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Port 42509 (candrp)

IANA assigns port 42509 to candrp: CA discovery response.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
candrp TCP CA discovery response Jon Press -
candrp UDP CA discovery response Jon Press -

Registry dates for this port: registered 2005-08.

Check what is listening on port 42509

Linux: is anything listening on TCP 42509?
sudo ss -tlnp 'sport = :42509'
Linux: UDP 42509
sudo ss -ulnp 'sport = :42509'
Linux / macOS: which process owns the port
sudo lsof -nP -i :42509
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':42509 '
Windows (Command Prompt)
netstat -ano | findstr :42509
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 42509 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 42509
From Windows
Test-NetConnection HOST -Port 42509
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 42509

Block port 42509 in the firewall

ufw
sudo ufw deny 42509/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 42509 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 42509 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 42509' -Direction Inbound -Protocol TCP -LocalPort 42509 -Action Block
ufw
sudo ufw deny 42509/udp
iptables
sudo iptables -A INPUT -p udp --dport 42509 -j DROP
nftables
sudo nft add rule inet filter input udp dport 42509 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 42509' -Direction Inbound -Protocol UDP -LocalPort 42509 -Action Block

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