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Port 3127 (ctx-bridge)

IANA assigns port 3127 to ctx-bridge: CTX Bridge Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
ctx-bridge TCP CTX Bridge Port Alexander Dubrovsky -
ctx-bridge UDP CTX Bridge Port Alexander Dubrovsky -

Check what is listening on port 3127

Linux: is anything listening on TCP 3127?
sudo ss -tlnp 'sport = :3127'
Linux: UDP 3127
sudo ss -ulnp 'sport = :3127'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3127
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3127 '
Windows (Command Prompt)
netstat -ano | findstr :3127
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3127 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3127
From Windows
Test-NetConnection HOST -Port 3127
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3127

Block port 3127 in the firewall

ufw
sudo ufw deny 3127/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3127 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3127 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3127' -Direction Inbound -Protocol TCP -LocalPort 3127 -Action Block
ufw
sudo ufw deny 3127/udp
iptables
sudo iptables -A INPUT -p udp --dport 3127 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3127 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3127' -Direction Inbound -Protocol UDP -LocalPort 3127 -Action Block

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