Port 1433 MS-SQL-S
Port 1433 is the IANA-assigned ms-sql-s port for Microsoft-SQL-Server (TCP / UDP).
What is port 1433 used for?
Port 1433 is assigned to ms-sql-s and uses TCP / UDP. The IANA registry describes the service as “Microsoft-SQL-Server”. 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 ms-sql-s connects to port 1433. 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 1433 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.
Check and block port 1433
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 = :1433'
sudo ss -ulpn 'sport = :1433'Check on Windows
Get-NetTCPConnection -LocalPort 1433 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 1433 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 1433/tcp
sudo ufw deny 1433/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 1433 -j DROP
sudo iptables -A INPUT -p udp --dport 1433 -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 1433 safe to expose?
SQL Server should be reachable only from application hosts. Restrict it with firewall rules and enforce strong database authentication. 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 1433 is usually used
Most traffic on port 1433 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Application servers opening pooled connections to the database on port 1433.
- Migrations, backups and reporting jobs run from a trusted bastion host.
- Local Docker or test environments that publish the database on the host.
Open a SQL Server session
Ask for the server version. Use a least-privilege login rather than a production administrator account.
sqlcmd -S 127.0.0.1,1433 -Q "SELECT @@VERSION"Common problems with port 1433
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 1433. 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 = :1433' or sudo lsof -i :1433. 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 1433 at a glance
| Port number | 1433 |
|---|---|
| Service name | ms-sql-s |
| Transport protocol | TCP / UDP |
| Category | Databases |
| Range | Registered port (1024–10000) |
| Risk level | Medium |
| Registered assignee | Not listed |
| Registry description | Microsoft-SQL-Server |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 1433 used for?
Port 1433 (MS-SQL-S) uses TCP / UDP. Microsoft-SQL-Server.
How do I check if port 1433 is open?
On Linux run `sudo ss -tlpn 'sport = :1433'` and `sudo ss -ulpn 'sport = :1433'`. On Windows run `Get-NetTCPConnection -LocalPort 1433 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 1433 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 1433; empty output usually means nothing is bound to it.
How do I block port 1433?
With UFW run `sudo ufw deny 1433/tcp` and `sudo ufw deny 1433/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 1433 -j DROP` and `sudo iptables -A INPUT -p udp --dport 1433 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 1433 safe to expose?
SQL Server should be reachable only from application hosts. Restrict it with firewall rules and enforce strong database authentication. 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.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy