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