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Port 550 (new-rwho)

IANA assigns port 550 to new-rwho: new-who.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
new-rwho TCP new-who - n/a
new-rwho UDP new-who - n/a

Check what is listening on port 550

Linux: is anything listening on TCP 550?
sudo ss -tlnp 'sport = :550'
Linux: UDP 550
sudo ss -ulnp 'sport = :550'
Linux / macOS: which process owns the port
sudo lsof -nP -i :550
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':550 '
Windows (Command Prompt)
netstat -ano | findstr :550
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 550 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 550
From Windows
Test-NetConnection HOST -Port 550
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 550

Block port 550 in the firewall

ufw
sudo ufw deny 550/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 550 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 550 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 550' -Direction Inbound -Protocol TCP -LocalPort 550 -Action Block
ufw
sudo ufw deny 550/udp
iptables
sudo iptables -A INPUT -p udp --dport 550 -j DROP
nftables
sudo nft add rule inet filter input udp dport 550 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 550' -Direction Inbound -Protocol UDP -LocalPort 550 -Action Block

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