Port 3128 NDL-AAS
Port 3128 is the IANA-assigned ndl-aas port for Active API Server Port (TCP / UDP).
What is port 3128 used for?
Port 3128 is assigned to ndl-aas and uses TCP / UDP. The IANA registry describes the service as “Active API Server Port”. 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. A client that wants to reach ndl-aas connects to port 3128. 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 3128 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: [Martin_Norman].
Check and block port 3128
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 = :3128'
sudo ss -ulpn 'sport = :3128'Check on Windows
Get-NetTCPConnection -LocalPort 3128 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 3128 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 3128/tcp
sudo ufw deny 3128/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 3128 -j DROP
sudo iptables -A INPUT -p udp --dport 3128 -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 3128 safe to expose?
An unauthenticated forward proxy can be used as an open relay for abuse. Require authentication and restrict client networks. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 3128 is usually used
Most traffic on port 3128 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 3128.
- 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 3128Common problems with port 3128
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 3128. 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 = :3128' or sudo lsof -i :3128. 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 3128 at a glance
| Port number | 3128 |
|---|---|
| Service name | ndl-aas |
| Transport protocol | TCP / UDP |
| Category | Networking and proxies |
| Range | Registered port (1024–10000) |
| Risk level | Medium |
| Registered assignee | [Martin_Norman] |
| Registry description | Active API Server Port |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 3128 used for?
Port 3128 (NDL-AAS) uses TCP / UDP. Active API Server Port.
How do I check if port 3128 is open?
On Linux run `sudo ss -tlpn 'sport = :3128'` and `sudo ss -ulpn 'sport = :3128'`. On Windows run `Get-NetTCPConnection -LocalPort 3128 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 3128 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 3128; empty output usually means nothing is bound to it.
How do I block port 3128?
With UFW run `sudo ufw deny 3128/tcp` and `sudo ufw deny 3128/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 3128 -j DROP` and `sudo iptables -A INPUT -p udp --dport 3128 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 3128 safe to expose?
An unauthenticated forward proxy can be used as an open relay for abuse. Require authentication and restrict client networks. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
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Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy