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Port 261 (nsiiops)

IANA assigns port 261 to nsiiops: IIOP Name Service over TLS/SSL.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
nsiiops TCP IIOP Name Service over TLS/SSL Jeff Stewart -
nsiiops UDP IIOP Name Service over TLS/SSL Jeff Stewart -

Check what is listening on port 261

Linux: is anything listening on TCP 261?
sudo ss -tlnp 'sport = :261'
Linux: UDP 261
sudo ss -ulnp 'sport = :261'
Linux / macOS: which process owns the port
sudo lsof -nP -i :261
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':261 '
Windows (Command Prompt)
netstat -ano | findstr :261
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 261 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 261
From Windows
Test-NetConnection HOST -Port 261
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 261

Block port 261 in the firewall

ufw
sudo ufw deny 261/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 261 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 261 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 261' -Direction Inbound -Protocol TCP -LocalPort 261 -Action Block
ufw
sudo ufw deny 261/udp
iptables
sudo iptables -A INPUT -p udp --dport 261 -j DROP
nftables
sudo nft add rule inet filter input udp dport 261 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 261' -Direction Inbound -Protocol UDP -LocalPort 261 -Action Block

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