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Port 4990 (busycal)

IANA assigns port 4990 to busycal: BusySync Calendar Synch. Protocol.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
busycal TCP BusySync Calendar Synch. Protocol
Defined TXT keys: Proprietary David Riggle -
busycal UDP BusySync Calendar Synch. Protocol
Defined TXT keys: Proprietary David Riggle -

Registry dates for this port: registered 2008-01-07.

Check what is listening on port 4990

Linux: is anything listening on TCP 4990?
sudo ss -tlnp 'sport = :4990'
Linux: UDP 4990
sudo ss -ulnp 'sport = :4990'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4990
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':4990 '
Windows (Command Prompt)
netstat -ano | findstr :4990
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 4990 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 4990
From Windows
Test-NetConnection HOST -Port 4990
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4990

Block port 4990 in the firewall

ufw
sudo ufw deny 4990/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 4990 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 4990 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 4990' -Direction Inbound -Protocol TCP -LocalPort 4990 -Action Block
ufw
sudo ufw deny 4990/udp
iptables
sudo iptables -A INPUT -p udp --dport 4990 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4990 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4990' -Direction Inbound -Protocol UDP -LocalPort 4990 -Action Block

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