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// Network services

Port 433: NNTP for transit servers (NNSP)

Port 433 is assigned by IANA to nnsp (NNTP for transit servers (NNSP)).

433

How nnsp works

Network services
client:433nnspresponse client:433nnspresponse

The client sends a request to port 433 and the nnsp service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Network services
01 Security

Is port 433 dangerous?

We have no specific security note for port 433. Its risk depends on the program actually listening on it. General advice for network services services:

  • Answer only the networks that need the service (no open resolvers, relays or reflectors).
  • UDP services can be abused for DDoS amplification: rate-limit or block them at the edge.
  • Disable legacy test services that have no use in production.
02 Registry

IANA registry entries for port 433

ServicePortTransportDescriptionReferenceUpdated
nnsp 433 TCP, UDP NNTP for transit servers (NNSP) RFC 3977 2018-07-06

// source: IANA service name and port number registry

03 Check it

Which program uses port 433, and is it reachable?

Linux

sudo ss -ltnup 'sport = :433'      # process listening on 433
sudo lsof -nP -i :433

macOS

sudo lsof -nP -i :433

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 433 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 433
netstat -ano | findstr :433

From another machine

nmap -p 433 -sV <host>
sudo nmap -sU -p 433 <host>
nc -zv <host> 433

Block it in the firewall

sudo ufw deny 433/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 433" -Direction Inbound -LocalPort 433 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 433 used for?

Port 433 is assigned by IANA to nnsp (NNTP for transit servers (NNSP)).

Is port 433 TCP or UDP?

Port 433 is registered for TCP and UDP.

Is it safe to open port 433?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Answer only the networks that need the service (no open resolvers, relays or reflectors).

How do I check if port 433 is open?

On Linux run "ss -ltnup 'sport = :433'", on macOS "lsof -nP -i :433", and on Windows "netstat -ano | findstr :433" to see the local process. From another machine, "nmap -p 433 <host>" shows whether it is reachable.

05 Related
:53 DNS tcp · udp · moderate risk :135 MS RPC tcp · udp · high risk :123 NTP udp · tcp · moderate risk :67 DHCP server udp · tcp · moderate risk :111 rpcbind tcp · udp · high risk :9100 Raw printing (JetDirect) tcp · udp · high risk :631 IPP / CUPS tcp · udp · high risk :5353 mDNS udp · tcp · moderate risk

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