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Port 6123 (backup-express)

IANA assigns port 6123 to backup-express: Backup Express.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
backup-express TCP Backup Express Catalogic Software Inc -
backup-express UDP Backup Express Catalogic Software Inc -

Registry dates for this port: registered 2008-03-19, last modified 2015-02-04.

Check what is listening on port 6123

Linux: is anything listening on TCP 6123?
sudo ss -tlnp 'sport = :6123'
Linux: UDP 6123
sudo ss -ulnp 'sport = :6123'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6123
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':6123 '
Windows (Command Prompt)
netstat -ano | findstr :6123
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 6123 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 6123
From Windows
Test-NetConnection HOST -Port 6123
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6123

Block port 6123 in the firewall

ufw
sudo ufw deny 6123/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 6123 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 6123 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 6123' -Direction Inbound -Protocol TCP -LocalPort 6123 -Action Block
ufw
sudo ufw deny 6123/udp
iptables
sudo iptables -A INPUT -p udp --dport 6123 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6123 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6123' -Direction Inbound -Protocol UDP -LocalPort 6123 -Action Block

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