In this guide, we will delve into the process of fixing LDAP server lookup issues when joining Ubuntu 18.04 to an Active Directory (AD). This issue often presents itself as a “Can’t contact LDAP server” or “No such realm found” error when running the realm join command. We will discuss various troubleshooting steps and solutions to resolve this problem.
To fix LDAP server lookup issues when joining Ubuntu 18.04 to Active Directory, you need to verify the DNS configuration, check firewall settings, verify Kerberos configuration, check SSSD configuration, debug SSSD, check Realmd configuration, verify time synchronization, and test connectivity. By following these steps, you should be able to resolve the issue.
Table of Contents
- Verify DNS Configuration
- Check Firewall Settings
- Verify Kerberos Configuration
- Check SSSD Configuration
- Debug SSSD
- Check Realmd Configuration
- Verify Time Synchronization
- Test Connectivity
Verify DNS Configuration <a name=”dns-configuration”></a>
The first step in troubleshooting this issue is to verify the DNS configuration on your Ubuntu machine. The file /etc/resolv.conf should contain the correct DNS server IP addresses. You can open this file using a text editor like nano:
sudo nano /etc/resolv.confEnsure that the nameserver entries in this file match the IP addresses of your AD DNS servers. You can also test DNS resolution by pinging the AD server by its hostname:
ping ad_server_hostnameIf the ping command returns an unknown host error, there might be a DNS resolution issue.
Check Firewall Settings <a name=”firewall-settings”></a>
Next, verify that your firewall allows LDAP communication. Typically, LDAP uses port 389. You can check the status of this port using the ufw command:
sudo ufw statusIf port 389 is not open, you can open it using the following command:
sudo ufw allow 389Verify Kerberos Configuration <a name=”kerberos-configuration”></a>
Kerberos is a network authentication protocol that uses secret-key cryptography for secure communication. The configuration for Kerberos is stored in the krb5.conf file. Open this file using a text editor:
sudo nano /etc/krb5.confEnsure that the kdc and admin_server values match your AD server’s hostname or IP address. Also, verify that the default_realm value matches your AD domain name.
Check SSSD Configuration <a name=”sssd-configuration”></a>
The System Security Services Daemon (SSSD) is a software package that provides access to remote directories and authentication mechanisms. It allows a local system to connect to an external back-end system like an LDAP directory or an AD domain.
The configuration for SSSD is stored in the sssd.conf file. Open this file using a text editor:
sudo nano /etc/sssd/sssd.confEnsure that the ad_domain and krb5_realm values match your AD domain name. The id_provider and access_provider should be set to “ad”. The ldap_id_mapping parameter should be set to “True” to enable LDAP ID mapping.
Debug SSSD <a name=”debug-sssd”></a>
If you’re still facing issues, you can enable debug logging for SSSD to get more detailed information. Edit the sssd.conf file and add the following line under the [sssd] section:
debug_level = 9After making this change, restart the SSSD service:
sudo systemctl restart sssd.serviceCheck the logs in /var/log/sssd/ for any relevant error messages.
Check Realmd Configuration <a name=”realmd-configuration”></a>
Realmd provides a clear and simple way to discover and join an AD domain. The configuration for Realmd is stored in the realmd.conf file. Open this file using a text editor:
sudo nano /etc/realmd.confEnsure that the default-client is set to “sssd”. The os-name and os-version should match your Ubuntu version.
Verify Time Synchronization <a name=”time-synchronization”></a>
Time synchronization is crucial for Kerberos authentication. Ensure that your Ubuntu machine’s clock is synchronized with your AD server’s clock. You can use the ntp or chrony service to synchronize the time:
sudo systemctl status ntpor
sudo systemctl status chronyTest Connectivity <a name=”test-connectivity”></a>
Lastly, test the connectivity between your Ubuntu machine and your AD server. You can use the ping command to test basic connectivity:
ping ad_server_ipYou can also use the telnet command to test connectivity on the LDAP port:
telnet ad_server_ip 389If you can’t establish a connection, there might be a network issue.
By following these steps, you should be able to resolve the LDAP server lookup issue when joining Ubuntu 18.04 to an AD domain. If you’re still facing issues, consider seeking assistance from the Ubuntu or AD community forums.
LDAP stands for Lightweight Directory Access Protocol. It is a protocol used to access and manage directory information services, such as user authentication and authorization, in a networked environment.
The "Can’t contact LDAP server" error typically occurs when there is an issue with the DNS configuration, firewall settings, Kerberos configuration, SSSD configuration, or connectivity between your Ubuntu machine and the Active Directory server. The guide provides troubleshooting steps to resolve these issues.
You can verify the DNS configuration by checking the /etc/resolv.conf file. Open it using a text editor like nano and ensure that the nameserver entries match the IP addresses of your Active Directory DNS servers. You can also test DNS resolution by pinging the AD server by its hostname.
You can check the firewall settings using the ufw command. Run sudo ufw status to see the status of the firewall. If port 389, which is used for LDAP communication, is not open, you can open it using sudo ufw allow 389.
SSSD, or System Security Services Daemon, is a software package that allows a local system to connect to an external back-end system like an LDAP directory or an Active Directory domain. It is important in joining Ubuntu to Active Directory as it provides access to remote directories and authentication mechanisms.
To enable debug logging for SSSD, edit the sssd.conf file and add the line debug_level = 9 under the [sssd] section. After making this change, restart the SSSD service using sudo systemctl restart sssd.service. You can then check the logs in /var/log/sssd/ for any relevant error messages.
Time synchronization is crucial for Kerberos authentication, which is used for secure communication with Active Directory. It ensures that the clocks on your Ubuntu machine and the Active Directory server are synchronized. This synchronization is necessary for successful authentication.
You can test basic connectivity using the ping command to ping the Active Directory server’s IP address. Additionally, you can use the telnet command to test connectivity on the LDAP port by running telnet ad_server_ip 389. If you can’t establish a connection, there might be a network issue.
