BRev Port 137 — NetBIOS Name Service
Port 137/UDP is how NetBIOS names are resolved: a machine broadcasts “who has this name?” and the owner answers. It predates DNS being universal on Windows networks, and it survives because Windows still enables NetBIOS over TCP/IP on adapters by default in many configurations, and because some legacy applications resolve names no other way.
The exposure is a reconnaissance one. A single nbtstat -A <address> query against a reachable host returns its NetBIOS name, its workgroup or domain, and sometimes the logged-on user — free, unauthenticated inventory of a network, which is why the first phase of most internal assessments is exactly this query in a loop. Being UDP and broadcast-based, it is also trivially spoofable, and it is a known DDoS amplification vector.
Turning it off is usually painless: disable NetBIOS over TCP/IP in the adapter’s advanced TCP/IP settings (or via DHCP option 001 across a fleet), which stops 137 and 138 and leaves modern SMB on 445 working. The related name-poisoning attacks — LLMNR and NBT-NS spoofing, the standard route to captured NTLM hashes on a corporate LAN — go away with it, which is the bigger win.
Port 137 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 137 | Well-known |
| Service | NetBIOS name | Legacy Windows name resolution. (UDP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Never | Never expose to the internet |
Should port 137 face the internet? Leaks machine and workgroup names; amplification vector.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
Your ISP or provider reports amplification traffic
NetBIOS name service answers spoofed queries with larger replies, so a reachable 137 can be used to flood a third party. It should never be reachable from outside the local segment — block 137/138 UDP at the perimeter, and disable NetBIOS on any host that does not need it.
Name resolution breaks after disabling NetBIOS
Something was relying on it — usually a legacy application resolving short names, or browsing “Network” in Explorer, which uses the old browser service. Fix it with proper DNS suffixes and DNS records rather than turning NetBIOS back on; WSD handles modern network discovery without it.
Check port 137 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 137 — swap example.com for the host you are actually testing.
Is port 137 reachable on another host?
nc -vz example.com 137"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 137 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 137Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 137A 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:137Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 137?
sudo ss -tlnp | grep :137Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :137The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :137The 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 | |
|---|---|---|---|---|---|
| 137 | udp | NetBIOS name | Legacy Windows name resolution.Leaks machine and workgroup names; amplification vector. | Neverbrowser-blocked |
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
What does port 137 do?
It resolves NetBIOS names to addresses on a local segment, by broadcast. It is the naming half of legacy Windows networking, paired with 138 for datagrams and 139 for the actual session.
Is it safe to leave port 137 open?
Not to the internet — it hands out machine and workgroup names to anyone who asks and amplifies DDoS traffic. Even internally, disabling NetBIOS removes a whole class of name-poisoning attacks used to capture credentials.
Can I disable NetBIOS over TCP/IP safely?
On a network with working DNS, usually yes. Modern SMB on 445 does not need it. Test the legacy applications that resolve short hostnames first — those are the ones that notice.