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Port 688 (realm-rusd)

IANA assigns port 688 to realm-rusd: ApplianceWare managment protocol.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
realm-rusd TCP ApplianceWare managment protocol Stacy Kenworthy -
realm-rusd UDP ApplianceWare managment protocol Stacy Kenworthy -

Check what is listening on port 688

Linux: is anything listening on TCP 688?
sudo ss -tlnp 'sport = :688'
Linux: UDP 688
sudo ss -ulnp 'sport = :688'
Linux / macOS: which process owns the port
sudo lsof -nP -i :688
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':688 '
Windows (Command Prompt)
netstat -ano | findstr :688
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 688 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 688
From Windows
Test-NetConnection HOST -Port 688
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 688

Block port 688 in the firewall

ufw
sudo ufw deny 688/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 688 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 688 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 688' -Direction Inbound -Protocol TCP -LocalPort 688 -Action Block
ufw
sudo ufw deny 688/udp
iptables
sudo iptables -A INPUT -p udp --dport 688 -j DROP
nftables
sudo nft add rule inet filter input udp dport 688 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 688' -Direction Inbound -Protocol UDP -LocalPort 688 -Action Block

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