HTTPfy

Port 243 (sur-meas)

IANA assigns port 243 to sur-meas: Survey Measurement.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
sur-meas TCP Survey Measurement Dave Clark -
sur-meas UDP Survey Measurement Dave Clark -

Check what is listening on port 243

Linux: is anything listening on TCP 243?
sudo ss -tlnp 'sport = :243'
Linux: UDP 243
sudo ss -ulnp 'sport = :243'
Linux / macOS: which process owns the port
sudo lsof -nP -i :243
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':243 '
Windows (Command Prompt)
netstat -ano | findstr :243
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 243 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 243
From Windows
Test-NetConnection HOST -Port 243
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 243

Block port 243 in the firewall

ufw
sudo ufw deny 243/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 243 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 243 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 243' -Direction Inbound -Protocol TCP -LocalPort 243 -Action Block
ufw
sudo ufw deny 243/udp
iptables
sudo iptables -A INPUT -p udp --dport 243 -j DROP
nftables
sudo nft add rule inet filter input udp dport 243 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 243' -Direction Inbound -Protocol UDP -LocalPort 243 -Action Block

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All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.