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Port 4135 (cl-db-attach)

IANA assigns port 4135 to cl-db-attach: Classic Line Database Server Attach.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
cl-db-attach TCP Classic Line Database Server Attach - -
cl-db-attach UDP Classic Line Database Server Attach - -

Check what is listening on port 4135

Linux: is anything listening on TCP 4135?
sudo ss -tlnp 'sport = :4135'
Linux: UDP 4135
sudo ss -ulnp 'sport = :4135'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4135
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':4135 '
Windows (Command Prompt)
netstat -ano | findstr :4135
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 4135 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 4135
From Windows
Test-NetConnection HOST -Port 4135
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4135

Block port 4135 in the firewall

ufw
sudo ufw deny 4135/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 4135 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 4135 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 4135' -Direction Inbound -Protocol TCP -LocalPort 4135 -Action Block
ufw
sudo ufw deny 4135/udp
iptables
sudo iptables -A INPUT -p udp --dport 4135 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4135 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4135' -Direction Inbound -Protocol UDP -LocalPort 4135 -Action Block

Related ports

4133nuts-bootp, nuts_bootp4134nifty-hmi4136cl-db-request4137cl-db-remote

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