The IBM i *LOOPBACK Interface Problem
October 22, 2014 Hey, Joe
Help! My IBM i credit card software stopped talking to our outside credit card processor. One minute it was working, and the next minute it crashed. When I restart the package, it immediately ends. What’s going on? We can’t process our orders.
This is an unusual situation I’ve seen every once in a while, and it usually involves your TCP/IP setup.
Troubleshooting The Issue
The first thing to look at is whether all your TCP/IP interfaces are up and running, and whether anything changed with your TCP/IP setup. Before you call your software provider or IBM, check the status of your IP addresses by running the following commands on a green screen.
1. Start the Configure TCP/IP menu by running the following Go to Menu (GO) command:
2. Take option 1=Work with TCP/IP Interfaces. This will show you all the TCP/IP addresses on your system. Press F11=Display Interface status, and the display will show you which IP addresses are active and which IP addresses are inactive.
Pay particular attention to any IP addresses that are inactive, especially when the 127.0.0.1 interface is inactive (the *LOOPBACK interface). The *LOOPBACK interface is a non-routable IP address that resides on most computers. It allows administrators and software packages to test and send TCP/IP commands to the host machine, without accessing the local network or the Internet.
If *LOOPBACK is inactive, try restarting the *LOOPBACK interface and then try starting up your third-party software again to see if that solves your problem. When starting up, many software packages use the *LOOPBACK interface to check if TCP/IP is active and to perform other functions. When*LOOPBACK isn’t active, some packages will shut down or error out and the fix is to activate the *LOOPBACK interface and restart the software.
The inactive *LOOPBACK problem matches your problem profile, where a reliable piece of software suddenly stops working and refuses to restart. I’ve seen this happen several times where *LOOPBACK goes inactive and suddenly, all kinds of third-party software stop working. If your problem isn’t the *LOOPBACK interface, then you’ll have to call IBM or your third-party provider to troubleshoot a solution.
Author’s Note: At this point, Henry wrote back and said that he found the *LOOPBACK interface was inactive. When he reactivated *LOOPBACK, the software was able to run again.
Determining How *LOOPBACK Was Deactivated
When a TCP/IP interface suddenly turns off or turns on, you’ll want to determine what happened in order to prevent it from happening again. It’s easy to determine which user turned on or turned off a TCP/IP interface by looking at the job log for the QTCPWRK program.
QTCPWRK is the TCP/IP system control job for IBM i 6.1 and above. It performs all system TCP/IP interface activations and it writes log entries each time an IP interface is started or stopped. You can find which user and job turned off your *LOOPBACK interface by going into the QTCPWRK job log and looking for the following message.
TCP1B67 - User &7 in job &6/%5/&4 ended TCP/IP &3 interface &1
Where &7 is the user name who ended your interface. The &6/&5/&4 combination is the job that ended the interface. And the &1 parameter is the IP address of the interface. You can use this information to track down the user and job that ended your *LOOPBACK interface.
Check out this article for more information on using QTCPWRK to determine when your TCP/IP interfaces were last started or stopped.
Stopping the *LOOPBACK interface is a somewhat rare problem but I have seen it happen on occasion. with similar results. It may or may not be the result of a rogue user. The last time I encountered this issue, it occurred because a user was trying to start an IBM i software package without proper authority. The package’s startup routine performed some checks on the *LOOPBACK interface. When the program ran an operation on *LOOPBACK without proper authority, the software errored out and left the *LOOPBACK interface in an inactive state, causing an issue.
As I said, *LOOPBACK deactivation is rare, but possible. It’s worth adding a TCP/IP check to your troubleshooting procedures when a normally reliable package suddenly crashes.
Joe Hertvik is an IBM i subject matter expert (SME) and the owner of Hertvik Business Services, a content strategy organization servicing the computer industry. Email Joe for a free quote for any upcoming projects. He also runs a data center for two companies outside Chicago, featuring multiple IBM i ERP systems. Joe is a contributing editor for IT Jungle and has written the Admin Alert column since 2002. Check out his blog where he features practical information for tech users at joehertvik.com.