Port 4990 BUSYCAL
Port 4990 is the IANA-assigned busycal port for BusySync Calendar Synch. Protocol (TCP / UDP).
What is port 4990 used for?
Port 4990 is assigned to busycal and uses TCP / UDP. The IANA registry describes the service as “BusySync Calendar Synch. Protocol”. It sits in the registered range, the block used by databases, application servers and developer tooling. Port conflicts here are common when several local services start at once.
Ports are the transport-layer address a service listens on. A client that wants to reach busycal connects to port 4990. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 4990 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [David_Riggle].
Check and block port 4990
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :4990'
sudo ss -ulpn 'sport = :4990'Check on Windows
Get-NetTCPConnection -LocalPort 4990 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 4990 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 4990/tcp
sudo ufw deny 4990/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 4990 -j DROP
sudo iptables -A INPUT -p udp --dport 4990 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 4990 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 4990 is usually used
Most traffic on port 4990 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Application backends and supporting services that use a fixed registered number.
- Containers or local stacks that publish the port on the host.
- Firewall and exposure reviews before the service goes to production.
Probe the port
Confirm something is listening before you debug the client. This checks a TCP handshake on the port.
nc -vz 127.0.0.1 4990Common problems with port 4990
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 4990. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :4990' or sudo lsof -i :4990. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 4990 at a glance
| Port number | 4990 |
|---|---|
| Service name | busycal |
| Transport protocol | TCP / UDP |
| Category | Application services |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | [David_Riggle] |
| Registry description | BusySync Calendar Synch. Protocol |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 4990 used for?
Port 4990 (BUSYCAL) uses TCP / UDP. BusySync Calendar Synch. Protocol.
How do I check if port 4990 is open?
On Linux run `sudo ss -tlpn 'sport = :4990'` and `sudo ss -ulpn 'sport = :4990'`. On Windows run `Get-NetTCPConnection -LocalPort 4990 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 4990 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 4990; empty output usually means nothing is bound to it.
How do I block port 4990?
With UFW run `sudo ufw deny 4990/tcp` and `sudo ufw deny 4990/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 4990 -j DROP` and `sudo iptables -A INPUT -p udp --dport 4990 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 4990 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
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Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy