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Port 433 (nnsp)

IANA assigns port 433 to nnsp: NNTP for transit servers (NNSP).

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
nnsp TCP NNTP for transit servers (NNSP) IESG RFC 3977
nnsp UDP NNTP for transit servers (NNSP) IESG RFC 3977

Registry dates for this port: last modified 2018-07-06.

Check what is listening on port 433

Linux: is anything listening on TCP 433?
sudo ss -tlnp 'sport = :433'
Linux: UDP 433
sudo ss -ulnp 'sport = :433'
Linux / macOS: which process owns the port
sudo lsof -nP -i :433
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':433 '
Windows (Command Prompt)
netstat -ano | findstr :433
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 433 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 433
From Windows
Test-NetConnection HOST -Port 433
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 433

Block port 433 in the firewall

ufw
sudo ufw deny 433/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 433 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 433 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 433' -Direction Inbound -Protocol TCP -LocalPort 433 -Action Block
ufw
sudo ufw deny 433/udp
iptables
sudo iptables -A INPUT -p udp --dport 433 -j DROP
nftables
sudo nft add rule inet filter input udp dport 433 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 433' -Direction Inbound -Protocol UDP -LocalPort 433 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.