BRev Port 636 — LDAPS
Port 636 is LDAP inside TLS, with the handshake completed before any directory operation. It is the port every domain-joined application and appliance should use, because it removes both the cleartext credential exposure of port 389 and the ambiguity of a StartTLS upgrade that may or may not have happened.
Encryption changes the transport and nothing about the value of the target. This is still the directory that defines who exists in the organisation, what they can access and how they are grouped — so our reference marks it care rather than safe: restrict it by source address, give applications narrowly scoped service accounts, and keep it off the public internet. A well-encrypted channel to a directory that permits anonymous bind is still an enumeration surface.
In an Active Directory forest there are two more ports worth recognising in a scan: 3268 and 3269 are the Global Catalog, which serves a partial, forest-wide replica — and 3269 is its TLS version. Applications that need to search across domains use those rather than 389/636, which is why they appear in firewall requests that otherwise look wrong.
Port 636 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 636 | Well-known |
| Service | LDAPS | Directory lookups over TLS. (TCP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Care | Expose with care |
Should port 636 face the internet? Encrypted, but it is still your user directory. Restrict by source.
Legitimately public in some setups, but only hardened — current version, real authentication, rate limits, and ideally an allow-list.
What commonly goes wrong
LDAPS fails while 389 works
Almost always the certificate. Domain controllers need a certificate with Server Authentication EKU and the correct name, and the client must trust the issuing CA — a self-signed or expired DC certificate fails the handshake before LDAP is spoken. openssl s_client -connect dc:636 shows the chain and the verification result, which is faster than reading the client’s error.
It works from Windows and fails from a Linux appliance
The Windows client trusts the internal CA through the domain; the appliance has its own trust store and does not. Install the CA certificate on the appliance rather than disabling verification — a client configured to ignore certificate errors on a directory connection is back to trusting the network.
Check port 636 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 636 — swap example.com for the host you are actually testing.
Is port 636 reachable on another host?
nc -vz example.com 636"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 636 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 636Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 636A 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:636Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 636?
sudo ss -tlnp | grep :636Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :636The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :636The 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 | |
|---|---|---|---|---|---|
| 636 | tcp | LDAPS | Directory lookups over TLS.Encrypted, but it is still your user directory. Restrict by source. | Carebrowser-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 LDAPS on 636 safe to expose?
Safer than 389, not safe to publish. TLS protects the credentials in transit; it does not stop enumeration, password spraying or an exposed anonymous bind. Restrict by source and keep it internal or behind a VPN.
LDAPS on 636 or StartTLS on 389?
636, when you have the choice. Implicit TLS cannot be stripped and cannot silently fall back; StartTLS depends on the client actually requiring the upgrade, and many do not.
What are ports 3268 and 3269?
The Active Directory Global Catalog — a forest-wide partial replica used for cross-domain searches. 3268 is plaintext, 3269 is TLS. Applications that search the whole forest need those instead of 389/636.