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Port 3681 (bts-x73)

IANA assigns port 3681 to bts-x73: BTS X73 Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
bts-x73 TCP BTS X73 Port Todd Cooper -
bts-x73 UDP BTS X73 Port Todd Cooper -

Registry dates for this port: registered 2003-01.

Check what is listening on port 3681

Linux: is anything listening on TCP 3681?
sudo ss -tlnp 'sport = :3681'
Linux: UDP 3681
sudo ss -ulnp 'sport = :3681'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3681
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3681 '
Windows (Command Prompt)
netstat -ano | findstr :3681
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3681 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3681
From Windows
Test-NetConnection HOST -Port 3681
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3681

Block port 3681 in the firewall

ufw
sudo ufw deny 3681/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3681 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3681 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3681' -Direction Inbound -Protocol TCP -LocalPort 3681 -Action Block
ufw
sudo ufw deny 3681/udp
iptables
sudo iptables -A INPUT -p udp --dport 3681 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3681 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3681' -Direction Inbound -Protocol UDP -LocalPort 3681 -Action Block

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