The Views Expressed Below Do not in any way reflect Internal Doctorine or Official Statements of Netgear Inc. These are just my notes – Use at your own Risk.
THIS ARTCILE APPLIES TO READYNAS 4.2.x INTEL UNITS and 4.1.x SPARC UNITS and READYNAS 5.x (cant remember if they support domain joining)

The Domain Controller (DC) should also be the DNS. If not adjust accordingly.
You need to know the domain name and the netbios name of the domain.
For example here its:
Domain Name: netgear.com
Netbios Name: NETGEAR
* To check Netbios name of anything go to “cmd” and type “nbtstat -a 192.168.1.10” where 192.168.1.10 is the IP of a pretend DC controller. If on

1) Make sure Clock on the NAS points to the DC and same timezone as the DC
2) Make sure the IP of the NAS is static and not DHCP
3) Without 2 this wont work, Make sure DNS is pointing to the DC with the DCs IP and have the correct DOMAIN name there (this should be identical to the DOMAIN name used in step 5)
with that said make sure the DC is also a DNS server
4) Security Mode Tab -> Domain Check -> Dropdown to ADS (Active Directory), The other option is Domain and its for old Win2K server.
5) On those settings – Set the DOMAIN name (this should be identical to the DOMAIN name used in step 3), the NETBIOS name of the domain, and the IP of the DC(instead of having it automatically look for it). Also put the admin username and password there of the domain to join it appropriately.
6) Hit Apply to join

If have troubles (try these steps and repead from step 4):
*Try Reenabling CIFS
*Make sure on latest firmware
*Try manually adding the READYNAS to the DNS server in its forward and reverse lookups
*OS Reinstall might help, but usually last restort
*Look at domain_join log to see whats wrong. This log only appears when the joining fails.

Another Example

Imagine a network: Domain name: domain.local, NETBIOS NAME: DOMAIN, Domain Controller: 10.1.1.10, Another Domain Controller: 10.1.1.11

1. Make sure under Interface settings – Global settings. Set the hostname to the full FQDN as would be seen from the DNS controller example: readynas1.domain.local, also set the DNS servers to your domain controller (hopefully your dc is also a DNS server), set the domain name to “domain.local” for the sake of this example

2. System-Clock use NTP servers and point the addresses to the IP of your dc

10.1.1.10, 10.1.1.11

3.Security mode – Domain – ADS (do not select the domain type of domain, select the ads type of domain – domain is for windows nt, and ads is for active directory based domains, I believe windows server 2003 +)

Fill out the domain name as “domain.local”, the NetBIOS name as “DOMAIN”, point the discovery process to a domain controller ip address instead of auto discovering the DC. If you are in a forest (a domain forest, multiple domain controllers and the like) leave the checkmark at the bottom checked, if not uncheck it. In the username field put a user that is part of the Domain Admins group.

Then click join. If it fails download the logs and check out the domain-join.log it helps in telling what the issue is, just parse through it for the error message. The domain-join log is only there on a failed domain join attempt. Note sometimes it takes a few attempts to join, I don’t know why, I think it’s because of Time synchronizations that’s why NTP settings are important. Finally, if all else fails try this: in your active directory user and computers right click on the Administrator user and select the copy option, it will then give you a window to make a new user, name him something simple like “readynasjoiner” with a password of “P@ssword123” (or anything else) and then try the joining again.