Port 647 DHCP-FAILOVER
Port 647 is the IANA-assigned dhcp-failover port for DHCP Failover (TCP / UDP).
What is port 647 used for?
Port 647 is assigned to dhcp-failover and uses TCP / UDP. The IANA registry describes the service as “DHCP Failover”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach dhcp-failover connects to port 647. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 647 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Browse all Networking and proxies ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [IESG]. Reference: [RFC 8156].
Check and block port 647
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :647'
sudo ss -ulpn 'sport = :647'Check on Windows
Get-NetTCPConnection -LocalPort 647 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 647 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 647/tcp
sudo ufw deny 647/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 647 -j DROP
sudo iptables -A INPUT -p udp --dport 647 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 647 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 647 is usually used
Most traffic on port 647 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Routers, switches and proxies exchanging routing or session information.
- Clients sending traffic through a forward or reverse proxy on port 647.
- Troubleshooting path selection, NAT or proxy rules during an outage.
Probe the network service
Check reachability from a host on the same path before you inspect device configuration.
nc -vz 127.0.0.1 647Common problems with port 647
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 647. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :647' or sudo lsof -i :647. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 647 at a glance
| Port number | 647 |
|---|---|
| Service name | dhcp-failover |
| Transport protocol | TCP / UDP |
| Category | Networking and proxies |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | [IESG] |
| Reference | [RFC 8156] |
| Registry description | DHCP Failover |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 647 used for?
Port 647 (DHCP-FAILOVER) uses TCP / UDP. DHCP Failover.
How do I check if port 647 is open?
On Linux run `sudo ss -tlpn 'sport = :647'` and `sudo ss -ulpn 'sport = :647'`. On Windows run `Get-NetTCPConnection -LocalPort 647 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 647 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 647; empty output usually means nothing is bound to it.
How do I block port 647?
With UFW run `sudo ufw deny 647/tcp` and `sudo ufw deny 647/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 647 -j DROP` and `sudo iptables -A INPUT -p udp --dport 647 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 647 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy