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190 lines
7.4 KiB
XML
190 lines
7.4 KiB
XML
<?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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<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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<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="http://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
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addresses assigned to that host. Now, glibc developers made a really good
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decision when implementing the internals of the function when they decided
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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
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make much sense to create one big implementation containing all possible
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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
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provided and the function then calls those plugins. There is just one
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configuration file that instructs the lookup function in which order should
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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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<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
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addresses for libvirt networks. The NSS plugin does no more than search the
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list trying to find matching record for given host name. When found,
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matching IP address is returned to the caller. If not found, translation
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process continues with the next plugin configured. At this point it is
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important to stress the order in which plugins are called. Users should be
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aware that a hostname might match in multiple plugins and right after first
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match, translation process is terminated and no other plugin is consulted.
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Therefore, if there are two different records for the same host name users
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should carefully chose the lookup order.
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</p>
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<h2><a id="Limitations">Limitations</a></h2>
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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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<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
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required. Doing that for every name translation request would be too
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costly. Fortunately, libvirt spawns dnsmasq for NATed networks. Not only
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that, it provides small executable that on each IP address space change
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updates an internal list of addresses thus keeping it in sync. The NSS
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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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<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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<pre>
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virsh domifaddr --source lease $domain
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</pre>
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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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