Libvirt NSS module

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() 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 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

  1. 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.
  2. 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.