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Port 43 (nicname)

IANA assigns port 43 to nicname: Who Is.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
nicname TCP Who Is - -
nicname UDP Who Is - -

Check what is listening on port 43

Linux: is anything listening on TCP 43?
sudo ss -tlnp 'sport = :43'
Linux: UDP 43
sudo ss -ulnp 'sport = :43'
Linux / macOS: which process owns the port
sudo lsof -nP -i :43
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':43 '
Windows (Command Prompt)
netstat -ano | findstr :43
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 43 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 43
From Windows
Test-NetConnection HOST -Port 43
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 43

Block port 43 in the firewall

ufw
sudo ufw deny 43/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 43 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 43 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 43' -Direction Inbound -Protocol TCP -LocalPort 43 -Action Block
ufw
sudo ufw deny 43/udp
iptables
sudo iptables -A INPUT -p udp --dport 43 -j DROP
nftables
sudo nft add rule inet filter input udp dport 43 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 43' -Direction Inbound -Protocol UDP -LocalPort 43 -Action Block

Related ports

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