BRev Port 2375 — Docker daemon API
Port 2375 exposes the Docker daemon’s REST API without TLS and without authentication. Anyone who can reach it can create a container — and because a container can mount the host filesystem and run privileged, that single API call is root on the machine. There is no credential to guess, no login to brute-force: reachability is the compromise.
This is not theoretical. Automated campaigns scan the entire IPv4 space for 2375, and an exposed daemon is typically running a cryptominer within minutes; some campaigns also use the API to escape into the host, install persistence and pivot. It is one of the fastest exploitation paths that exists, which is why our reference marks it never in the strongest terms.
Port 2376 is the TLS variant with mutual certificate authentication, and it is better but not a different category — a client certificate is still root-equivalent on the host, so it belongs on a private network with tightly controlled certificate distribution. The safest arrangement is the default one: no TCP listener at all, the daemon on its Unix socket, and remote access over SSH (docker context create --docker "host=ssh://user@host"), which reuses SSH’s authentication instead of inventing a new trust path.
Port 2375 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 2375 | Registered |
| Service | Docker API | The Docker daemon’s unauthenticated HTTP API. (TCP) |
| Category | Dev & tooling | dev servers, build tools, admin consoles |
| Exposure | Never | Never expose to the internet |
Should port 2375 face the internet? Reaching it is root on the host — one API call mounts the filesystem. Cryptominers scan for it continuously.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
You enabled the TCP socket to make a CI job work
That is the usual origin. Replace it with an SSH-based Docker context, which needs no open port and authenticates with keys you already manage. If a TCP listener is unavoidable, use 2376 with mutual TLS, bind it to a private interface, and firewall it to the specific CI addresses — never 0.0.0.0.
Unexpected containers, or the host CPU pinned at 100%
Check docker ps for containers you did not start, and the daemon logs for API calls from unfamiliar addresses. Assume host compromise rather than a container problem: an attacker with the API can mount the host filesystem, so removing the container does not remove them. Rebuild, and rotate every credential the host held.
Check port 2375 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 2375 — swap example.com for the host you are actually testing.
What on this machine is holding port 2375?
sudo ss -tlnp | grep :2375Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :2375The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :2375The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
Is port 2375 reachable on another host?
nc -vz example.com 2375"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 2375 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 2375Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 2375A 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:2375Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
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 | |
|---|---|---|---|---|---|
| 2375 | tcp | Docker API | The Docker daemon’s unauthenticated HTTP API.Reaching it is root on the host — one API call mounts the filesystem. Cryptominers scan for it continuously. | 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 ever safe to expose port 2375?
No. Unauthenticated access to the Docker API is root on the host, and it is scanned for constantly. If you need remote Docker access, use an SSH context; if you need a TCP socket, use 2376 with mutual TLS on a private network.
Is 2376 safe because it has TLS?
Safer, not safe. Mutual TLS authenticates the client, but any authorised client is still root-equivalent on the host. Keep it private, distribute certificates carefully, and prefer SSH.
How do I check whether my Docker socket is exposed?
Look for a -H tcp:// flag in the daemon configuration or DOCKER_HOST, and check what is actually listening with the local commands on this page. A listener on 0.0.0.0:2375 on a host with a public address should be treated as an incident.