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Port 20999 (athand-mmp)

IANA assigns port 20999 to athand-mmp: At Hand MMP.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
athand-mmp TCP At Hand MMP Stepan Riha -
athand-mmp UDP AT Hand MMP Stepan Riha -

Check what is listening on port 20999

Linux: is anything listening on TCP 20999?
sudo ss -tlnp 'sport = :20999'
Linux: UDP 20999
sudo ss -ulnp 'sport = :20999'
Linux / macOS: which process owns the port
sudo lsof -nP -i :20999
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':20999 '
Windows (Command Prompt)
netstat -ano | findstr :20999
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 20999 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 20999
From Windows
Test-NetConnection HOST -Port 20999
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 20999

Block port 20999 in the firewall

ufw
sudo ufw deny 20999/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 20999 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 20999 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 20999' -Direction Inbound -Protocol TCP -LocalPort 20999 -Action Block
ufw
sudo ufw deny 20999/udp
iptables
sudo iptables -A INPUT -p udp --dport 20999 -j DROP
nftables
sudo nft add rule inet filter input udp dport 20999 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 20999' -Direction Inbound -Protocol UDP -LocalPort 20999 -Action Block

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