Files Reference

irs.conf File

Purpose

Specifies the ordering of certain name resolution services.

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:

host names	gethostbyname, gethostaddr, gethostent
networks	getnetbyname, getnetbyaddr, getnetent
services	getservbyname, getservbyaddr, getservent
protocols	getprotobyname, getprotobyaddr, getprotoent
netgroups	getnetgrent

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:

Domain Name Server (DNS)
Network Information Service (NIS), if active
local files

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

DNS
NIS, if active
local files

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

NIS, if active
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:

services	Lists the port numbers, transport protocols, and names of well-known services
protocols	Retrieves official names and protocol numbers of protocol aliases
hosts	Defines the mappings between host names and their IP addresses
networks	Retrieves network names and addresses
netgroup	Retrieves groups of hosts, networks, and users in this group

The following values are available for the mechanism parameter:

local	Examines local configuration files (/etc/hosts, /etc/protocols, /etc/services, /etc/netgroup, and /etc/networks files)
dns	Queries DNS; the /etc/resolv.conf file must be configured for this mechanism to work
nis	Queries NIS; the NIS client must be active on the system for this mechanism to work
nis+	Queries NIS+; The NIS+ client must be active on the system for this mechanism to work
ldap	Queries the LDAP server; the resolv.ldap file must be configured for this mechanism to work Note: You can only assign the value hosts to the map parameter for ldap.
local4	Examines local configuration files for IPv4 addresses
local6	Examines local configuration files for IPv6 addresses
dns4	Queries DNS for A records (IPv4 addresses); the /etc/resolv.conf file must be configured for this mechanism to work
dns6	Queries DNS for AAAA records (IPv6 addresses); the /etc/resolv.conf file must be configured for this mechanism to work
nis4	Queries NIS for information about IPv4 addresses; the NIS client must be active on the system to use this mechanism
nis6	Queries NIS for information about IPv6 addresses; the NIS client must be active on the system to use this mechanism
ldap4	Queries the LDAP server for information about IPv4 addresses
ldap6	Queries the LDAP server for information about IPv6 addresses

The following values are available for the option parameter:

continue	If the information is not found in the specified mechanism, then instructs the resolver to continue to the next line, which should list another mechanism for the same map
merge	Merges all responses from multiple mechanism parameters into one response

Examples

To use only the local /etc/hosts file for host name resolution, enter:
```
hosts local
```
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
```
To use only DNS to resolve host names and to use NIS to resolve protocols, enter:
```
hosts dns
protocols nis
```
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
```
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
```
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.
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.
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
```
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.
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

/etc/hosts	Contains the Internet Protocol (IP) name and addresses of hosts on the local network
/etc/protocols	Contains official names and protocol numbers of protocol aliases
/etc/services	Contains lists of the port numbers, transport protocols, and names of well-known services
/etc/netgroup	Contains a list of groups of hosts, networks, and users in these groups
/etc/networks	Contains a list of network names and addresses
/etc/resolv.conf	Contains Domain Name Protocol (DOMAIN) name-server information for local resolver subroutines
/etc/netsvc.conf	Specifies the ordering of certain name resolution services
/etc/resolv.ldap	Contains the IP address of the LDAP server

Related Information

The hosts file format for TCP/IP, netgroup file for NIS, netsvc.conf file, networks file format for TCP/IP, protocols file format for TCP/IP, resolv.conf file format for TCP/IP, resolv.ldap file format for TCP/IP, services file format for TCP/IP.

The ypbind daemon.

The gethostbyname, gethostbyaddr, and gethostent subroutines for host names.

The getnetbyname, getnetbyaddr, getnetent subroutines for networks.

The getservbyname, getservbyport, getservent subroutines for services.

The getprotobyname, getprotobynumber, getprotoent subroutines for protocols.