The nsswitch.conf File
Version 2 of the GNU standard library includes a more powerful and flexible replacement for the older host.conf mechanism. The concept of the name service has been extended to include a variety of different types of information. Configuration options for all of the different functions that query these databases have been brought back into a single configuration file; the nsswitch.conf file.
The nsswitch.conf file allows the system administrator to configure a wide variety of different databases. We'll limit our discussion to options that relate to host and network IP address resolution. You can easily find more information about the other features by reading the GNU standard library documentation.
Options in nsswitch.conf must appear on separate lines. Fields may be separated by whitespace (spaces or tabs). A hash sign (#) introduces a comment that extends to the next newline. Each line describes a particular service; hostname resolution is one of these. The first field in each line is the name of the database, ending with a colon. The database name associated with host address resolution is hosts. A related database is networks, which is used for resolution of network names into network addresses. The remainder of each line stores options that determine the way lookups for that database are performed.
The following options are available:
dns
Use the Domain Name System (DNS) service to resolve the address. This makes sense only for host address resolution, not network address resolution. This mechanism uses the /etc/resolv.conf file that we'll describe later in the chapter.
files
Search a local file for the host or network name and its corresponding address. This option uses the traditional /etc/hosts and /etc/network files.
nis or nisplus
Use the Network Information System (NIS) to resolve the host or network address. NIS and NIS+ are discussed in detail in Chapter 13, The Network Information System.
The order in which the services to be queried are listed determines the order in which they are queried when attempting to resolve a name. The query-order list is in the service description in the /etc/nsswitch.conf file. The services are queried from left to right and by default searching stops when a resolution is successful.
A simple example of host and network database specification that would mimic our configuration using the older libc standard library is shown in Example 6.2.
Example 6.2: Sample nsswitch.conf File
# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# Information about this file is available in the `libc6-doc' package.
hosts: dns files
networks: files
This example causes the system to look up hosts first in the Domain Name System, and the /etc/hosts file, if that can't find them. Network name lookups would be attempted using only the /etc/networks file.
You are able to control the lookup behavior more precisely using "action items" that describe what action to take given the result of the previous lookup attempt. Action items appear between service specifications, and are enclosed within square brackets, []. The general syntax of the action statement is:
[[!] status = action…]
There are two possible actions:
return
Controls returns to the program that attempted the name resolution. If a lookup attempt was successful, the resolver will return with the details, otherwise it will return a zero result.
continue
The resolver will move on to the next service in the list and attempt resolution using it.
The optional (!) character specifies that the status value should be inverted before testing; that is, it means "not."
The available status values on which we can act are:
success
The requested entry was found without error. The default action for this status is return.
notfound
There was no error in the lookup, but the target host or network could not be found. The default action for this status is continue.
unavail
The service queried was unavailable. This could mean that the hosts or networks file was unreadable for the files service or that a name server or NIS server did not respond for the dns or nis services. The default action for this status is continue.
tryagain
This status means the service is temporarily unavailable. For the files files service, this would usually indicate that the relevant file was locked by some process. For other services, it may mean the server was temporarily unable to accept connections. The default action for this status is continue.
A simple example of how you might use this mechanism is shown in Example 6.3.
Example 6.3: Sample nsswitch.conf File Using an Action Statement
# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# Information about this file is available in the `libc6-doc' package.
hosts: dns [!UNAVAIL=return] files
networks: files
This example attempts host resolution using the Domain Name Service system. If the return status is anything other than unavailable, the resolver returns whatever it has found. If, and only if, the DNS lookup attempt returns an unavailable status, the resolver attempts to use the local /etc/hosts. This means that we should use the hosts file only if our name server is unavailable for some reason.