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Port 3491 (swr-port)

IANA assigns port 3491 to swr-port: SWR Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
swr-port TCP SWR Port Ian Manning -
swr-port UDP SWR Port Ian Manning -

Registry dates for this port: registered 2002-05.

Check what is listening on port 3491

Linux: is anything listening on TCP 3491?
sudo ss -tlnp 'sport = :3491'
Linux: UDP 3491
sudo ss -ulnp 'sport = :3491'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3491
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3491 '
Windows (Command Prompt)
netstat -ano | findstr :3491
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3491 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3491
From Windows
Test-NetConnection HOST -Port 3491
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3491

Block port 3491 in the firewall

ufw
sudo ufw deny 3491/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3491 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3491 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3491' -Direction Inbound -Protocol TCP -LocalPort 3491 -Action Block
ufw
sudo ufw deny 3491/udp
iptables
sudo iptables -A INPUT -p udp --dport 3491 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3491 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3491' -Direction Inbound -Protocol UDP -LocalPort 3491 -Action Block

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