Server Message Block (SMB) is a core protocol for file sharing, printer access, and network communication, widely used in Windows environments and cross-platform systems (such as Linux through Samba). However, SMB configuration errors, version compatibility, security policy, or network issues can result in failed connections, poor performance, or abnormal functionality. This document describes the fault location, common fault scenarios, and solutions to help users quickly resolve SMB problems.

First take a look at SMB problem classification and preliminary troubleshooting!

1. The connection is faulty

Symptom: The shared folder cannot be accessed, the connection times out, and the message "The network path does not exist" or "Access Denied" is displayed.

Troubleshooting steps:

Check network connectivity: Run ping or TestNetConnection to check whether the target IP address is reachable.

Confirm that the port is open: The SMB uses TCP port 445 by default. Test the Port status by telnet <IP> 445 or TestNetConnection Port 445.

Verify host name resolution: If the host name is used for access, check whether the DNS or local hosts file is correctly configured.

2. The authentication fails

Symptom: The user name or password is incorrect or Login failed is displayed.

Troubleshooting steps:

Check credentials: Ensure that the account you enter has access to the shared resource.

Authentication Protocol: NTLMv1 is disabled on Windows by default (insecure). If the client or server forcibly uses the old protocol, you need to adjust the group policy (for example, enabling NTLMv2).

Domain environment problem: If the communication between the domain controller is abnormal, Kerberos may fail. Check the status of domain members and time synchronization.

3. The permission configuration is incorrect

Symptom: Can connect but cannot read or write files.

Troubleshooting steps:

Share permission: Check whether the Share permission of the shared folder allows users to access it.

NTFS permission: Ensure that the file system's ACL (access control list) does not overwrite the share permission.

User group nesting: Complex user group nesting can cause confusion in permission inheritance. Use the icacls command to check file permissions.

4. The SMB version is incompatible

Symptom: Windows 10/11 cannot connect to an old NAS device or Linux Samba server.

Key points:

SMB1 vulnerability and deprecation: SMB1 (CIFS) is disabled on Windows by default. If an old device relies on SMB1, manually enable it (not recommended) or upgrade the device.

Negotiation protocol version: through GetSmbConnection (PowerShell) to check the active connection version, mandatory use SMB2/3 (such as SetSmbClientConfiguration RequireSecuritySignature $true).

Second, share with you about advanced issues and fixes.

Scenario 1: The SMB transmission speed is slow

Possible causes:

The network bandwidth is insufficient or the MTU is incorrectly configured.

Encryption cost (for example, enabling SMB3 encryption).

Client/server hardware resource bottlenecks.

Solution:

Disable encryption (not sensitive to environment) : SetSmbServerConfiguration EncryptData $false.

To adjust the MTU, run the netsh interface ipv4 show subinterfaces command to ensure that there are no fragments.

Enable SMB Direct connection (RDMA) : Compatible network adapters and configurations are required.

Scenario 2: The message "Access Denied" is displayed repeatedly.

Check process:

1. Ensure that the user is not in the Access Deny list (the share permission and NTFS permission must be checked).

2. Check firewall rules: Ensure that the File and Printer Sharing exception is enabled.

3. Clear the credential cache: Run cmdkey /delete:< destination address > or delete old credentials through Credential Manager.

Scenario 3: Linux and Windows SMB interoperability

Common problem: Samba is incorrectly configured, causing Windows to fail to connect.

Repair steps:

1. Edit the Samba configuration file (smb.conf) :

ini

[global]

client min protocol = SMB2 Disables SMB1

server min protocol = SMB2

ntlm auth = yes Indicates that NTLM is allowed

2. Run the systemctl restart smbd command to restart the service.

3. Run smbclient L <IP> U < user > to test the connection.

Learn more about security hardening and best practices.

1. Disable SMB1:

PowerShell: DisableWindowsOptionalFeature Online FeatureName SMB1Protocol

Linux Samba: Set min protocol to SMB2.

2. Enable SMB3 encryption:

Suitable for sensitive data transfer: SetSmbServerConfiguration EncryptData $true.

3. Network isolation:

Restrict SMB traffic only to Intranet channels.

4. Regular audit:

Monitoring logs (Windows event ID 5140/5145, Linux /var/log/samba/).

Finally, share some typical fault cases

Example: Windows 11 cannot access NAS shares

Background: The NAS supports SMB2, but the connection to Windows 11 fails.

Resolved:

1. Check the SMB Settings of the NAS to ensure that SMB1 is disabled.

2. Run the following command on the Windows client:

powershell

SetItemProperty Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters" Name  "AllowInsecureGuestAuth" Value 1 Allow anonymous access (test environment only)

3. If the failure persists, enable SMB3 on the NAS and disable signature (security needs to be balanced).

Most SMB problems are caused by configuration, rights, or version compatibility. By checking the network, protocol, rights, and security at different levels, most problems can be quickly located. Keeping the system up to date, unifying SMB versions, and following the principle of least permission are key to avoiding failures. For complex environments, you are advised to back up and test the configuration before modifying it.

End-of-article benefits:

Recommended tools

Wireshark: Captures packets to analyze SMB protocol interactions.

Microsoft SMB Client: smbclient (Linux) and GetSmbSession (Windows).

Nmap script: nmap script smbprotocols <IP> Detects the supported SMB version.