Port 6511 DCCP-UDP
Port 6511 is the IANA-assigned dccp-udp port for Datagram Congestion Control Protocol Encapsulation for NAT Traversal (UDP).
What is port 6511 used for?
Port 6511 is assigned to dccp-udp and uses UDP. The IANA registry describes the service as “Datagram Congestion Control Protocol Encapsulation for NAT Traversal”. It sits in the registered range, the block used by databases, application servers and developer tooling. Port conflicts here are common when several local services start at once.
Ports are the transport-layer address a service listens on. This entry has no TCP assignment: UDP is connectionless, so a client that gets no reply may be hitting either an idle service or a firewall drop. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 6511 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.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [IESG]. Reference: [RFC 6773].
Check and block port 6511
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 -ulpn 'sport = :6511'Check on Windows
Get-NetUDPEndpoint -LocalPort 6511 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 6511/udpBlock with iptables
sudo iptables -A INPUT -p udp --dport 6511 -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 6511 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 6511 is usually used
Most traffic on port 6511 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Application backends and supporting services that use a fixed registered number.
- Containers or local stacks that publish the port on the host.
- Firewall and exposure reviews before the service goes to production.
Confirm the UDP listener
This port is not registered for TCP, so an nc TCP probe cannot confirm it. Run this on the host that runs the service; empty output means nothing is bound to port 6511.
sudo ss -ulpn 'sport = :6511'Common problems with port 6511
No reply from the service
Because UDP has no handshake, a missing reply can mean the service is idle, the packet was dropped, or the client sent it to the wrong interface. Check the listener locally before blaming the network.
Port already in use
Find the process holding the port with sudo ss -ulpn 'sport = :6511' or sudo lsof -i :6511. 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 6511 at a glance
| Port number | 6511 |
|---|---|
| Service name | dccp-udp |
| Transport protocol | UDP |
| Category | Application services |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | [IESG] |
| Reference | [RFC 6773] |
| Registry description | Datagram Congestion Control Protocol Encapsulation for NAT Traversal |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 6511 used for?
Port 6511 (DCCP-UDP) uses UDP. Datagram Congestion Control Protocol Encapsulation for NAT Traversal.
How do I check if port 6511 is open?
On Linux run `sudo ss -ulpn 'sport = :6511'`. On Windows run `Get-NetUDPEndpoint -LocalPort 6511 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 6511; empty output usually means nothing is bound to it.
How do I block port 6511?
With UFW run `sudo ufw deny 6511/udp`. With iptables run `sudo iptables -A INPUT -p udp --dport 6511 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 6511 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.
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Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy