Strange DNS resolution.

60 pts.
Tags:
AS/400
DNS
Host table
PTFs
TCP/IP
Hello. This week one of our AS400 got me all surprised. I do not know what to do, where to look, so I would like to ask here...just in case. On this AS400, among others there are four host table entries. These entries are set to 2.xxx.xxx.xxx addresses. The DNS has 9.xxx.xxx.xxx addresses. Customer does not want to use those 9* ones,so they decided to bypass it with the local host table. So: Host table contains four names with 2.xxx.xxx.xxx addresses. DNS server table contains the same four names with 9.xxx.xxx.xxx addresses. Host name search priority *LOCAL. 24 hours after an IPL, one of the four stopped communicating with the AS400 (was unreachable from the AS400) PING to 2.xxx.xxx.xxx address ok. PING to 9.xxx.xxx.xxx address timed out (obviously) PING to the name returned 9.xxx.xxx.xxx address - therefore could not communicate. Last PTFs installed 2 weeks ago onto other 20 partitions. No reports from the others. No recent changes to DNS or host table. Nothing in history log since the IPL. Nothing in QSYSOPR since the IPL. (searched for QTCP, not resolved, communication, TCPIP). TCPIP started successfully during the IPL. Not restarted since the IPL. We run on V6R1. I did not run SST Communications Trace (pitty, I could have) When I removed the host table entry, waited a few seconds and added it back, the communication was established. PING to the name returned 2.xxx.xxx.xxx address. It is working since then. My questions are: - what could have caused it? - is there any kind of fix to this? - how to prevent it from happening again? Any reponse and help are highly appreciated.

Software/Hardware used:
AS400 V6R1

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  • TomLiotta
    I have no clue what happened to my comment. I'll wait a while to see if it shows up before trying to enter all of it again.   Tom
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  • TomLiotta
    Well, I tried a second time and it's also not showing up. Apparently you're destined not to get help. -- Tom
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  • Champs
    Hi.Thank you Tom, anyway.Here is what I figured out together with IBM guys.It shows there was probably wrong record in the DNS cache.Apparently -according to APAR MA39638- this was caused with TCPIP groups lvl 6 or 7 and is known issue on 610. Fixed with  MF50442.It was not supposed to show up on SLIC 611 (which is the one customer server uses).If this problem ever occurs (on SLIC 611) to anyone of you, then- launch SST Communications trace and try pinging the name- run SST TCP macros- use CALL QSOMAINT PARM('2' '2') to dump the cache=== all this only collects the data you can use to see if it is this problem ===- and then remove and add the local host table entry - apprently this is enough to empty the cache :-/
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  • TomLiotta
    Since neither post is showing, so I didn't have much to do with it. I didn't have an answer, only questions. Cache clearing would probably have been the result. I have ILE code to do it rather than going into SST, but it could have been helpful. Glad you have it cleared. -- Tom
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