BRev Port 53 — DNS
Port 53 carries DNS on UDP for ordinary queries and on TCP for anything that does not fit — large responses, DNSSEC-signed answers, and zone transfers. The old advice that “DNS is UDP and TCP 53 is only for zone transfers” is wrong and causes real outages: a resolver that cannot fall back to TCP fails on truncated responses, which is exactly what DNSSEC and modern record sets produce.
The distinction that decides your exposure is authoritative versus recursive. An authoritative server publishes your zones and must answer the whole internet — that is its function. A recursive resolver looks answers up on behalf of clients, and must only serve your own clients. An open recursive resolver is a DDoS amplifier: a small spoofed query returns a much larger answer to the victim, and the ANY-query and DNSSEC-response variants multiply it further. Our reference marks 53 care for exactly this split.
Encrypted DNS moved the traffic rather than replacing it: DNS-over-TLS runs on 853 and DNS-over-HTTPS on 443, both marked safe to expose because encryption is the point of them. Plain 53 remains what everything else on the network uses, including the query your browser makes before any of this page loads.
Port 53 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 53 | Well-known |
| Service | DNS | The name-to-address lookup every connection starts with. (TCP and UDP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Care | Expose with care |
Should port 53 face the internet? Authoritative servers must be public; a public RECURSIVE resolver gets abused for amplification.
Legitimately public in some setups, but only hardened — current version, real authentication, rate limits, and ideally an allow-list.
What commonly goes wrong
Queries work sometimes and fail for certain domains
Very often TCP 53 or large-UDP being blocked. A response too large for the client’s UDP buffer is truncated, the resolver retries over TCP, and a firewall that only permits UDP 53 breaks that retry — so DNSSEC-signed zones and long TXT records fail while ordinary lookups succeed. Permit both transports.
Your resolver appears in an abuse report
It is answering recursive queries for strangers. Restrict recursion to your own address ranges (allow-recursion in BIND, the equivalent in Unbound or dnsmasq), or split the roles onto separate servers entirely — authoritative for the world, recursive for the LAN. Rate limiting helps but is not the fix.
Check port 53 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 53 — swap example.com for the host you are actually testing.
Is port 53 reachable on another host?
nc -vz example.com 53"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 53 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 53Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 53A 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:53Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 53?
sudo ss -tlnp | grep :53Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :53The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :53The 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 | |
|---|---|---|---|---|---|
| 53 | tcp/udp | DNS | The name-to-address lookup every connection starts with.Authoritative servers must be public; a public RECURSIVE resolver gets abused for amplification. | Carebrowser-blocked | |
| 853 | tcp | DNS over TLS | Encrypted DNS (DoT).A public resolver on 853 is intended; the encryption is the point. | Safe | |
| 5351 | udp | NAT-PMP | Automatic port mapping on a router.Lets anything on the network punch holes in your firewall. | Never | |
| 5353 | udp | mDNS | Multicast DNS / Bonjour discovery.Local link only; publicly it becomes an amplifier and a device inventory. | Never | |
| 5355 | udp | LLMNR | Windows fallback name resolution.Spoofing it harvests NTLM hashes. Disable it entirely. | Never | |
| 5357 | tcp | WSDAPI | Windows web-services device API.LAN discovery service with a poor security history. | Never | |
| 8530 | tcp | WSUS | Windows Server Update Services.It decides what patches your fleet installs. | 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 DNS UDP or TCP?
Both. UDP 53 handles the common case; TCP 53 handles responses too large for UDP, zone transfers, and any query the resolver retries after truncation. Blocking TCP 53 breaks DNSSEC and large record sets.
Should port 53 be open to the internet?
For an authoritative nameserver, yes — that is its job. For a recursive resolver, no: an open resolver is abused as a DDoS amplifier. The two roles should be separated, or recursion restricted to your own networks.
What about DNS over TLS and DNS over HTTPS?
DoT uses port 853 and DoH uses 443. They encrypt the query between client and resolver, which is why both are safe to expose. They do not replace port 53 between resolvers and authoritative servers.