So directory manager is the *value* of the attribute, the attribute is CN (common name) the DN, what it's explicitly asking for is the Distinguished Name, which is the absolute location of the user in the directory you are trying to use. For example

CN = directory manager
DN = cn=directory manager, cn=config

so

db2bak.pl -D "cn=directory manager,cn=config"  etc etc

will be the correct syntax you want to use. Hope this helps.

Dan

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