IBM Support

PowerVM/VIOS: Understanding hostname resolution options (local,bind4/local4,bind4/local,bind/local6,bind6)

Question & Answer


Question

Virtual I/O Server (VIOS) relies heavily on hostname resolution during system startup and for the operation of internal components such as change manager, CMDB, clustering services, and local databases.
Incorrect resolver configuration in /etc/netsvc.conf can lead to hostname resolution failures that impact internal VIOS services, even when external network connectivity appears healthy.
This technote explains how hostname resolution works on VIOS, describes the available resolver options, and provides clear guidance on selecting the appropriate configuration for common environments.

 

Cause

Applies to:

  • IBM Power Systems
  • Virtual I/O Server (VIOS) 3.x and later
  • IPv4-only, IPv6-only, and dual-stack environments

 

Background: Hostname resolution on VIOS:

Hostname resolution on VIOS is controlled by the resolver configuration file:

/etc/netsvc.conf

This file determines:

  • The order of hostname resolution
  • Whether resolution is local or external
  • Which IP protocol families (IPv4 / IPv6) are used

VIOS performs hostname resolution during:

  • System boot
  • Change manager and CMDB initialization
  • Internal service communication
  • Cluster and SSP operations

 

Unlike general AIX systems, VIOS relies extensively on local hostname resolution during system startup. Several internal services require the VIOS hostname to resolve successfully before external networking is fully available. As a result, correct resolver configuration is critical to stable startup behavior.

Resolution sources

local

  • Uses /etc/hosts
  • No network traffic
  • Required for internal and loopback resolution

bind

  • Uses DNS (BIND)
  • Requires network connectivity
  • Used for external hostname resolution

 

IP protocol qualifiers:

Resolver keywords maybe suffixed with protocol qualifiers:

QualifierMeaning
4IPv4 only
6IPv6 only
NoneIPv4 and IPv6
 
These qualifiers apply equally to:
 
  • External addresses
  • Loopback addresses (127.0.0.1, ::1)
  • Entries in /etc/hosts
 
IPv6 loopback on VIOS
VIOS enables IPv6 loopback (::1) by default, even in environments that are otherwise IPv4-only.
 
IPv6 loopback:
 
  • Is used internally by VIOS services
  • Does not require IPv6 routing or IPv6 DNS
  • Is commonly selected by system libraries during hostname resolution
Disabling IPv6 at the resolver level can prevent internal services from resolving the VIOS hostname to itself.

 

Common resolver configurations
 

 

hosts=local,bind4 (Recommended for most environments)

Behavior

  • /etc/hosts: IPv4 and IPv6
  • DNS: IPv4 only

Benefits

  • IPv6 loopback (::1) available for internal services
  • No dependency on IPv6 DNS

  • Stable startup behavior

     

Recommended use:

This is the preferred and recommended configuration for the majority of VIOS deployments, including fresh installations and IPv4-only networks.

 

hosts=local4,bind4

Behavior

  • /etc/hosts: IPv4 only
  • DNS: IPv4 only
  • IPv6 (including loopback ::1) is ignored

Impact on VIOS

  • IPv6 loopback cannot be used for hostname resolution
  • Local hostname resolution may fail
  • Internal services may exit during startup

Recommended use:

This configuration is not recommended for most VIOS environments.
It should be used only when IPv6 is explicitly disabled by policy at all levels.

 

hosts=local,bind

Behavior

  • /etc/hosts: IPv4 and IPv6
  • DNS: IPv4 and IPv6

Considerations:

  • Requires fully functional IPv6 networking
  • DNS must support AAAA records reliably

  • May introduce delays if IPv6 DNS is misconfigured

     

Recommended use:

Use this configuration only in environments where IPv6 networking and DNS are fully validated.

 

hosts=local6,bind6

Behavior

  • /etc/hosts: IPv6 only
  • DNS: IPv6 only
  • IPv4 is ignored

 

Risks

  • Many VIOS components assume IPv4 availability

  • Limited real-world testing on VIOS

     

Recommended use:

This configuration is intended primarily for IPv6-only lab or test environments and is not recommended for general VIOS production use.

 

Decision flow: Which resolver should I choose?
 

Start
|
|-- Is this a VIOS system?
|       |
|       |-- No --> Use standard AIX resolver guidance
|       |
|       |-- Yes
|             |
|             |-- Is IPv6 fully routed and DNS AAAA records validated?
|                     |
|                     |-- Yes --> hosts=local,bind
|                     |
|                     |-- No
|                           |
|                           |-- Is IPv6 disabled by policy?
|                                   |
|                                   |-- Yes --> hosts=local4,bind4
|                                   |
|                                   |-- No
|                                         |
|                                         |--> hosts=local,bind4
|
End

 

IPv6 loopback on VIOS

  • IPv6 loopback (::1) is enabled by default on VIOS
  • Used internally by system services
  • Does not imply IPv6 external networking
IPv6 loopback is an internal OS mechanism, not an external networking requirement.
Resolver settings that disable it may unintentionally disrupt VIOS startup and internal services.

