BRev Port 445 — SMB
Port 445 carries SMB directly over TCP — Windows file shares, printer sharing, named pipes, and a large amount of the machinery Active Directory depends on. It is essential inside a network and catastrophic outside one, which is why many ISPs block it at their borders by default and why our reference gives it the flattest verdict in the set: never.
The reason is specific and historical. SMBv1 contained a pre-authentication remote code execution flaw (EternalBlue, MS17-010) that WannaCry and NotPetya both weaponised in 2017 — worms that spread machine to machine over 445 with no user interaction, across hospitals, shipping companies and manufacturing lines. SMBv1 should be removed entirely; modern SMB 3.x with signing and encryption is a different and much better protocol, but it is still not something to publish.
Port 139 is the older path to the same service over NetBIOS, and hosts often listen on both. Blocking one and not the other is a common half-measure. For remote file access the correct shapes are a VPN, an SFTP server, or a purpose-built sync service — never an SMB share reachable from the internet, and never a share exposed to a guest network.
Port 445 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 445 | Well-known |
| Service | SMB | Windows file and printer sharing. (TCP) |
| Category | File & print | file sharing, transfer and printing |
| Exposure | Never | Never expose to the internet |
Should port 445 face the internet? Never. Ransomware worms scan for it — WannaCry and NotPetya both spread on this port.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
A share works on the LAN and not over the VPN
Name resolution rather than the port. SMB needs to resolve the server name, and NetBIOS broadcast does not cross a VPN — so \\server fails while \\10.0.0.5 works. Fix DNS for the internal names, or use the address. Confirm 445 is reachable across the tunnel with the commands on this page before touching share permissions.
You need file access from outside and are about to forward 445
Do not. This is the port ISPs block on purpose, and the one ransomware worms scan for. A VPN gives you the same shares with authentication in front; SFTP or a managed sync service gives you file transfer without SMB at all. There is no hardening configuration that makes public 445 reasonable.
Check port 445 yourself
No browser can open a raw TCP socket, so no web page can honestly tell you whether this port is open on a remote host. These are the real commands, generated for port 445 — swap example.com for the host you are actually testing.
Is port 445 reachable on another host?
nc -vz example.com 445"succeeded!" means the TCP handshake completed — the port is open. "Connection refused" is closed; a hang until timeout means filtered (a firewall dropped it silently).
nmap -Pn -p 445 example.comThe only common tool that distinguishes open / closed / filtered. -Pn skips the ping sweep, so it still works against hosts that ignore ICMP.
Test-NetConnection example.com -Port 445Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 445A blank screen or a banner means it connected. "Connection refused" means closed. Quit with Ctrl-] then `quit`.
curl -v --max-time 5 telnet://example.com:445Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 445?
sudo ss -tlnp | grep :445Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :445The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :445The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
No web page can tell you whether a port is open on a remote host
A browser cannot open a raw TCP socket — there is no API for it, by design. So a site that reports “port 3389 is OPEN on 203.0.113.10” is not checking from your browser; it is asking its own server to scan on your behalf. That works, but it means their server sees the host you asked about, scans from their address rather than yours, and answers a question about their network path, not yours.
We don't have a server here and won't pretend to. So this page does three honest things instead:
| Port | Proto | Service | What it is | Exposure | |
|---|---|---|---|---|---|
| 445 | tcp | SMB | Windows file and printer sharing.Never. Ransomware worms scan for it — WannaCry and NotPetya both spread on this port. | Never |
http://host:port/ and times it. A connection that is established proves a socket is open; an instant refusal usually means nothing is listening; anything else is honestly reported as “can't tell”. Each attempt gives up after 2.5s.nc -vz example.com 443
nmap -Pn -p 443 example.com
telnet example.com 443
curl -v --max-time 5 telnet://example.com:443
This page cannot answer that, and neither can any other page — from your browser. Opening a TCP connection to an arbitrary host and port is not something JavaScript is allowed to do. Every “open port checker” that gives you a green tick is running the scan on their server. Which is fine, as long as you know what you are agreeing to: they learn which host and port you were worried about, and the answer they give you is about the route from their data centre — not from the internet as your users see it.
What to do instead
Run the check from outside your own network. Copy an nmap or nc line from the card above and run it from a machine that is not behind the same router — a phone on mobile data with a terminal app, a cheap VPS, a colleague's laptop. Running it from inside the LAN tests the LAN, not the internet, and will happily say “open” while the world sees nothing.
Then check the thing that is actually blocking it, in this order: the service is bound to0.0.0.0 and not 127.0.0.1 · the host firewall (ufw, firewalld, Windows Defender Firewall) · the cloud security group or the router's port-forward rule · and finally your ISP, which commonly blocks 25, 80 and 445 on residential lines whatever you configure.
What a browser genuinely can verify
One thing: that an HTTPS service answers. If what you actually want to know is “is my site up and how fast is it”, the latency test measures real round trips to a CORS-open URL. If the name doesn't resolve, or resolves somewhere unexpected, that is a DNS problem, not a port problem — and it is the more common cause of “my port is closed” than an actual firewall.
An IP address finds the machine; a port picks which program on it gets the traffic. TCP sets up a connection first — a handshake, ordered delivery, retransmission — which is why the web, email and SSH use it. UDP just sends the packet with no handshake and no guarantee it arrives, which is why DNS, video calls and games use it: late data is worse than lost data.
That difference is why a UDP port is so much harder to test. TCP either completes a handshake or it doesn't, so a tool can tell you. A UDP service that has nothing to say to an unexpected packet stays silent — and silence looks identical to a firewall dropping it.
The three ranges
0–1023Assigned by IANA to core services. On Unix, binding one needs root (or CAP_NET_BIND_SERVICE).1024–49151Registered by vendors for their applications. Any user can bind these — which is why every dev server lives here.49152–65535Never assigned. The OS hands these out as the SOURCE port of your outgoing connections. (Linux actually uses 32768–60999 by default.)Closed is not the same as filtered
Closed means the host answered: the packet arrived, nothing is listening, and you got an immediate refusal (a TCP RST). Filtered means nothing answered at all — a firewall dropped your packet in silence, so you wait for a timeout and learn nothing. The difference matters: closed proves the host is reachable and the port is free; filtered tells you only that something between you and it is discarding traffic.nmap reports the two separately. A browser cannot distinguish them, which is why this page reports a timeout as “can't tell” rather than guessing.
The one habit worth keeping
Bind services to 127.0.0.1 instead of 0.0.0.0 unless something outside the machine genuinely needs them, and reach them over SSH or a VPN. Every database on the never-expose list above ships with that advice, and the leaks all begin the same way: a default bind address, a cloud instance with a public IP, and a scanner that found it within the hour.
The blocked-port list this page uses is the WHATWG Fetch Standard's “bad port” table — the ports every browser refuses to connect to (Chrome calls it ERR_UNSAFE_PORT). Local probing is additionally gated by Chrome's Local Network Access permission, which replaced Private Network Access and ships in Chrome 142.
FAQ
Is it safe to open port 445?
No — internally it is necessary, externally it is indefensible. It has a history of wormable pre-authentication vulnerabilities and is scanned constantly. Use a VPN for remote access to shares.
What is the difference between port 139 and 445?
139 is SMB over the legacy NetBIOS session service; 445 is SMB directly over TCP, introduced with Windows 2000. Modern clients prefer 445, but many hosts still listen on both — block both at the perimeter.
Why is my ISP blocking port 445?
To contain worms. Because SMB is so widely enabled and has produced wormable vulnerabilities, consumer ISPs commonly filter it at the network edge, which stops infected machines scanning the internet for neighbours.