Port 3900 (udt-os, udt_os)

IANA assigns port 3900 to udt-os, udt_os: Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os"..

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference

udt-os TCP Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os". James Powell -

udt_os TCP Unidata UDT OS
This entry is an alias to "udt-os". This entry is now historic, not usable for use with many common service discovery mechanisms. James Powell -

udt-os UDP Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os". James Powell -

udt_os UDP Unidata UDT OS
This entry is an alias to "udt-os". This entry is now historic, not usable for use with many common service discovery mechanisms. James Powell -

Check what is listening on port 3900

Linux: is anything listening on TCP 3900?

sudo ss -tlnp 'sport = :3900'

Linux: UDP 3900

sudo ss -ulnp 'sport = :3900'

Linux / macOS: which process owns the port

sudo lsof -nP -i :3900

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':3900 '

Windows (Command Prompt)

netstat -ano | findstr :3900

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3900 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 3900

From Windows

Test-NetConnection HOST -Port 3900

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 3900

Block port 3900 in the firewall

ufw

sudo ufw deny 3900/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 3900 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 3900 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 3900' -Direction Inbound -Protocol TCP -LocalPort 3900 -Action Block

ufw

sudo ufw deny 3900/udp