The page contains some confusing information, especially around
limitations that supposedly only affect one of the two variants,
and goes into what is arguably an unnecessary amount of detail
when it comes to its inner workings.
We can make the page a lot shorter and snappier without
affecting its usefulness, so let's do just that.
Signed-off-by: Andrea Bolognani <abologna@xxxxxxxxxx>
---
docs/nss.rst | 189 +++++++++++++++++++--------------------------------
1 file changed, 70 insertions(+), 119 deletions(-)
diff --git a/docs/nss.rst b/docs/nss.rst
index 53955a3278..eeb48ea1f2 100644
--- a/docs/nss.rst
+++ b/docs/nss.rst
@@ -1,161 +1,112 @@
+.. role:: since
+
==================
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:
+Effectively managing guests often requires connecting to them via SSH, same as
+you would for any remote machine. Ideally
+
+::
+
+ $ ssh user@mydomain
-``ssh user@virtualMachine``
+would work out of the box, but depending on the network configuration that
+might not be the case. Setting up the libvirt NSS module is a one-time
+operation that makes the process of connecting from the host to the guests
+running on it much more convenient.
-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.
+Note that this feature only works in certain scenarios. See the
+`limitations <#limitations>`__ section for more information.
Installation
------------
-Installing the module is really easy:
+Installing the module on Fedora or RHEL is really easy:
::
# yum install libvirt-nss
+The package might have a different name on other distributions, but the process
+of installing it will be similar.
+
Configuration
-------------
-Enabling the module is really easy. Just add **libvirt** into
-``/etc/nsswitch.conf`` file. For instance:
+To enable the module, modify ``/etc/nsswitch.conf`` so that the ``hosts`` line
+looks similar to
::
- $ 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
-----------------------
+ hosts: files libvirt libvirt_guest dns
-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).
+With this configuration, whenever SSH (or any other application)
+tries to contact a guest, the ``files`` module will be consulted first (this
+boils down to searching for a matching line in ``/etc/hosts``); if no IP
+address could be found that way, the ``libvirt`` and ``libvirt_guest`` modules
+(see `below <#variants>`_ for differences between the two) will be used
+instead. Finally, if no previous attempt at resolving the hostname was
+successful, a DNS query will be performed.
-To enable either of the modules put their name into the ``nsswitch.conf`` file.
-For instance, to enable ``libvirt_guest`` module:
+Variants
+--------
-::
+There are two different variants of the module:
- $ cat /etc/nsswitch.conf
- # /etc/nsswitch.conf:
- hosts: files libvirt_guest dns
- # ...
+* ``libvirt`` (:since:`since 1.3.3`) resolves hostnames based on the
+ information that the guest OS itself has reported to the DHCP server when
+ asking for an IP address, so it won't work if the guest OS hasn't been fully
+ configured yet;
-Or users can enable both at the same time:
+* ``libvirt_guest`` (:since:`since 3.0.0`) resolves hostnames by mapping them
+ directly to libvirt domain names, so it will work regardless of how the guest
+ OS is configured and will have more predictable results.
-::
+The recommended configuration seen above enables both of them and gives
+priority to the former but it's also possible to enable only a single one, or
+to alter the precedence by simply changing the order in which they are listed.
- $ 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.
+Implementation details
+----------------------
-Limitations
------------
+Whenever a Unix process needs to convert a hostname into an IP address, it will
+call the `gethostbyname() <https://linux.die.net/man/3/gethostbyname>`__ libc
+function or one of its variants.
-#. 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:
+Since multiple sources for this information are possible (for example the
+contents of ``/etc/hosts``, DNS, LDAP, etc.) a mechanism called
+`NSS <https://en.wikipedia.org/wiki/Name_Service_Switch>`__ has been created to
+make the name resolution process extensible. This allows each source to be
+implemented as a separate plugin that can be enabled or disabled based on the
+administrator's preferences.
-::
+In the case of libvirt, the lookup is performed by inspecting the DHCP leases
+handed out by ``dnsmasq``, the software used to implement NATed networks. The
+results will be the same that would be reported by
- virsh net-dhcp-leases $network
+::
-where ``$network`` iterates through all running networks. So the module does
-merely the same as
+ $ virsh domifaddr --source lease mydomain
-::
+except that things will work transparently for any application that uses the
+libc resolver, without it needing to link against libvirt or even be aware of
+its existence.
- virsh domifaddr --source lease $domain
+Limitations
+-----------
-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.
+Since the libvirt NSS module works by looking at ``dnsmasq``, it can only work
+for guests that are connected to a NATed libvirt network. Guests that obtain
+their IP addresses in any other way (usermode networking, assigned network
+devices and so on) will not be able to have their hostnames resolved through
+it.
Alternatives
------------
-As of ``v10.3.0`` libvirt implements an `SSH proxy <ssh-proxy.html>`__ which
-doesn't require any network interface to SSH into the guest as SSH flows
-through a VSOCK device.
+:since:`Since 10.3.0`, libvirt implements an `SSH proxy <ssh-proxy.html>`__.
+This allows the use of SSH even for guests that have no network connectivity,
+by communicating over VSOCK.
--
2.47.0
