DCSync is a technique that uses Windows Domain Controller's API to simulate the replication process from a remote domain controller. This attack can lead to the compromise of major credential material such as the Kerberos krbtgt keys used legitimately for tickets creation, but also for tickets forging by attackers. The consequences of this attack are similar to an NTDS.dit dump and parsing but the practical aspect differ. A DCSync is not a simple copy & parse of the NTDS.dit file, it's a DsGetNCChanges operation transported in an RPC request to the DRSUAPI (Directory Replication Service API) to replicate data (including credentials) from a domain controller.
This attack requires domain admin privileges to succeed (more specifically, it needs the following extended privileges: DS-Replication-Get-Changes and DS-Replication-Get-Changes-All). Members of the Administrators, Domain Admins, Enterprise Admins, and Domain Controllers groups have these privileges by default. In some cases, over-privileged accounts can be abused to grant controlled objects the right to DCSync.
A setting exists in the account policy or when creating users telling the domain controller to store the user's password using reversible encryption instead of irreversible hashing. This allows attackers to retrieve the passwords in clear-text.
On Windows, mimikatz (C) can be used to operate a DCSync and recover the krbtgt keys for a golden ticket attack for example. For this attack to work, the following mimikatz command should run in an elevated context (i.e. through runas with plaintext password, pass-the-hash or pass-the-ticket).
# Extract a specific user, in this case the krbtgt