Port 4321 ASTRO
Port 4321 is most commonly used by Astro. The IANA registry lists TCP / UDP for this number.
What is port 4321 used for?
Port 4321 is most commonly used by Astro. Astro's development server commonly listens on port 4321; the IANA registry assigns the number to rwhois. 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 rwhois connects to port 4321. 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 4321 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: [Mark_Kosters]. Reference: [RFC 2167].
Check and block port 4321
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 = :4321'
sudo ss -ulpn 'sport = :4321'Check on Windows
Get-NetTCPConnection -LocalPort 4321 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 4321 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 4321/tcp
sudo ufw deny 4321/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 4321 -j DROP
sudo iptables -A INPUT -p udp --dport 4321 -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 4321 safe to expose?
Astro's dev server is intended for local development and may expose debugging features. Keep it on localhost. 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 4321 is usually used
Most traffic on port 4321 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Starting a framework development server and opening the localhost URL it prints.
- Testing a frontend against a local API, proxy or mock service.
- Hot-reload sessions that should stay bound to 127.0.0.1 instead of a public interface.
Start the Astro dev server
Astro serves its development site on 4321 by default.
npm run dev -- --port 4321Common problems with port 4321
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 4321. 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 = :4321' or sudo lsof -i :4321. 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 4321 at a glance
| Port number | 4321 |
|---|---|
| IANA service name | rwhois |
| Commonly used by | Astro |
| Transport protocol | TCP / UDP |
| Category | Development servers |
| Range | Registered port (1024–10000) |
| Risk level | Medium |
| Registered assignee | [Mark_Kosters] |
| Reference | [RFC 2167] |
| Registry description | Remote Who Is |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 4321 used for?
Port 4321 (ASTRO). The IANA registry lists TCP / UDP for this number. Astro's development server commonly listens on port 4321; the IANA registry assigns the number to rwhois.
How do I check if port 4321 is open?
On Linux run `sudo ss -tlpn 'sport = :4321'` and `sudo ss -ulpn 'sport = :4321'`. On Windows run `Get-NetTCPConnection -LocalPort 4321 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 4321 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 4321; empty output usually means nothing is bound to it.
How do I block port 4321?
With UFW run `sudo ufw deny 4321/tcp` and `sudo ufw deny 4321/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 4321 -j DROP` and `sudo iptables -A INPUT -p udp --dport 4321 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 4321 safe to expose?
Astro's dev server is intended for local development and may expose debugging features. Keep it on localhost. 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