Installing Oracle and the HOSTS file

After re-installing Oracle DB 10g (10.2.0) and OAS 10g (10.1.2) on Windows boxes that had been removed from a child domain and re-joined to the main domain I discovered that Enterprise Manager and the TNSnames.ora files were still referring to the old fully-qualified domain names, eg. foo1.childdomain.maindomain.com and foo2.childdomain.maindomain.com. I wondered why this happened because the computer names were changed to foo.maindomain.com when they were re-joined.

The reason was entries in the HOSTS file located in the Windows\System32\drivers\etc folder that referred to the previous fqdns. I forgot the HOSTS file is the first place checked on a DNS lookup; apparently the Oracle install scripts look here first too. A good thing to remember when re-configuring a development set-up