 

Common symptoms of resolver misconfiguration:

  • VIO_INFO messages indicating hostname resolution failures
  • Change manager exiting during startup
  • CMDB fatal export bundles created under:

/home/ios/logs/cmdb.fatal.exp/

  • PostgreSQL-related messages during boot
  • System reaches multi-user mode but internal services are degraded

 

Verification steps:

Test resolution of names and IPs using nslookup if a name server is used in /etc/resolv.conf. 


 

(1) Test should be for both VIOS’s IP and hostname. 


 

(2) The return from name server should match what is configured on the /etc/hosts file.


 

(3) Fully qualified domain name (FQDN) is critical if using DNS so make sure  /etc/hosts file has FQDN since that will also be required on DNS servers.


 

(4) Use the “host” command on VIOS to check local resolver to see what its returning for hostname and IP if there is no /etc/resolv.conf file being used to designate a name server.


 

(5) Syntax for using nslookup and host command:
# nslookup 1.2.3.4
# nslookup myhostname.mydomain
# host 1.2.3.4
# host myhostname.mydomain


 

EXAMPLE: vios2 has IP 9.19.176.117 and domain is dfw.ibm.com
-- Run a reverse name lookup (query for IP in DNS database)
# nslookup 9.19.176.117
Server:         9.0.130.50
Address:        9.0.130.50#53
Non-authoritative answer:
117.176.19.9.in-addr.arpa       name = vios2.dfw.ibm.com.


 

-- Run a hostname lookup using FQDN
# nslookup vios2.dfw.ibm.com
Server:         9.0.130.50
Address:        9.0.130.50#53
Non-authoritative answer:
Name:   vios2.dfw.ibm.com
Address: 9.19.176.117
NOTE: both the hostname lookup and reverse name lookup returned the same information and this is what you need to see if DNS is configured (/etc/resolv.conf file exists and is configured properly)



 

-- Also check local name resolution using the host command


 

# host 9.19.176.117
vios2.dfw.ibm.com is 9.19.176.117
# host vios2.dfw.ibm.com
vios2.dfw.ibm.com is 9.19.176.117
NOTE: both queries returned same information.

 

Answer

Hostname resolution is a foundational dependency on VIOS.
Understanding the distinction between local vs bind and IPv4 vs IPv6 qualifiers is essential when configuring /etc/netsvc.conf.
Allowing IPv6 locally while controlling external DNS usage provides the safest and most robust configuration for most VIOS deployments.

 

Best practice recommendation:
For the majority of VIOS environments:

hosts=local,bind4

 

This configuration:
 
  • Preserves IPv6 loopback for internal services
  • Prevents dependency on IPv6 DNS
  • Aligns with stable VIOS startup behavior

 

 

 

Additional Information

SUPPORT

If you require more assistance, use the following step-by-step instructions to contact IBM to open a case for software with an active and valid support contract.  

1. Document (or collect screen captures of) all symptoms, errors, and messages related to your issue.

2. Capture any logs or data relevant to the situation.

3. Contact IBM to open a case:

  -For electronic support, see the IBM Support Community:
    https://www.ibm.com/mysupport
  -If you require telephone support, see the web page:
     https://www.ibm.com/planetwide/

4. Provide a clear, concise description of the issue.

- For more information, see: Working with IBM AIX Support: Describing the problem.

5. If the system is accessible, collect a system snap, and upload all of the details and data for your case.

- For more information, see: Working with IBM AIX Support: Collecting snap data

 

Author: Ahmed Deif - ahmed.deif@ibm.com

[{"Type":"MASTER","Line of Business":{"code":"LOB57","label":"Power"},"Business Unit":{"code":"BU058","label":"IBM Infrastructure w\/TPS"},"Product":{"code":"SSPHKW","label":"PowerVM Virtual I\/O Server"},"ARM Category":[{"code":"a8m3p000000PCZsAAO","label":"CMDB"}],"ARM Case Number":"","Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"1.1.0;1.2.0;1.3.0;1.4.0;1.5.0;3.1.0;3.1.1;3.1.2;3.1.3;3.1.4;4.1.0;4.1.1;4.1.2"}]

Document Information

Modified date:
02 February 2026

UID

ibm17259383

Page Feedback