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Registered port (1024–10000) Medium risk

Port 4000 PHOENIX / DEV SERVER

Quick answer

Port 4000 is most commonly used by Phoenix / dev server. The IANA registry lists TCP / UDP for this number.

Check and block Is it safe? Troubleshooting FAQ
TransportTCP / UDP
RiskMedium
Typical useDevelopment servers

What is port 4000 used for?

Port 4000 is most commonly used by Phoenix / dev server. Phoenix and other development servers commonly listen on port 4000; the IANA registry assigns the number to terabase. 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 terabase connects to port 4000. 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 4000 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 Development servers ports

Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Thor_Olson].

Check and block port 4000

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 = :4000' sudo ss -ulpn 'sport = :4000'

Check on Windows

Get-NetTCPConnection -LocalPort 4000 -State Listen -ErrorAction SilentlyContinue Get-NetUDPEndpoint -LocalPort 4000 -ErrorAction SilentlyContinue

Block with UFW

sudo ufw deny 4000/tcp sudo ufw deny 4000/udp

Block with iptables

sudo iptables -A INPUT -p tcp --dport 4000 -j DROP sudo iptables -A INPUT -p udp --dport 4000 -j DROP

These 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 4000 safe to expose?

Medium

Development servers usually run with debug features and no hardening. Keep them on localhost and never expose them to the internet. 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 4000 is usually used

Most traffic on port 4000 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.

Demo

Fetch the dev-server homepage

Phoenix and similar development servers usually serve on localhost; ask for the homepage first.

curl -s http://127.0.0.1:4000/ | head

Common problems with port 4000

Connection refused or timed out

Refused usually means the host is reachable but nothing is listening on port 4000. 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 = :4000' or sudo lsof -i :4000. 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 4000 at a glance

Port number4000
IANA service nameterabase
Commonly used byPhoenix / dev server
Transport protocolTCP / UDP
CategoryDevelopment servers
RangeRegistered port (1024–10000)
Risk levelMedium
Registered assignee[Thor_Olson]
Registry descriptionTerabase
SourceIANA Service Name and Transport Protocol Port Number Registry

Frequently asked questions

What is port 4000 used for?

Port 4000 (PHOENIX / DEV SERVER). The IANA registry lists TCP / UDP for this number. Phoenix and other development servers commonly listen on port 4000; the IANA registry assigns the number to terabase.

How do I check if port 4000 is open?

On Linux run `sudo ss -tlpn 'sport = :4000'` and `sudo ss -ulpn 'sport = :4000'`. On Windows run `Get-NetTCPConnection -LocalPort 4000 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 4000 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 4000; empty output usually means nothing is bound to it.

How do I block port 4000?

With UFW run `sudo ufw deny 4000/tcp` and `sudo ufw deny 4000/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 4000 -j DROP` and `sudo iptables -A INPUT -p udp --dport 4000 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.

Is port 4000 safe to expose?

Development servers usually run with debug features and no hardening. Keep them on localhost and never expose them to the internet. 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.

See all Development servers ports

Primary data: IANA Service Name and Transport Protocol Port Number Registry.

Last reviewed 2026-10-30 · Methodology · About · Privacy