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Port 33 (dsp)

IANA assigns port 33 to dsp: Display Support Protocol.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
dsp TCP Display Support Protocol Ed Cain -
dsp UDP Display Support Protocol Ed Cain -

Check what is listening on port 33

Linux: is anything listening on TCP 33?
sudo ss -tlnp 'sport = :33'
Linux: UDP 33
sudo ss -ulnp 'sport = :33'
Linux / macOS: which process owns the port
sudo lsof -nP -i :33
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':33 '
Windows (Command Prompt)
netstat -ano | findstr :33
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 33 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 33
From Windows
Test-NetConnection HOST -Port 33
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 33

Block port 33 in the firewall

ufw
sudo ufw deny 33/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 33 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 33 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 33' -Direction Inbound -Protocol TCP -LocalPort 33 -Action Block
ufw
sudo ufw deny 33/udp
iptables
sudo iptables -A INPUT -p udp --dport 33 -j DROP
nftables
sudo nft add rule inet filter input udp dport 33 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 33' -Direction Inbound -Protocol UDP -LocalPort 33 -Action Block

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