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Port 3551 (apcupsd)

IANA assigns port 3551 to apcupsd: Apcupsd Information Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
apcupsd TCP Apcupsd Information Port Riccardo Facchetti -
apcupsd UDP Apcupsd Information Port Riccardo Facchetti -

Registry dates for this port: registered 2002-07.

Check what is listening on port 3551

Linux: is anything listening on TCP 3551?
sudo ss -tlnp 'sport = :3551'
Linux: UDP 3551
sudo ss -ulnp 'sport = :3551'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3551
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3551 '
Windows (Command Prompt)
netstat -ano | findstr :3551
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3551 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3551
From Windows
Test-NetConnection HOST -Port 3551
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3551

Block port 3551 in the firewall

ufw
sudo ufw deny 3551/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3551 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3551 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3551' -Direction Inbound -Protocol TCP -LocalPort 3551 -Action Block
ufw
sudo ufw deny 3551/udp
iptables
sudo iptables -A INPUT -p udp --dport 3551 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3551 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3551' -Direction Inbound -Protocol UDP -LocalPort 3551 -Action Block

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3549tellumat-nms3550ssmpp3552taserver3553rbr-discovery

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