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For instructions how to connect JIRA to Active Directory please refer to https://confluence.atlassian.com/display/JIRA/Connecting+to+an+LDAP+Directory.
For example, to To enable automatic mapping for "Assignee" field please click the "Values" link for "Assignee" mapping and select Mapping Mode "Copy Value (Attempt Type Conversion)". Please see the screenshots below for details.
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