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libvirt 5.6.0-2
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
  <body>
    <h1>Libvirt NSS module</h1>

    <ul id="toc"></ul>

    <p>
    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:
    </p>

    <code>ssh user@virtualMachine</code>

    <p>
    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
    <code>/etc/hosts</code> file in sync. But this puts needless burden onto
    users. This is where NSS module comes handy.
    </p>

    <h2><a id="Installation">Installation</a></h2>

    <p>
    Installing the module is really easy:
    </p>

<pre>
# yum install libvirt-nss
</pre>

    <h2><a id="Configuration">Configuration</a></h2>

    <p>
    Enabling the module is really easy. Just add <b>libvirt</b> into
    <code>/etc/nsswitch.conf</code> file. For instance:
    </p>

<pre>
$ cat /etc/nsswitch.conf
# /etc/nsswitch.conf:
passwd:      compat
shadow:      compat
group:       compat
hosts:       files libvirt dns
# ...
</pre>

    <p>
    So, in this specific case, whenever ssh program is looking up the host user
    is trying to connect to, <b>files</b> module is consulted first (which
    boils down to looking up the host name in <code>/etc/hosts</code> file), if
    not found <b>libvirt</b> 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.
    </p>

    <h2><a id="Sources">Sources of information</a></h2>

    <p>
    As of <code>v3.0.0</code> release, libvirt offers two NSS modules
    implementing two different methods of hostname translation. The first and
    older method is implemented by <code>libvirt</code> 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 <code>libvirt_guest</code> module which does libvirt guest
    name to IP address translation (regardless of hostname set in the guest).
    </p>

    <p>
    To enable either of the modules put their name into the
    <code>nsswitch.conf</code> file. For instance, to enable
    <code>libvirt_guest</code> module:
    </p>

<pre>
$ cat /etc/nsswitch.conf
# /etc/nsswitch.conf:
hosts:       files libvirt_guest dns
# ...
</pre>
    <p>Or users can enable both at the same time:</p>
<pre>
$ cat /etc/nsswitch.conf
# /etc/nsswitch.conf:
hosts:       files libvirt libvirt_guest dns
# ...
</pre>

    <p>
    This configuration will mean that if hostname is not found by the
    <code>libvirt</code> module (e.g. because a guest did not sent hostname
    during DHCP transaction), the <code>libvirt_guest</code> module is
    consulted (and if the hostname matches libvirt guest name it will be
    resolved).
    </p>

    <h2><a id="Internals">How does it work?</a></h2>

    <p>
    Whenever an Unix process wants to do a host name translation
    <a href="http://linux.die.net/man/3/gethostbyname"><code>gethostbyname()</code></a>
    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. <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
    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 <a href="https://en.wikipedia.org/wiki/Name_Service_Switch">wiki
    page</a> is recommended.
    </p>

    <p>
    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.
    </p>

    <h2><a id="Limitations">Limitations</a></h2>

    <ol>
      <li>The <code>libvirt</code> 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 <code>v3.0.0</code> there are two libvirt NSS modules
        translating both hostnames provided by guest and libvirt guest names.</li>
      <li>The module works only in that cases where IP addresses are assigned by
        dnsmasq spawned by libvirt. Libvirt NATed networks are typical
        example.</li>
    </ol>

    <p>
    <i>The following paragraph describes implementation limitation of the
    <code>libvirt</code> NSS module.</i>
    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:
    </p>

<pre>
virsh net-dhcp-leases $network
</pre>

    <p>
    where <code>$network</code> iterates through all running networks. So the module
    does merely the same as
    </p>

<pre>
virsh domifaddr --source lease $domain
</pre>

    <p>
    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 <code>v3.0.0</code> libvirt provides <code>libvirt_guest</code> NSS
    module that doesn't have this limitation. However, the statement is still
    true for the <code>libvirt</code> NSS module.
    </p>
  </body>
</html>