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Configuring resolver nsswitch.conf file

nsswitch.conf file

Configuring the resolver with nsswitch.conf

The nsswitch.conf file tells the resolver library[1] which of the three possible methods to use to convert host names to IP addresses and back.
Here is a sample file, taken from a Linux machine:

#
# /etc/nsswitch.conf
# An example Name Service Switch config file.  
# This file should be sorted with the most-used  
# services at the beginning.
#
# The entry '[NOTFOUND=return]' means that the  
# search for an entry should stop if the search  
# in the previous entry turned up nothing. Note  
# that if the search failed due to some other reason  
# (like no NIS server responding) then the search 
# continues with the next entry.
#
# Legal entries are:
#
# nisplus or nis+ Use NIS+ (NIS version 3)
# nis or yp Use NIS (NIS version 2), also called 
# YP dns Use DNS (Domain Name Service)
# files Use the local files
# [NOTFOUND=return] Stop searching if not found 
# so far
#
passwd: files nisplus nis
shadow: files nisplus nis
group: files nisplus nis
hosts: files nisplus nis dns
services: nisplus [NOTFOUND=return] files
networks: nisplus [NOTFOUND=return] files
protocols: nisplus [NOTFOUND=return] files
rpc: nisplus [NOTFOUND=return] files
ethers: nisplus [NOTFOUND=return] files
netmasks: nisplus [NOTFOUND=return] files
bootparams: nisplus [NOTFOUND=return] files
netgroup: nisplus
publickey: nisplus
automount: files nisplus
aliases: files nisplus

We will study this file in more detail later, but for now, focus on the line marked hosts:
hosts: files nisplus nis dns
This line tells the resolver library to first look in the /etc/hosts file (files), then try to contact an NIS+ server (nisplus), then an NIS server (nis), and finally to try the Domain Name Service.

[1]Resolver library: The TCP/IP protocol library software that formats requests to be sent to the Domain Name Server for hostname to Internet address conversion.