Port 6699 BABEL-DTLS
Port 6699 is the IANA-assigned babel-dtls port for Babel Routing Protocol over DTLS (UDP).
What is port 6699 used for?
Port 6699 is assigned to babel-dtls and uses UDP. The IANA registry describes the service as “Babel Routing Protocol over DTLS”. 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 6699 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 8968].
Check and block port 6699
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 = :6699'Check on Windows
Get-NetUDPEndpoint -LocalPort 6699 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 6699/udpBlock with iptables
sudo iptables -A INPUT -p udp --dport 6699 -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 6699 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 6699 is usually used
Most traffic on port 6699 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 6699.
- Troubleshooting path selection, NAT or proxy rules during an outage.
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 6699.
sudo ss -ulpn 'sport = :6699'Common problems with port 6699
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 = :6699' or sudo lsof -i :6699. 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 6699 at a glance
| Port number | 6699 |
|---|---|
| Service name | babel-dtls |
| Transport protocol | UDP |
| Category | Networking and proxies |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | [IESG] |
| Reference | [RFC 8968] |
| Registry description | Babel Routing Protocol over DTLS |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 6699 used for?
Port 6699 (BABEL-DTLS) uses UDP. Babel Routing Protocol over DTLS.
How do I check if port 6699 is open?
On Linux run `sudo ss -ulpn 'sport = :6699'`. On Windows run `Get-NetUDPEndpoint -LocalPort 6699 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 6699; empty output usually means nothing is bound to it.
How do I block port 6699?
With UFW run `sudo ufw deny 6699/udp`. With iptables run `sudo iptables -A INPUT -p udp --dport 6699 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 6699 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