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Port 12121 (nupaper-ss)

IANA assigns port 12121 to nupaper-ss: NuPaper Session Service.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
nupaper-ss TCP NuPaper Session Service David Warden 2 -
nupaper-ss UDP NuPaper Session Service David Warden 2 -

Registry dates for this port: registered 2005-11.

Check what is listening on port 12121

Linux: is anything listening on TCP 12121?
sudo ss -tlnp 'sport = :12121'
Linux: UDP 12121
sudo ss -ulnp 'sport = :12121'
Linux / macOS: which process owns the port
sudo lsof -nP -i :12121
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':12121 '
Windows (Command Prompt)
netstat -ano | findstr :12121
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 12121 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 12121
From Windows
Test-NetConnection HOST -Port 12121
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 12121

Block port 12121 in the firewall

ufw
sudo ufw deny 12121/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 12121 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 12121 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 12121' -Direction Inbound -Protocol TCP -LocalPort 12121 -Action Block
ufw
sudo ufw deny 12121/udp
iptables
sudo iptables -A INPUT -p udp --dport 12121 -j DROP
nftables
sudo nft add rule inet filter input udp dport 12121 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 12121' -Direction Inbound -Protocol UDP -LocalPort 12121 -Action Block

Related ports

12013vipera-ssl12109rets-ssl12168cawas12172hivep

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