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Last update: 2022-03-11

NTLM and Kerberos troubleshooting

Troubleshoot common Kerberos and NTLM issues.


Client devices fail authentication when Kerberos and NTLM are configured.


Some common issues for authentication failure are: Configuration errors, domain join failures, and in the case of Kerberos the key version number (KVNO) not matching between endpoints and Sophos Firewall.


  1. Go to Authentication > Servers.
  2. Click on your AD server and then click Test connection.

    If the connection fails, you must resolve the AD connectivity issues. If the connection is successful, continue the steps below.

  3. Check a firewall rule is in place to allow Kerberos and NTLM traffic for the affected clients under Rules and policies > Firewall rules.

  4. Go to Administration > Device access and make sure AD SSO is configured for the zone that clients are authenticating from. This will typically be your LAN zone.
  5. If you have configured Sophos Firewall as an explicit proxy, make sure the hostname has been used in the browser settings. If you have used an IP address, the client allows only NTLM authentication.
  6. Sign in to the Sophos Firewall command line interface.
  7. Select option 5. Device Management then option 3. Advanced Shell.
  8. Use the following command to check the nasm service is running: service -S | grep -i "nasm"
  9. If the proxy name doesn't match between the client and Sophos Firewall, make sure the host record in AD for Sophos Firewall matches the hostname configured under: Administration > Admin and user settings > Hostname.
  10. If the KVNO doesn't match, the user must sign out and back in to their account, or you must rejoin Sophos Firewall to the domain. This issue is normally caused when the hostname of Sophos Firewall is changed.

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