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docs: Convert 'nss' page to rST
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
This commit is contained in:
Peter Krempa
parent
98ae4c3102
commit
b4ae5dbe4a
@ -51,7 +51,6 @@ docs_html_in_files = [
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'internals',
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'internals',
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'java',
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'java',
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'logging',
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'logging',
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'nss',
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'pci-hotplug',
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'pci-hotplug',
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'php',
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'php',
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'python',
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'python',
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@ -103,6 +102,7 @@ docs_rst_files = [
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'macos',
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'macos',
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'migration',
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'migration',
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'newreposetup',
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'newreposetup',
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'nss',
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'pci-addresses',
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'pci-addresses',
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'platforms',
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'platforms',
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'programming-languages',
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'programming-languages',
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189
docs/nss.html.in
189
docs/nss.html.in
@ -1,189 +0,0 @@
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Libvirt NSS module</h1>
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<ul id="toc"></ul>
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<p>
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When it comes to managing guests and executing commands inside them, logging
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into guest operating system and doing the job is convenient. Users are used
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to ssh in this case. Ideally:
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</p>
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<code>ssh user@virtualMachine</code>
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<p>
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would be nice. But depending on virtual network configuration it might not
|
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be always possible. For instance, when using libvirt NATed network it's
|
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dnsmasq (spawned by libvirt) who assigns IP addresses to domains. But by
|
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default, the dnsmasq process is then not consulted when it comes to host
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name translation. Users work around this problem by configuring their
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libvirt network to assign static IP addresses and maintaining
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<code>/etc/hosts</code> file in sync. But this puts needless burden onto
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users. This is where NSS module comes handy.
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</p>
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<h2><a id="Installation">Installation</a></h2>
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<p>
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Installing the module is really easy:
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</p>
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<pre>
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# yum install libvirt-nss
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</pre>
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<h2><a id="Configuration">Configuration</a></h2>
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<p>
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Enabling the module is really easy. Just add <b>libvirt</b> into
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<code>/etc/nsswitch.conf</code> file. For instance:
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</p>
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<pre>
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$ cat /etc/nsswitch.conf
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# /etc/nsswitch.conf:
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passwd: compat
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shadow: compat
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group: compat
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hosts: files libvirt dns
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# ...
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</pre>
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<p>
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So, in this specific case, whenever ssh program is looking up the host user
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is trying to connect to, <b>files</b> module is consulted first (which
|
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boils down to looking up the host name in <code>/etc/hosts</code> file), if
|
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not found <b>libvirt</b> module is consulted then. The DNS is the last
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effort then, if none of the previous modules matched the host in question.
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Therefore users should consider the order in which they want the modules to
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lookup given host name.
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</p>
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<h2><a id="Sources">Sources of information</a></h2>
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<p>
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As of <code>v3.0.0</code> release, libvirt offers two NSS modules
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implementing two different methods of hostname translation. The first and
|
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older method is implemented by <code>libvirt</code> plugin and
|
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basically looks up the hostname to IP address translation in DHCP server
|
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records. Therefore this is dependent on hostname provided by guests. Thing
|
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is, not all the guests out there provide one in DHCP transactions, or not
|
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every sysadmin out there believes all the guests. Hence libvirt implements
|
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second method in <code>libvirt_guest</code> module which does libvirt guest
|
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name to IP address translation (regardless of hostname set in the guest).
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</p>
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|
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<p>
|
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To enable either of the modules put their name into the
|
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<code>nsswitch.conf</code> file. For instance, to enable
|
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<code>libvirt_guest</code> module:
|
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</p>
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<pre>
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$ cat /etc/nsswitch.conf
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# /etc/nsswitch.conf:
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hosts: files libvirt_guest dns
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# ...
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</pre>
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<p>Or users can enable both at the same time:</p>
|
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<pre>
|
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$ cat /etc/nsswitch.conf
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# /etc/nsswitch.conf:
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hosts: files libvirt libvirt_guest dns
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# ...
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</pre>
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|
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<p>
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This configuration will mean that if hostname is not found by the
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<code>libvirt</code> module (e.g. because a guest did not sent hostname
|
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during DHCP transaction), the <code>libvirt_guest</code> module is
|
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||||||
consulted (and if the hostname matches libvirt guest name it will be
|
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resolved).
|
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</p>
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<h2><a id="Internals">How does it work?</a></h2>
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<p>
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Whenever an Unix process wants to do a host name translation
|
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<a href="https://linux.die.net/man/3/gethostbyname"><code>gethostbyname()</code></a>
|
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or some variant of it is called. This is a glibc function that takes a
|
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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
|
|
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records (e.g. <code>/etc/hosts</code> 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
|
|
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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
|
|
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reading <a href="https://en.wikipedia.org/wiki/Name_Service_Switch">wiki
|
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page</a> is recommended.
|
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</p>
|
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|
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<p>
|
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And this is point where libvirt comes in. Libvirt provides plugin for the
|
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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.
|
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</p>
|
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|
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<h2><a id="Limitations">Limitations</a></h2>
|
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|
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<ol>
|
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<li>The <code>libvirt</code> NSS module matches only hostnames provided by guest.
|
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If the libvirt name and one advertised by guest differs, the latter is
|
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||||||
matched. However, as of <code>v3.0.0</code> there are two libvirt NSS modules
|
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||||||
translating both hostnames provided by guest and libvirt guest names.</li>
|
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<li>The module works only in that cases where IP addresses are assigned by
|
|
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dnsmasq spawned by libvirt. Libvirt NATed networks are typical
|
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example.</li>
|
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</ol>
|
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|
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<p>
|
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<i>The following paragraph describes implementation limitation of the
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<code>libvirt</code> NSS module.</i>
|
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These limitation are result of libvirt's internal implementation. While
|
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libvirt can report IP addresses regardless of their origin, a public API
|
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||||||
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
|
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view the list themselves:
|
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</p>
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|
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<pre>
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virsh net-dhcp-leases $network
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</pre>
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<p>
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where <code>$network</code> iterates through all running networks. So the module
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does merely the same as
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</p>
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|
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<pre>
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virsh domifaddr --source lease $domain
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</pre>
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|
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<p>
|
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If there's no record for either of the aforementioned commands, it's
|
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||||||
very likely that NSS module won't find anything and vice versa.
|
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||||||
As of <code>v3.0.0</code> libvirt provides <code>libvirt_guest</code> NSS
|
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||||||
module that doesn't have this limitation. However, the statement is still
|
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||||||
true for the <code>libvirt</code> NSS module.
|
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</p>
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</body>
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</html>
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154
docs/nss.rst
Normal file
154
docs/nss.rst
Normal file
@ -0,0 +1,154 @@
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|
==================
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|
Libvirt NSS module
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|
==================
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|
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|
.. contents::
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|
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||||||
|
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.
|
||||||
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