BRev Port 161 — SNMP
SNMP is how switches, routers, printers, UPS units and servers report their state to a monitoring system — interface counters, CPU, temperature, serial numbers, routing tables, sometimes running configuration. Its problem is the authentication in versions 1 and 2c: a community string, sent in the clear, functioning as a shared password that is very often still the factory default of “public” for reads and “private” for writes.
What that yields is a complete device inventory to anyone who can reach the port. An snmpwalk against a reachable device returns the interface list, addresses, ARP and routing tables, installed software on some systems, and the model and firmware version — which is the map an attacker wants before doing anything else. Where a writable community string is left enabled, SNMP can reconfigure the device or trigger a configuration download, which is full control of network equipment.
It is also UDP and therefore spoofable, making it a well-known DDoS amplification vector. SNMPv3 fixes the protocol properly with per-user authentication and encryption, and is what any current deployment should use. Our reference marks 161 never: monitoring belongs on a management network, not on a routable interface.
Port 161 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 161 | Well-known |
| Service | SNMP | Device monitoring and configuration. (UDP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Never | Never expose to the internet |
Should port 161 face the internet? v1/v2c "authentication" is a community string, usually still "public". Full device inventory, sometimes write access.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
The monitoring system times out and the device looks fine
UDP gives you no error to read. Check the device’s SNMP access-control list — most restrict which source addresses may query, and monitoring servers move. Then check the community string and the version: a v2c poller against a v3-only device times out identically to a firewall drop, which is why this takes so long to diagnose.
You inherited devices and do not know which have SNMP enabled
Assume all of them do, with default strings. Network hardware, printers and UPS units ship with SNMP on. Scan your own management ranges for UDP 161, replace v1/v2c with v3 where the hardware supports it, disable the write community everywhere, and restrict source addresses on the device itself rather than relying on the perimeter.
Check port 161 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 161 — swap example.com for the host you are actually testing.
Is port 161 reachable on another host?
nc -vz example.com 161"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 161 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 161Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 161A 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:161Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 161?
sudo ss -tlnp | grep :161Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :161The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :161The 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 | |
|---|---|---|---|---|---|
| 161 | udp | SNMP | Device monitoring and configuration.v1/v2c "authentication" is a community string, usually still "public". Full device inventory, sometimes write access. | Neverbrowser-blocked | |
| 8161 | tcp | ActiveMQ console | ActiveMQ web console.Default admin/admin, and CVE-2023-46604 was pre-auth RCE. | Never | |
| 61616 | tcp | ActiveMQ OpenWire | ActiveMQ broker protocol.CVE-2023-46604 was pre-auth RCE over 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 SNMP secure?
v1 and v2c are not — the community string is a cleartext shared secret, commonly left at the default. v3 adds per-user authentication and encryption and is the only version worth exposing to any untrusted segment.
What can someone do with a read-only community string?
Enumerate interfaces, addresses, routing and ARP tables, hardware models, firmware versions and often installed software — a full map of the network, unauthenticated. Read-only is not harmless.
What is port 162 for?
SNMP traps: devices pushing alerts to a manager, rather than the manager polling. It shares the same weak authentication model in v1/v2c and is additionally spoofable, so a forged trap can raise or suppress alarms.