How to Test if a UDP Port Is Open for a Game Server

Most online port checkers only test TCP, and UDP has no handshake, so a "closed" result for a UDP game port often means nothing. The reliable test is to watch for packets arriving on the server PC: run pktmon on Windows or tcpdump on Linux filtered to your game's port, then have someone outside your network send traffic to your public IP on that port. If packets show up, the router forward works and any remaining problem is on the PC or in the game. If nothing arrives, the block is at the router or your ISP.

Why UDP is hard to test

TCP connections start with a handshake: the checker sends a SYN, an open port replies with SYN-ACK, and the checker knows instantly. UDP has no handshake. A checker sends a packet and either:

• gets a reply, which only happens if the program listening understands that packet, or
• gets nothing, which can mean open (the game ignored a packet it did not understand) or blocked.

Game servers ignore random packets, so an open UDP port and a blocked one look identical from outside. That is why port checker sites say "closed" for working Valheim, Palworld and Rust servers every day.

Method 1: watch for incoming packets (best)

This tells you whether traffic reaches the PC, which is the question you actually have.

On Windows with pktmon

pktmon is built into Windows 10 and 11. Open Command Prompt as administrator:

pktmon filter remove
pktmon filter add -p 2456
pktmon start --capture --comp nics -m real-time

Replace 2456 with your game's port. Now have a friend try to join (or send test traffic as in Method 2). Lines appearing in the window mean packets are arriving. Press Ctrl + C to stop. Clear the filter afterwards with pktmon filter remove.

If you prefer a GUI, Wireshark with the filter udp.port == 2456 does the same.

On Linux with tcpdump

sudo tcpdump -ni any udp port 2456

Each line is a packet. Source addresses from outside your network mean the forward works.

Reading the result

Capture tools see packets before Windows Firewall filters them, which is exactly why this method separates router problems from PC problems.

Method 2: a netcat listener

Stop the game server first so the port is free, then listen with netcat on the server:

Linux:

nc -u -l 2456

(On some distros the syntax is nc -u -l -p 2456.)

From outside (a friend's PC, or a cheap VPS, or a phone with a terminal app on mobile data):

nc -u YOUR.PUBLIC.IP 2456

Type a word and press Enter. If it appears on the server's listener, the port is open all the way through. On Windows, the Nmap project's ncat works the same way: ncat -u -l 2456 and ncat -u IP 2456.

Remember to allow the port in the firewall, or the listener will never see anything even with a working forward.

Method 3: ask the server a question it answers

Steam based servers answer A2S queries on their query port. If the server replies, the port is definitely open.

• In the Steam client, open View, Game Servers, Favorites, Add a server, enter publicIP:queryport and click Find games at this address. If it lists your server, the query port is reachable.
• Do this from outside your network. Inside, NAT loopback can make it fail even when it works; see friends can't connect but I can.

Query ports for some games:

For Minecraft Bedrock, server status websites send a real Bedrock ping on UDP 19132, so they do give a meaningful answer.

Method 4: nmap (with care)

nmap -sU -p 2456 YOUR.PUBLIC.IP

Run from outside your network. Results:

• open: something replied. Good.
• closed: an ICMP "port unreachable" came back. The packet reached a machine that has nothing listening, so the forward works but the server is not running on that port.
• open|filtered: no reply. Could be open, could be blocked. This is the most common result for game servers and tells you little.

Testing from outside your network

Every method above must send traffic from outside your home. Options:

• A friend's PC.
• Your phone on mobile data with a terminal app, or tethering a laptop to it.
• A cheap cloud VPS for a few minutes.

Do not test from inside your house using your public IP. Many routers do not loop that traffic back.

Once you know where the block is

• Nothing arrives: check the router rule and protocol (port forwarding guide), then double NAT and CGNAT.
• Arrives, no reply: Windows Firewall rules for a game server or ufw and firewalld, and check the server listens on 0.0.0.0 rather than 127.0.0.1.

Skip the packet capture

If you would rather never run tcpdump to find out why friends cannot join, a hosted server has its ports open and reachable from the start. GamezServers servers have their own address, a live console where you can watch players connect, and DDoS protection. See rent a game server.

FAQ

Why does a port checker say my UDP port is closed?

Most checkers only test TCP, and those that try UDP cannot tell open from blocked because game servers do not reply to random packets.

Can I test a UDP port while the game server is running?

Yes, with a packet capture (pktmon or tcpdump) or a Steam query. Netcat listeners need the server stopped so the port is free.

How do I check if a port is open on Windows without installing anything?

Use pktmon, which is built into Windows 10 and 11, to watch for packets arriving on the port while a friend tries to connect.

Does Windows Firewall block packets before pktmon sees them?

No. pktmon captures at the network adapter, before the firewall decides. That is useful: it shows whether traffic reached the PC at all.