BRev Port 27015 — Source engine game servers
Port 27015 is the default for Valve’s Source engine — Counter-Strike, Team Fortress 2, Garry’s Mod, Left 4 Dead and the rest — carrying both gameplay traffic and the A2S query protocol that server browsers use. Related ports commonly appear alongside it: 27005 for the client, 27020 for SourceTV spectating, and 27036 for Steam’s local network discovery, which is why a scan of a gaming machine shows a cluster rather than one number.
The A2S query protocol is the interesting part. It answers unauthenticated UDP requests with the server name, map, player count and often the player list — which is what makes server browsers work, and also what made Source servers a reflection and amplification vector. A spoofed A2S_INFO request produces a much larger reply sent to the victim, and the technique was used widely enough that Valve added rate limiting and response changes to mitigate it.
Marked care, because a public game server is a legitimate public service. The conditions are the usual ones for something that parses hostile input from strangers: keep the engine and any plugins current (SourceMod and MetaMod extensions are a real attack surface), set an RCON password that is long and not reused — RCON is remote command execution on the server by design — and isolate the host, because game servers are attacked for the machine as often as for the game.
Port 27015 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 27015 | Registered |
| Service | Source engine | Counter-Strike, Team Fortress and other Valve servers. (TCP and UDP) |
| Category | Games & voice | game servers and voice chat |
| Exposure | Care | Expose with care |
Should port 27015 face the internet? Public game hosting; also a known UDP amplification vector, so rate-limit it.
Legitimately public in some setups, but only hardened — current version, real authentication, rate limits, and ideally an allow-list.
What commonly goes wrong
The server does not appear in the browser but direct connect works
The query port is being blocked while the gameplay port passes, or the server is not registered with the master servers. A2S queries arrive on the same UDP port by default but can be moved with -sport, so a partial firewall rule breaks discovery while play still works. Check the game server token too — Valve requires one for public listing.
Players lag or get dropped under load
Before tuning tickrate, check for query flooding: unauthenticated A2S requests cost the server work, and a flood degrades gameplay without ever touching the game logic. Rate-limit queries at the firewall, keep the engine current for its built-in mitigations, and separate the query port from the game port if the platform allows.
Check port 27015 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 27015 — swap example.com for the host you are actually testing.
Is port 27015 reachable on another host?
nc -vz example.com 27015"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 27015 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 27015Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 27015A 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:27015Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 27015?
sudo ss -tlnp | grep :27015Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :27015The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :27015The 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 | |
|---|---|---|---|---|---|
| 27015 | tcp/udp | Source engine | Counter-Strike, Team Fortress and other Valve servers.Public game hosting; also a known UDP amplification vector, so rate-limit it. | Care |
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 port 27015 UDP or TCP?
Primarily UDP for gameplay and queries; TCP 27015 is used by RCON. Firewall rules that permit only TCP break the game entirely, which is a common mistake.
How do I test whether my game server port is open?
A TCP check on 27015 only tests RCON. For the UDP side, nmap -sU reports open|filtered honestly, and the real proof is a client or a server-browser query completing — UDP gives no handshake to observe.
What are ports 27005, 27020 and 27036?
27005 is the client’s outgoing port, 27020 is SourceTV, and 27036 is Steam local network discovery and remote play. They show up together on a machine that runs or plays Source games.