BRev Port 1723 — PPTP
PPTP listens on 1723/TCP for control and then carries the actual data in GRE, IP protocol 47 — not a port at all. That split is the source of half its operational pain: a firewall rule allowing “TCP 1723” lets the tunnel negotiate and then silently carries no traffic, because GRE was never permitted, and many NAT devices cannot track GRE without a dedicated helper.
The security position is not a matter of configuration. PPTP’s authentication, MS-CHAPv2, was reduced in 2012 to the difficulty of a single DES key — recoverable in hours by a commercial service — and MPPE derives its session keys from that same password material. The practical summary in our reference is blunt: PPTP traffic is decryptable. Not “weak”, not “deprecated pending review”. Captured traffic can be read.
It survives because it is built into every operating system and needs no client install, which is exactly the property that keeps legacy appliances offering it. The migration targets are WireGuard (UDP 51820), OpenVPN (UDP 1194) or IKEv2/IPsec (UDP 500 and 4500) — all of which authenticate with certificates or modern PSKs and none of which have a comparable break.
Port 1723 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 1723 | Registered |
| Service | PPTP | Microsoft’s legacy VPN protocol. (TCP) |
| Category | Remote access | shells, desktops and VPNs |
| Exposure | Never | Never expose to the internet |
Should port 1723 face the internet? Never bare. MS-CHAPv2 is broken — PPTP traffic is decryptable. Use WireGuard or OpenVPN.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
Error 619 / 806 — the tunnel authenticates and then dies
That is GRE being blocked or un-NATted. PPTP negotiates on TCP 1723 and then needs IP protocol 47 to pass in both directions; firewalls that only know about ports drop it, and NAT gateways need a PPTP passthrough helper to track it. Opening “port 47” does nothing — 47 is a protocol number, not a port.
Modern clients refuse to connect at all
macOS removed the PPTP client in Sierra and it has not returned; iOS dropped it at the same time; several Linux distributions ship it disabled. This is deliberate deprecation rather than a bug, and it is the practical signal to migrate rather than to hunt for a workaround.
Check port 1723 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 1723 — swap example.com for the host you are actually testing.
Is port 1723 reachable on another host?
nc -vz example.com 1723"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 1723 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 1723Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 1723A 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:1723Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 1723?
sudo ss -tlnp | grep :1723Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :1723The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :1723The 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 | |
|---|---|---|---|---|---|
| 1723 | tcp | PPTP | Microsoft’s legacy VPN protocol.Never bare. MS-CHAPv2 is broken — PPTP traffic is decryptable. Use WireGuard or OpenVPN. | 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
Is PPTP safe if I use a very long password?
No. The MS-CHAPv2 break reduces the handshake to a single DES keyspace regardless of password length, so a long password does not help the way it would against ordinary guessing. The protocol needs replacing, not tuning.
What should replace PPTP?
WireGuard for new deployments, OpenVPN where you need TCP fallback or richer authentication integration, IKEv2/IPsec where native OS clients matter (it is built into Windows, macOS and iOS and reconnects well on mobile).
Why is port 1723 open on my router?
Many consumer routers ship a PPTP server or a PPTP passthrough enabled by default. If nobody uses it, turn it off — it is an authentication surface with a known break, listening for anyone who scans you.