BRev Port 2049 — NFS
NFS mounts a remote directory as if it were local, and its default authentication tells you most of what you need to know: AUTH_SYS means the client simply asserts which user ID it is acting as, and the server believes it. Anyone with root on a machine that can reach the export can therefore become any user on it — the trust boundary is the network, not the credential.
The version distinction matters for firewalling. NFSv3 needed the portmapper on 111 plus mountd, lockd and statd on dynamically assigned ports, which is why NFSv3 rules were notoriously fiddly and why 111 is itself an enumeration and amplification surface. NFSv4 consolidated everything onto port 2049 and dropped the portmapper dependency, so a single rule now covers it — and it supports Kerberos (sec=krb5p) for real cryptographic authentication rather than asserted user IDs.
Exports are the other half. A share exported to * with no_root_squash and no address restriction is readable and writable by anyone who reaches the port, and exposed NFS has leaked backups, home directories and virtual machine images repeatedly. Keep root_squash on, export to specific hosts or subnets, and keep 2049 inside a private network. Our reference marks it never.
Port 2049 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 2049 | Registered |
| Service | NFS | Unix network file system. (TCP and UDP) |
| Category | File & print | file sharing, transfer and printing |
| Exposure | Never | Never expose to the internet |
Should port 2049 face the internet? Exports are commonly readable by anyone who can reach the port. Never public.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
“access denied by server while mounting”
The export list, not the port. The server checks the client address against /etc/exports, so an address that is not listed — or a client behind NAT presenting a different address than expected — is refused after a successful TCP connection. showmount -e (v3) or checking the server’s export configuration resolves it faster than firewall changes.
Mounts hang forever instead of failing
A hard mount blocks indefinitely when the server becomes unreachable — by design, so writes are not lost — and it makes anything touching the mount point unkillable. For non-critical mounts use soft with a timeout, or intr where supported, and put the mount behind autofs so an unreachable server does not wedge every process that runs ls.
Check port 2049 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 2049 — swap example.com for the host you are actually testing.
Is port 2049 reachable on another host?
nc -vz example.com 2049"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 2049 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 2049Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 2049A 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:2049Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 2049?
sudo ss -tlnp | grep :2049Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :2049The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :2049The 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 | |
|---|---|---|---|---|---|
| 2049 | tcp/udp | NFS | Unix network file system.Exports are commonly readable by anyone who can reach the port. Never public. | Neverbrowser-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 it safe to expose NFS to the internet?
No. With the default AUTH_SYS the client asserts its own user identity, so the network is the only real access control. Keep it on a private network, export to specific hosts, and use Kerberos if you need cryptographic authentication.
What is root_squash?
A server-side setting that maps requests arriving as root to an unprivileged user, so a client’s root cannot act as root on the exported files. Turning it off (no_root_squash) is what turns a reachable export into a full compromise of that filesystem.
Do I still need port 111 for NFS?
Only for NFSv3, which uses the portmapper to locate mountd, lockd and statd on dynamic ports. NFSv4 uses port 2049 alone, which makes both the firewall rules and the exposure much easier to reason about.