HTTPfy

Port 4321 (rwhois)

IANA assigns port 4321 to rwhois: Remote Who Is.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
rwhois TCP Remote Who Is Mark Kosters RFC 2167
rwhois UDP Remote Who Is Mark Kosters RFC 2167

Check what is listening on port 4321

Linux: is anything listening on TCP 4321?
sudo ss -tlnp 'sport = :4321'
Linux: UDP 4321
sudo ss -ulnp 'sport = :4321'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4321
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':4321 '
Windows (Command Prompt)
netstat -ano | findstr :4321
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 4321 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 4321
From Windows
Test-NetConnection HOST -Port 4321
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4321

Block port 4321 in the firewall

ufw
sudo ufw deny 4321/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 4321 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 4321 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 4321' -Direction Inbound -Protocol TCP -LocalPort 4321 -Action Block
ufw
sudo ufw deny 4321/udp
iptables
sudo iptables -A INPUT -p udp --dport 4321 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4321 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4321' -Direction Inbound -Protocol UDP -LocalPort 4321 -Action Block

Related ports

4319skytale4320fdt-rcatp4322trim-event4323trim-ice

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