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Port 17 (qotd)

IANA assigns port 17 to qotd: Quote of the Day.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
qotd TCP Quote of the Day Jon Postel -
qotd UDP Quote of the Day Jon Postel -

Check what is listening on port 17

Linux: is anything listening on TCP 17?
sudo ss -tlnp 'sport = :17'
Linux: UDP 17
sudo ss -ulnp 'sport = :17'
Linux / macOS: which process owns the port
sudo lsof -nP -i :17
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':17 '
Windows (Command Prompt)
netstat -ano | findstr :17
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 17 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 17
From Windows
Test-NetConnection HOST -Port 17
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 17

Block port 17 in the firewall

ufw
sudo ufw deny 17/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 17 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 17 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 17' -Direction Inbound -Protocol TCP -LocalPort 17 -Action Block
ufw
sudo ufw deny 17/udp
iptables
sudo iptables -A INPUT -p udp --dport 17 -j DROP
nftables
sudo nft add rule inet filter input udp dport 17 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 17' -Direction Inbound -Protocol UDP -LocalPort 17 -Action Block

Related ports

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