Port 42509 CANDRP
Port 42509 is the IANA-assigned candrp port for CA discovery response (TCP / UDP).
What is port 42509 used for?
Port 42509 is assigned to candrp and uses TCP / UDP. The IANA registry describes the service as “CA discovery response”. It sits above the registered range, where most numbers are transient client ports. A fixed high port is worth documenting precisely because tooling often assumes the default.
Ports are the transport-layer address a service listens on. A client that wants to reach candrp connects to port 42509. 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 42509 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: [Jon_Press].
Check and block port 42509
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 = :42509'
sudo ss -ulpn 'sport = :42509'Check on Windows
Get-NetTCPConnection -LocalPort 42509 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 42509 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 42509/tcp
sudo ufw deny 42509/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 42509 -j DROP
sudo iptables -A INPUT -p udp --dport 42509 -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 42509 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 42509 is usually used
Most traffic on port 42509 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Custom or vendor-specific services configured to use a fixed high number.
- Short-lived container or test environments that publish the port on the host.
- Firewall planning, since most other high ports are transient client ports.
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 42509Common problems with port 42509
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 42509. 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 = :42509' or sudo lsof -i :42509. 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 42509 at a glance
| Port number | 42509 |
|---|---|
| Service name | candrp |
| Transport protocol | TCP / UDP |
| Category | Custom high ports |
| Range | High port (10001–65535) |
| Risk level | Standard/Low |
| Registered assignee | [Jon_Press] |
| Registry description | CA discovery response |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 42509 used for?
Port 42509 (CANDRP) uses TCP / UDP. CA discovery response.
How do I check if port 42509 is open?
On Linux run `sudo ss -tlpn 'sport = :42509'` and `sudo ss -ulpn 'sport = :42509'`. On Windows run `Get-NetTCPConnection -LocalPort 42509 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 42509 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 42509; empty output usually means nothing is bound to it.
How do I block port 42509?
With UFW run `sudo ufw deny 42509/tcp` and `sudo ufw deny 42509/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 42509 -j DROP` and `sudo iptables -A INPUT -p udp --dport 42509 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 42509 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.
Related ports
- Port 42508 · candpComputer Associates network discovery protocol
- Port 42999 · curiosityAPI endpoint for search application
- Port 41797 · crestron-ctpsCrestron Secure Terminal Port
- Port 43000 · recvr-rc-discReceiver Remote Control Discovery
- Port 41796 · crestron-cipsCrestron Secure Control Port
- Port 43191 · noit-transportReconnoiter Agent Data Transport
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy