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Port 647 (dhcp-failover)

IANA assigns port 647 to dhcp-failover: DHCP Failover.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
dhcp-failover TCP DHCP Failover IESG RFC 8156
dhcp-failover UDP DHCP Failover Bernard Volz -

Registry dates for this port: last modified 2018-05-21.

Check what is listening on port 647

Linux: is anything listening on TCP 647?
sudo ss -tlnp 'sport = :647'
Linux: UDP 647
sudo ss -ulnp 'sport = :647'
Linux / macOS: which process owns the port
sudo lsof -nP -i :647
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':647 '
Windows (Command Prompt)
netstat -ano | findstr :647
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 647 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 647
From Windows
Test-NetConnection HOST -Port 647
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 647

Block port 647 in the firewall

ufw
sudo ufw deny 647/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 647 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 647 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 647' -Direction Inbound -Protocol TCP -LocalPort 647 -Action Block
ufw
sudo ufw deny 647/udp
iptables
sudo iptables -A INPUT -p udp --dport 647 -j DROP
nftables
sudo nft add rule inet filter input udp dport 647 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 647' -Direction Inbound -Protocol UDP -LocalPort 647 -Action Block

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