Network Configuration Files

Linux name resolution relies on three key files: /etc/hosts, /etc/resolv.conf, and /etc/nsswitch.conf.Together, they define where and in what order to look for hostname information.

1. /etc/hosts

  • Contains a table of hostnames and their IP addresses.
  • Can supplement DNS for name resolution.
  • Example:

127.0.0.1   localhost
* Lookups in this file are checked first if /etc/nsswitch.conf prioritizes files.

2. /etc/resolv.conf

  • Stores DNS server IP addresses for hostname resolution.
  • Can include additional keywords like domain and search.
  • Example:

nameserver 10.0.2.3
nameserver 10.0.2.4
* Resolution process:

  1. Tries the first nameserver.
  2. If unavailable or timeout, tries the second nameserver.

  3. Keywords:

    • domain: Appends the domain for partial hostnames (e.g., polaris → polaris.snowblower.example.com).
    • search: Lists multiple domains to try in order for name resolution.

3. /etc/nsswitch.conf

  • Configures the order of hostname lookup sources.
  • Example entry:
hosts: files dns
  • files → /etc/hosts is checked first.
  • dns → DNS server(s) in /etc/resolv.conf are used second.

  • Order affects resolution behavior:

  • If a hostname is found in /etc/hosts, DNS may not be used, even if the IP is incorrect.

Name Resolution Flow

  1. System consults /etc/nsswitch.conf to determine lookup order.
  2. Checks /etc/hosts for a matching entry.
  3. If no match, queries the DNS server(s) listed in /etc/resolv.conf.
  4. Result is cached for a configurable period.