BRev Port 5432 — PostgreSQL
Port 5432 carries the PostgreSQL frontend/backend protocol. Unlike most databases, Postgres has two independent gates in front of it, and understanding the pair is most of the operational knowledge worth having: listen_addresses in postgresql.conf decides which interfaces the server binds at all, and pg_hba.conf decides — per source address, per database, per user — which connections are accepted and by what authentication method.
The default on a source build is listen_addresses = localhost, which is the safe posture: the port is simply not reachable from another machine. Distribution packages and container images vary, and Docker in particular publishes the port on all interfaces the moment you write -p 5432:5432, which is the most common way a development database ends up on the public internet. Our reference marks 5432 never, and exposed instances get ransomed exactly like MySQL.
For remote human access, an SSH tunnel is the answer: ssh -L 5432:localhost:5432 user@dbhost and connect to 127.0.0.1. For application access, keep the database on a private network and use pg_hba.conf with scram-sha-256 (not md5, which is legacy) plus TLS. A managed service’s allow-list is the same idea expressed in a console.
Port 5432 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 5432 | Registered |
| Service | PostgreSQL | The PostgreSQL wire protocol. (TCP) |
| Category | Databases | data stores and their wire protocols |
| Exposure | Never | Never expose to the internet |
Should port 5432 face the internet? Never public — bind to localhost or a private network, and check pg_hba.conf. Exposed instances get ransomed.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
“no pg_hba.conf entry for host …”
This is the good failure. It means the network reached Postgres and the server’s own access rules refused the connection — the second gate doing its job. Add a specific host line for the source, database and user with a real authentication method; do not add a 0.0.0.0/0 trust line, which is how a database becomes unauthenticated to the whole internet.
“could not connect to server: Connection refused” on a machine where psql works locally
listen_addresses is still localhost, so the server never binds the external interface. Change it in postgresql.conf and restart (it is not reloadable), then confirm with the local commands on this page that the listener is on the address you expect — and make sure the firewall now restricts what you have just opened.
Check port 5432 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 5432 — swap example.com for the host you are actually testing.
Is port 5432 reachable on another host?
nc -vz example.com 5432"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 5432 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 5432Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 5432A 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:5432Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 5432?
sudo ss -tlnp | grep :5432Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :5432The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :5432The 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 | |
|---|---|---|---|---|---|
| 5432 | tcp | PostgreSQL | The PostgreSQL wire protocol.Never public — bind to localhost or a private network, and check pg_hba.conf. Exposed instances get ransomed. | 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 expose port 5432?
No. Even with strong authentication it is an unnecessary attack surface holding all of your data behind one credential. Keep it on a private network or loopback and reach it through a tunnel, a VPN or the application tier.
What is the difference between listen_addresses and pg_hba.conf?
listen_addresses controls which network interfaces the server binds — a network-level gate. pg_hba.conf controls which client addresses may connect to which database as which user, and how they authenticate. Both must permit a connection for it to succeed.
How do I connect to a remote PostgreSQL database?
Prefer an SSH tunnel: ssh -L 5432:localhost:5432 user@dbhost, then connect to 127.0.0.1:5432. If the server must accept remote connections, restrict pg_hba.conf to specific source addresses, require scram-sha-256, and enable TLS.