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what is SMB port and how does it work?

SMB is a networking protocol that allows systems on a network to share access. This article is all about SMB, how it works and what ports it uses.

Posted: 06 Dec, 22 Updated: 06 Dec, 22 by Susith Nonis 9 Min

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SMB, which stood for Server Message Block and was formerly known as Common Internet File System, is a networking protocol that allows systems on a network to share access.

At its core, it is a set of guidelines to share printers and files across a network. Computers use a local network to communicate with one another using the SMB file-sharing protocol.

This local network could consist of a single office for a small business or a global network of offices for a multinational corporation.

SMB uses a client-server architecture, in which the client submits requests and the server answers as necessary. A response-request protocol is what this is. This protocol makes it easier for networked computers to transfer files.

Once connected, it allows users or programs to send requests to a file server and gain access to resources on the distant server, such as mail slots, printer sharing, and named pipes.

A user of the app can now access files on the remote server and open, view, move, edit, and update them.

SMB functioned on top of the NetBIOS network architecture in earlier iterations of Windows. Microsoft modified SMB in Windows 2000 to use a dedicated IP port and run on top of TCP.

Windows versions running today still utilize that port.

Microsoft keeps improving SMBs for both performance and security. SMB2 lowered the protocol's overall chattiness, and SMB3 supported strong end-to-end encryption and performance improvements for virtualized situations.

The SMB protocol requires security measures, like any other connection, to ensure secure communication. SMB authentication at the user level needs a username and password to grant access to the server.

The system administrator is in charge; he or she can add or remove users and keep track of who is permitted access. Users must submit a one-time password at the share level to access the shared server or file, but identity authentication is unnecessary.

Computer programmers have invented SMB dialects that are used for various reasons, just like any language. As an illustration, the Common Internet File System (CIFS) is a particular SMB implementation that permits file sharing.

SMB and CIFS share the same fundamental design, even though many people think otherwise. Implementations of significant SMBs include:


Windows servers and compatible NAS devices use the common file-sharing protocol known as CIFS.


Authentication and authorization, name resolution, file sharing and print services, and service announcements among Linux/Unix servers and Windows clients are all supported by Samba, an open-source implementation of the SMB protocol and Microsoft Active Directory for Unix systems and Linux distributions. 


Visuality Systems created the NQ series of portable SMB client and server solutions. NQ supports the SMB 3.1.1 dialect and is adaptable to non-Windows platforms like Linux, iOS, and Android.


Ryussi Technologies' MoSMB is a proprietary SMB implementation.

Tuxera SMB

Another exclusive SMB implementation, Tuxera, can be used in kernel or user space.


In 2012, EMC acquired Likewise, a multi-protocol, identity-aware network file-sharing technology.

SMB 2.1 

It was introduced along with Windows 7 and Windows Server 2008 R2. Opportunistic locking was replaced with the client oplock leasing model to boost caching and speed.

Additionally, it included support for high maximum transmission units (MTU) and enhanced energy efficiency. Clients could now open files from an SMB server to go into sleep mode.

SMB 3.0 

Debuted in Windows Server 2012 and Windows 8, it brought about several important enhancements to management, performance, backups, security, and availability.


Ryussi Technologies created MoSMB, a proprietary SMB implementation for Linux and other Unix-like operating systems.

SMB 3.02 

Introduced in Windows 8.1 and Windows Server 2012 R2, this feature came with performance improvements and the option to turn off CIFS/SMB 1.0 support, which required removing any relevant binaries.

SMB 3.1.1

Support for enhanced encryption, pre-authentication integrity to thwart man-in-the-middle attacks, and cluster dialect fencing were added and released with Windows 10 and Windows Server 2016.

It knows which SMB protocol version your device employs is crucial, particularly if you run a business with interconnected Windows devices.

In a modern office, it would be difficult to find a PC running Windows 95 or XP (and utilizing SMBv1), yet they might still be doing so on outdated servers.

To provide file and print-sharing functions within a network, SMB uses several ports. However, 139 and 445 are the most often utilized SMB ports on a network when using file and print services.

Port 139

SMB dialects that interact over NetBIOS use port 139. It functions as an application layer protocol for device communication across a network in Windows operating systems.

For instance, Port 139 is used by printers and serial ports to connect.

Port 445

Simply put, Windows uses port 445 for file sharing across the network. Microsoft changed Windows 2000 to use port 445 for SMB.

Microsoft-DS also referred to as directory services from Microsoft, uses port 445. Both TCP and UDP protocols use port 445 for several Microsoft services.

Microsoft Active Directory and Domain Services use this port for file replication, user and device authentication, group policies, and trusts.

SMB, CIFS, LSARPC, SMB2, DFSN, NbtSS, SamR, NetLogonR, and SrvSvc protocols and services are most likely involved in the traffic on these ports.

Is SMB safe to use and secure? It appears that way for the time being. However, fresh vulnerabilities can appear at any time. It's better to stop SMB completely if you're not running any applications that use it to safeguard your system from potential threats.

Since SMB is not, by default, enabled in Windows 10 as of October 2017, you only have to take action if you're running an earlier version of Windows. The following actions are required to maintain the security of your SMB port:

Do not expose SMB ports

Since a decade ago, it has not been safe to open ports 135 through 139 and 445. Although opening ports 139 and 445 to the Internet isn't inherently harmful, doing so has several acknowledged drawbacks.

Using the netstat command, you may determine whether a port is open.

Fix everything

Keep your computers updated to protect against attacks like Main-in-the-Middle (MITM) and NetBIOS name service (NBNS) spoofing.

Leave no one point of failure.

Whether it's malware, hardware malfunction, hardware infection, database problem, or another issue, if your data is crucial, at least one other secure site should have a copy.

Make use of a firewall or endpoint security.

A blacklist of identified attacker IP addresses with their most frequently used ports is typically included of solutions.

Implement a virtual private network (VPN)

Network traffic is encrypted and protected via VPNs.


Business networks that use VLANs can better separate internal traffic based on recognized needs. One of the best measures to stop lateral movements and privilege escalation assaults from spreading is this control. To separate internal network traffic, utilize VLANs.

Take advantage of MAC Address Filtering.

This can stop unauthorized systems from connecting to your network. The above methods are the most typical for preventing malicious actors from exploiting SMB flaws.

That's not a complete list, though, and it's tough to compile one because attackers utilize a variety of tactics, like pretending to be a legitimate asset within a network on a hacked employee's workstation.

Therefore, when it comes to securing an organization, a proactive cyber security approach is necessary to ensure that the security strategy is built on solid fundamentals with the inclusion of a defence-in-depth approach, layered architecture that adheres to the least privilege principle, and collective effort from the people, process, and technology pillars.

The "inter-process communication" protocol, which enables programs and services on networked computers to communicate with one another, is made possible by the SMB protocol. SMB allows sharing of files, prints, and devices, among other essential network functions.

In other words, a Server Message Block (SMB) allows an application on a computer to read and write files and ask server software in a computer network for services.

However, it is inevitable for computers to link to one another over the internet, especially when resources are shared. It would help if you also kept an eye out to prevent attacks from malicious users.

Windows servers' exposed SMB ports are an open invitation to attackers and can give them access to a specific system or corporate network. By employing a few straightforward strategies, SMB administrators can lessen the risk and vulnerability of SMB ports to internet threats.

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Susith Nonis

I'm fascinated by the IT world and how the 1's and 0's work. While I venture into the world of Technology, I try to share what I know in the simplest way with you. Not a fan of coffee, a travel addict, and a self-accredited 'master chef'.