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Port 9 (discard)

IANA assigns port 9 to discard: Discard.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
discard TCP Discard Jon Postel -
discard UDP Discard Jon Postel -
discard SCTP Discard Randall Stewart RFC 9260
discard DCCP Discard Eddie Kohler RFC 4340

Registry dates for this port: last modified 2022-02-07.

Check what is listening on port 9

Linux: is anything listening on TCP 9?
sudo ss -tlnp 'sport = :9'
Linux: UDP 9
sudo ss -ulnp 'sport = :9'
Linux / macOS: which process owns the port
sudo lsof -nP -i :9
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':9 '
Windows (Command Prompt)
netstat -ano | findstr :9
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 9 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 9
From Windows
Test-NetConnection HOST -Port 9
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 9

Block port 9 in the firewall

ufw
sudo ufw deny 9/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 9 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 9 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 9' -Direction Inbound -Protocol TCP -LocalPort 9 -Action Block
ufw
sudo ufw deny 9/udp
iptables
sudo iptables -A INPUT -p udp --dport 9 -j DROP
nftables
sudo nft add rule inet filter input udp dport 9 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 9' -Direction Inbound -Protocol UDP -LocalPort 9 -Action Block

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