Port 4303 SRCP
Port 4303 is the IANA-assigned srcp port for Simple Railroad Command Protocol (TCP / UDP).
What is port 4303 used for?
Port 4303 is assigned to srcp and uses TCP / UDP. The IANA registry describes the service as “Simple Railroad Command Protocol”. 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 srcp connects to port 4303. 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 4303 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: [Matthias_Trute].
Check and block port 4303
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 = :4303'
sudo ss -ulpn 'sport = :4303'Check on Windows
Get-NetTCPConnection -LocalPort 4303 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 4303 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 4303/tcp
sudo ufw deny 4303/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 4303 -j DROP
sudo iptables -A INPUT -p udp --dport 4303 -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 4303 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 4303 is usually used
Most traffic on port 4303 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 4303Common problems with port 4303
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 4303. 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 = :4303' or sudo lsof -i :4303. 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 4303 at a glance
| Port number | 4303 |
|---|---|
| Service name | srcp |
| Transport protocol | TCP / UDP |
| Category | Application services |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | [Matthias_Trute] |
| Registry description | Simple Railroad Command Protocol |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 4303 used for?
Port 4303 (SRCP) uses TCP / UDP. Simple Railroad Command Protocol.
How do I check if port 4303 is open?
On Linux run `sudo ss -tlpn 'sport = :4303'` and `sudo ss -ulpn 'sport = :4303'`. On Windows run `Get-NetTCPConnection -LocalPort 4303 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 4303 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 4303; empty output usually means nothing is bound to it.
How do I block port 4303?
With UFW run `sudo ufw deny 4303/tcp` and `sudo ufw deny 4303/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 4303 -j DROP` and `sudo iptables -A INPUT -p udp --dport 4303 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 4303 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