AIX : Naming resolution

Following a recent conversation with IBM L2 support concerning general naming resolution, all affected parameter files and environment variables (and their relationship) will be explained.

AIX supports several mechanisms for naming resolution of hosts, networks, protocols, services, netgroups and rpc:

• dns - Domain Name Service
• nis - Network Information Service
• nis+ - Network Information Service Plus
• local - Local naming service. Searches the files in /etc directory for resolving
• nis_ldap - Provides naming resolution for host, networks, protocols, rpc, services, and netgroups. This mechanism works with any directory server which stores entity data using a schema defined in RFC 2307. Although the name of the mechanism is nis_ldap, this mechanism does not use or require any NIS services!

AIX can be configured to use a combination of the above services for naming resolution. There is a sequential order that AIX follows to use these services. The default ordering can be overridden in several ways:

• NSORDER environment variable
• /etc/netsvc.conf configuration file
• /etc/irs.conf configuration file

NSORDER
NSORDER is an environment variable that can be used to specify the order for resolving host names to addresses (gethostbyname) and vice versa (gethostbyaddr). NSORDER overrides the host settings in the netsvc.conf and irs.conf files. The supported mechanisms for NSORDER are bind, nis, local, which is also the default order.

/etc/netsvc.conf
The netsvc.conf file specifies the sequential order for resolving host names and aliases. It should be noted that sendmail ONLY uses netsvc.conf for resolution of host names and aliases. Other configuration files or environment variables are not consulted. The environment variable NSORDER overrides the host settings in the netsvc.conf file, which in turn overrides the host settings in the irs.conf file.

/etc/irs.conf
The irs.conf file is used to control the order of mechanisms that the resolver libraries use in searching for network-related data, including the resolving of host names, networks, services, protocols, and netgroups. The default order for resolving host names and networks is dns, nis, local. The default order for resolving services, protocols, and netgroups is nis, local. The order defined in irs.conf will override the default values. The settings in the netsvc.conf configuration file override the settings in the irs.conf file. The NSORDER environment variable overrides the settings in the irs.conf and netsvc.conf files.

Note:
AIX offers two LDAP naming services, ldap and nis_ldap. The ldap naming service uses the IBM specific schema and supports host name resolution only. The nis_ldap naming service implemented since AIX 5.2, uses the RFC 2307 schema and supports name resolution of hosts, services, networks, protocols, and netgroups.

Summary

Service	Precedence
hosts	NSORDER, netsvc.conf, /etc/irs.conf
networks	irs.conf
protocols	irs.conf
services	irs.conf
netgroups	irs.conf