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Port 42 (name, nameserver)

IANA assigns port 42 to name, nameserver: Host Name Server.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
name TCP Host Name Server - -
nameserver TCP Host Name Server - -
name UDP Host Name Server - -
nameserver UDP Host Name Server - -

Check what is listening on port 42

Linux: is anything listening on TCP 42?
sudo ss -tlnp 'sport = :42'
Linux: UDP 42
sudo ss -ulnp 'sport = :42'
Linux / macOS: which process owns the port
sudo lsof -nP -i :42
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':42 '
Windows (Command Prompt)
netstat -ano | findstr :42
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 42 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 42
From Windows
Test-NetConnection HOST -Port 42
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 42

Block port 42 in the firewall

ufw
sudo ufw deny 42/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 42 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 42 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 42' -Direction Inbound -Protocol TCP -LocalPort 42 -Action Block
ufw
sudo ufw deny 42/udp
iptables
sudo iptables -A INPUT -p udp --dport 42 -j DROP
nftables
sudo nft add rule inet filter input udp dport 42 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 42' -Direction Inbound -Protocol UDP -LocalPort 42 -Action Block

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