irs.conf File

Purpose

Specifies the ordering of certain name resolution services.

Description

The /etc/irs.conf file is used to control the order of mechanisms that the resolver libraries use in searching for network-related data. The following subroutines resolve host names, networks, services, protocols, and netgroups:

Because these subroutines are commonly used in many TCP/IP applications, using the irs.conf file can control the directions of the queries made by these applications as well.

By default, the subroutines use the lookup mechanisms to resolve host names in this order:

1. Domain Name Server (DNS)
2. Network Information Service (NIS), if active
3. local files

By default, the subroutines use the lookup mechanisms to resolve networks in this order:

1. DNS
2. NIS, if active
3. local files

By default, the subroutines use the lookup mechanisms to resolve other maps in this order:

1. NIS, if active
2. local files

You can override the default order by modifying the /etc/irs.conf configuration file and specifying the desired ordering.

The settings in the /etc/netsvc.conf configuration file override the settings in the /etc/irs.conf file. The NSORDER environment variable overrides the settings in the /etc/irs.conf and the /etc/netsvc.conf files.

To use DNS to obtain information concerning netgroups, protocols, and services, you must create and use a Hesiod DNS Zone in the following format:

map mechanism [option]

The following values are available for the map parameter:

The following values are available for the mechanism parameter:

The following values are available for the option parameter:

Examples

1. To use only the local /etc/hosts file for host name resolution, enter:

   hosts local

2. To use the LDAP server to resolve a host name that cannot be found in the the /etc/hosts file, enter:

   hosts local continue
   hosts ldap

3. To use only DNS to resolve host names and to use NIS to resolve protocols, enter:

   hosts dns
   protocols nis

4. To use only NIS to resolve host name addresses and netgroups and to use the local files to resolve services and networks, enter:

   hosts nis
   services local
   netgroup nis
   networks local

5. To try to resolve host names from the local /etc/hosts file and, after not finding them, try to resolve from DNS, then NIS, enter:

   hosts local continue
   hosts dns continue
   hosts nis continue

6. To try to resolve host names from the local /etc/hosts file, merge any information found with any DNS information found, and then merge this information to any NIS information found, enter:

   hosts local merge
   hosts dns merge
   hosts nis

   If the resolver finds no information, it returns none. If it finds information from more than one source, it returns that information as a merged response from all of the available sources.

7. To examine the local /etc/services file for port numbers before querying NIS, enter:

   services local continue
   services nis
   

   This entry in the /etc/irs.conf file could speed up the request; normally, querying NIS takes more time than querying the local file. If the resolver does not find the information in the local file, it searches NIS.

8. To query for IPv4 network addresses only from DNS and to query IPv6 host addresses only from the local file, enter:

   networks dns4
   hosts local6

9. In this example, assume the following presuppositions:
   • The /etc/hosts file contains the following information:

     1.2.3.4 host4
     1.2.3.5 host5
     1.2.3.6 host6

   • The information in DNS is the following:

     1.2.3.2 host2
     1.2.3.3 host3

   • The information in NIS is the following:

     1.2.3.1 host1

   To instruct the gethostbyname subroutine to look for the host name first in the local configuration files, then to continue to search in DNS if the host name is not found, and finally to continue searching in NIS if the host name is not found, create the following entry in the /etc/irs.conf file:

   hosts local continue
   hosts dns continue
   hosts nis

   In this example, the gethostbyname subroutine cannot find the host name in the /etc/hosts file and continues to search for the host name in DNS. After not finding it in DNS, it continues to search in NIS. After finding the address in NIS, it returns 1.2.3.1.

10. In this example, assume the following presuppositions:
    • The /etc/hosts file contains the following information:

      1.1.1.1 hostname

    • The information in DNS is the following:

      1.1.1.2 hostname

    To instruct the gethostbyname subroutine to merge all the answers from the specified mechanisms into one reply, create the following entry in the /etc/irs.conf file:

    hosts local merge
    hosts dns

    The gethostbyname subroutine returns 1.1.1.1 1.1.1.2.

Files