This error may appear if the DNS settings are not configured properly and the SonicWall is unable to access the LDAP server. We need to make sure that the DNS name resolution works correctly on the SonicWall.
In the SonicWall, specify the IP address of the local DNS server as the DNS server 1.
Navigate to Network | DNS, click Specify DNS manually and specify the IP address of the local DNS server as the DNS server 1 and the public DNS as DNS server 2.
Click Apply on the top of the web page.
NOTE:Your local DNS server should also forward the DNS requests to the Internet DNS servers. Check the forwarders settings in your local DNS server.
Try to ping the domain name of your LDAP server.
Navigate to System | Diagnostics | Ping | enter the domain name, click Go button. If the response is domain name is alive then it is confirmed that the DNS name resolution is working on the SonicWall.
If there is no reply from the domain name, then ensure the pphysical connectivity /IP address / Subnet mask of the LDAP server is correct. Try to ping the IP address of the LDAP server.
If you can ping the LDAP server, but are unable to connect to the LDAP server thru the LDAP configuration page then navigate to User Settings | Select ‘LDAP + Local Users’ and click Configure button.
Verify the parameters on the Settings tab.
IP Address of the LDAP server Port number Login User Name / Password Use TLS (SSL) is enabled.
We need to verify the configuration on the LDAP server side.
Check the following security settings on the Domain Controller Security Settings in the Active Directory.