Port 5911 ATS-ACARS
Port 5911 has a different IANA assignment for each transport protocol: TCP ats-acars; UDP ats-acars; SCTP cpdlc.
What is port 5911 used for?
Port 5911 carries a different IANA service on each transport protocol: TCP ats-acars; UDP ats-acars; SCTP cpdlc. 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. The same port is also registered under cpdlc.
Each transport reaches a different service on this number, so match the protocol to the service you are diagnosing. A TCP check says nothing about a UDP listener on the same port, and the reverse is also true. Both transports still bind to the same number, which is why two processes can collide even when they use different protocols.
Use the commands below to confirm whether port 5911 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: [Eivan_Cerasi].
Check and block port 5911
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 = :5911'
sudo ss -ulpn 'sport = :5911'
sudo ss -Slpn 'sport = :5911'Check on Windows
Get-NetTCPConnection -LocalPort 5911 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 5911 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 5911/tcp
sudo ufw deny 5911/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 5911 -j DROP
sudo iptables -A INPUT -p udp --dport 5911 -j DROP
sudo iptables -A INPUT -p sctp --dport 5911 -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. UFW and the built-in Windows cmdlets cover TCP and UDP only; this port also lists SCTP, so use nftables, iptables or the vendor's diagnostic tool for those protocols. SCTP and DCCP handling also depends on kernel support, including their diagnostic modules, so a missing result may mean the module is not loaded rather than that nothing is listening.
Is port 5911 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 5911 is usually used
Most traffic on port 5911 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.
Probe the port
Confirm something is listening before you debug the client. This checks a TCP handshake on the port.
nc -vz 127.0.0.1 5911Common problems with port 5911
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 5911. 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 = :5911' or sudo lsof -i :5911. 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 5911 at a glance
| Port number | 5911 |
|---|---|
| Service name | ats-acars |
| Aliases | cpdlc |
| Transport protocol | TCP / UDP / SCTP |
| Assignments by protocol |
|
| Category | Application services |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | [Eivan_Cerasi] |
| Registry description | Air Traffic Services applications using ACARS |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 5911 used for?
Port 5911 (ATS-ACARS). The IANA registry assigns a different service on each transport protocol: TCP ats-acars; UDP ats-acars; SCTP cpdlc.
How do I check if port 5911 is open?
On Linux run `sudo ss -tlpn 'sport = :5911'` and `sudo ss -ulpn 'sport = :5911'` and `sudo ss -Slpn 'sport = :5911'`. On Windows run `Get-NetTCPConnection -LocalPort 5911 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 5911 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 5911; empty output usually means nothing is bound to it. UFW and the built-in Windows cmdlets cover TCP and UDP only; SCTP needs nftables, iptables or the vendor's diagnostic tool.
How do I block port 5911?
With UFW run `sudo ufw deny 5911/tcp` and `sudo ufw deny 5911/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 5911 -j DROP` and `sudo iptables -A INPUT -p udp --dport 5911 -j DROP` and `sudo iptables -A INPUT -p sctp --dport 5911 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot. UFW covers TCP and UDP only; use iptables or nftables for SCTP.
Is port 5911 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
- Port 5910 · ats-atnAir Traffic Services applications using ATN
- Port 5912 · ais-metAeronautical Information Service/Meteorological applications using ACARS
- Port 5909 · agmaAir-ground media advisory
- Port 5913 · aoc-acarsAirline operational communications applications using ACARS
- Port 5908 · ipsmaIPS Management Application
- Port 5914 · ipsdtlsSecurity for Internet Protocol Suite
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy