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Port 1851 (ctcd)

IANA assigns port 1851 to ctcd.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
ctcd TCP ctcd John Ryan -
ctcd UDP ctcd John Ryan -

Check what is listening on port 1851

Linux: is anything listening on TCP 1851?
sudo ss -tlnp 'sport = :1851'
Linux: UDP 1851
sudo ss -ulnp 'sport = :1851'
Linux / macOS: which process owns the port
sudo lsof -nP -i :1851
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':1851 '
Windows (Command Prompt)
netstat -ano | findstr :1851
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 1851 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 1851
From Windows
Test-NetConnection HOST -Port 1851
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 1851

Block port 1851 in the firewall

ufw
sudo ufw deny 1851/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 1851 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 1851 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 1851' -Direction Inbound -Protocol TCP -LocalPort 1851 -Action Block
ufw
sudo ufw deny 1851/udp
iptables
sudo iptables -A INPUT -p udp --dport 1851 -j DROP
nftables
sudo nft add rule inet filter input udp dport 1851 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 1851' -Direction Inbound -Protocol UDP -LocalPort 1851 -Action Block

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All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.