Port 27999 TW-AUTH-KEY
Port 27999 is the IANA-assigned tw-auth-key port for TW Authentication/Key Distribution and (TCP / UDP).
What is port 27999 used for?
Port 27999 is assigned to tw-auth-key and uses TCP / UDP. The IANA registry describes the service as “TW Authentication/Key Distribution and”. It sits above the registered range, where most numbers are transient client ports. A fixed high port is worth documenting precisely because tooling often assumes the default.
Ports are the transport-layer address a service listens on. A client that wants to reach tw-auth-key connects to port 27999. 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 27999 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 Authentication and access ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Alex_Duncan_2].
Check and block port 27999
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 = :27999'
sudo ss -ulpn 'sport = :27999'Check on Windows
Get-NetTCPConnection -LocalPort 27999 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 27999 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 27999/tcp
sudo ufw deny 27999/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 27999 -j DROP
sudo iptables -A INPUT -p udp --dport 27999 -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 27999 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 27999 is usually used
Most traffic on port 27999 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Network devices authenticating users through a central service.
- Wi-Fi, VPN and remote-access gateways enforcing access policy.
- Accounting and session logs used for audits.
Probe the auth service
Authentication protocols are sensitive to shared secrets and source addresses, so test from an authorised client.
nc -vz 127.0.0.1 27999Common problems with port 27999
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 27999. 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 = :27999' or sudo lsof -i :27999. 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 27999 at a glance
| Port number | 27999 |
|---|---|
| Service name | tw-auth-key |
| Transport protocol | TCP / UDP |
| Category | Authentication and access |
| Range | High port (10001–65535) |
| Risk level | Standard/Low |
| Registered assignee | [Alex_Duncan_2] |
| Registry description | TW Authentication/Key Distribution and |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 27999 used for?
Port 27999 (TW-AUTH-KEY) uses TCP / UDP. TW Authentication/Key Distribution and.
How do I check if port 27999 is open?
On Linux run `sudo ss -tlpn 'sport = :27999'` and `sudo ss -ulpn 'sport = :27999'`. On Windows run `Get-NetTCPConnection -LocalPort 27999 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 27999 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 27999; empty output usually means nothing is bound to it.
How do I block port 27999?
With UFW run `sudo ufw deny 27999/tcp` and `sudo ufw deny 27999/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 27999 -j DROP` and `sudo iptables -A INPUT -p udp --dport 27999 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 27999 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