Specifies the ordering of certain name resolution services.
The /etc/irs.conf file is used to specify the ordering mechanism lookups for network related data by the resolver libraries of AIX. Resolution of hostname, networks, services, protocols, and netgroups can be achieved by using the subroutines listed in the following table.
| hostname | gethostbyname, gethostaddr, gethostent |
| networks | getnetbyname, getnetbyaddr, getnetent |
| services | getservbyname, getservbyaddr, getservent |
| protocols | getprotobyname, getprotobyaddr, getprotoent |
| netgroups | getnetgrent |
Because these routines 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.
The default order for lookup mechanisms for hosts is:
The default order for lookup mechanisms for networks is:
The default order for lookup mechanisms for other maps is:
The default order can be overwritten by creating the configuration file, /etc/irs.conf, and specifying the desired ordering.
The settings in the configuration file /etc/netsvc.conf override the settings in the /etc/irs.conf file. The environment variable NSORDER overrides the settings in the /etc/irs.conf and the /etc/netsvc.conf files.
To use DNS to get information concerning netgroups, protocols, and services you must create and use a Hesiod DNS Zone.
<map> <mechanism> [<option>]
Available maps:
| services | Lists the port numbers, transport protocols, and names of well known services. |
| protocol | Retrieves offical 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 these group. |
Available mechanisms:
| local | Examine local configuration files (/etc/hosts, /etc/protocols, /etc/services, /etc/netgroup, and /etc/networks files). |
| dns | Get information from DNS. The /etc/resolv.conf file must be configured for this mechanism. |
| nis | Get information from NIS. The NIS client must be active on the system to use this mechanism. |
| local4 | Same as local but only looks for IPv4 addresses. |
| local6 | Same as local but only looks IPv6 addresses. |
| dns4 | Same as dns but only queries for A records (IPv4 addresses). |
| dns6 | Same as dns but only queries for AAAA records (IPv6 addresses). |
| nis4 | Same as nis but only looks for IPv4 addresses. |
| nis6 | Same as nis but only looks for IPv6 addresses. |
Available options:
| continue | If not found in [mechanism], then continue to the next line (which should list another mechanism for the same map). |
| merge | Answers from all merged [mechanisms] into one response. |
An example of how to use [continue]:
Contents of /etc/irs.conf file:
hosts local continue
hosts dns continue
hosts nis
Contents of /etc/hosts file:
1.2.3.4 host4
1.2.3.5 host5
1.2.3.6 host6
Contents of DNS:
1.2.3.2 host2
1.2.3.3 host3
Contents of NIS:
1.2.3.1 host1
| Function Call: | gethostbyname(host1) |
| Result: | returns 1.2.3.1 |
| Reason: | The [continue] option will inform the resolver to first look for the information within the first mechanism, and if the requested information is not found, then continue to the next mechanism indicated in the /etc/irs.conf file. In this case, the resolver will first look at the local /etc/hosts file for host1 but will not find it. Next, it will look at the DNS, but again, will not find it. Finally, it will look at the NIS, find the address, and return 1.2.3.1. |
An example of how to use [merge]:
Contents of /etc/irc.conf file:
hosts local merge
hosts dns
Contents of /etc/hosts file:
1.1.1.1 hostname
Information in DNS:
1.1.1.2 hostname
| Function Call: | gethostbyname(hostname) |
| Result: | returns 1.1.1.1 1.1.1.2 |
| Reason: | The merge option indicates that the resolver should take all the answers from the selected mechanisms and merge the answers together into one reply. |
Examples of the /etc/irs.conf file:
The following examples show how the irs.conf file should look to control the query order of different records per mechanism and map. These examples are not an exhaustive list; any combination of mechanism and map can be used.
To use only the local /etc/hosts file for hostname resolution, enter:
hosts local
To use only the DNS for hostname resolution and NIS for protocols resolution, enter:
hosts dns
protocols nis
To use only NIS for hostname addresses and netgroup resolution and the local files for services and networks, enter:
hosts nis
services local
netgroup nis
networks local
To try to resolve host names from the local /etc/hosts file, then if they are not found, 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, if any, to any NIS information found. If no information is found, none is returned. If information is found in any source, it will return as merged information from all of the available sources. Enter:
hosts local merge
hosts dns merge
hosts nis
To examine the local /etc/services file for port numbers before going to NIS. This can speed up the request since going to the NIS for information is much slower than going to the local file. If the information is not found in the local file, NIS will be searched. Enter:
services local continue
services nis
To look for IPv4 network addresses only from DNS and IPv6 host addresses only from the local file, enter:
networks dns4
hosts local6
| /etc/hosts | Contains the Internet Protocol (IP) name and addresses of hosts on the local network. |
| /etc/protocols | Contains offical 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 routines. |
| /etc/netsvc.conf | Specifies the ordering of certain name resolution services. |
The ypbind daemon.
The gethostbyname, gethostbyaddr, and gethostent subroutines for hostnames.
The getnetbyname, getnetbyaddr, getnetent subroutines for networks.
The getservbyname, getservbyport, getservent subroutines for services.
The getprotobyname, getprotobynumber, getprotoent subroutines for protocols.
The getnetgrent subroutine for netgroups.