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Port 523 (ibm-db2)

IANA assigns port 523 to ibm-db2: IBM-DB2.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
ibm-db2 TCP IBM-DB2 Juliana Hsu -
ibm-db2 UDP IBM-DB2 Juliana Hsu -

Check what is listening on port 523

Linux: is anything listening on TCP 523?
sudo ss -tlnp 'sport = :523'
Linux: UDP 523
sudo ss -ulnp 'sport = :523'
Linux / macOS: which process owns the port
sudo lsof -nP -i :523
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':523 '
Windows (Command Prompt)
netstat -ano | findstr :523
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 523 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 523
From Windows
Test-NetConnection HOST -Port 523
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 523

Block port 523 in the firewall

ufw
sudo ufw deny 523/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 523 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 523 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 523' -Direction Inbound -Protocol TCP -LocalPort 523 -Action Block
ufw
sudo ufw deny 523/udp
iptables
sudo iptables -A INPUT -p udp --dport 523 -j DROP
nftables
sudo nft add rule inet filter input udp dport 523 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 523' -Direction Inbound -Protocol UDP -LocalPort 523 -Action Block

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