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Port 648 (rrp)

IANA assigns port 648 to rrp: Registry Registrar Protocol (RRP).

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
rrp TCP Registry Registrar Protocol (RRP) Scott Hollenbeck -
rrp UDP Registry Registrar Protocol (RRP) Scott Hollenbeck -

Check what is listening on port 648

Linux: is anything listening on TCP 648?
sudo ss -tlnp 'sport = :648'
Linux: UDP 648
sudo ss -ulnp 'sport = :648'
Linux / macOS: which process owns the port
sudo lsof -nP -i :648
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':648 '
Windows (Command Prompt)
netstat -ano | findstr :648
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 648 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 648
From Windows
Test-NetConnection HOST -Port 648
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 648

Block port 648 in the firewall

ufw
sudo ufw deny 648/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 648 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 648 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 648' -Direction Inbound -Protocol TCP -LocalPort 648 -Action Block
ufw
sudo ufw deny 648/udp
iptables
sudo iptables -A INPUT -p udp --dport 648 -j DROP
nftables
sudo nft add rule inet filter input udp dport 648 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 648' -Direction Inbound -Protocol UDP -LocalPort 648 -Action Block

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