Port 1433 MS SQL Server · TCP

What runs on port 1433

Microsoft SQL Server's default TCP port. The SQL Browser service on UDP 1434 tells clients which port a named instance actually uses. Exposed to the internet it is brute-forced for the sa account within hours; it belongs behind a firewall, a VPN or, in the cloud, a private endpoint. Encryption is optional unless "Force Encryption" is on.

Software that uses it: Microsoft SQL Server · Azure SQL · sqlcmd, SSMS, Azure Data Studio (clients)

Should port 1433 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• Named instances use dynamic ports unless you fix one in Configuration Manager; the firewall rule needs that port, not 1433.
• sqlcmd -S host,1433 -U user tests connectivity; note the comma.

Check port 1433

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :1433'
sudo lsof -i :1433

# from another machine: is it reachable?
nc -zv host 1433
nmap -p 1433 host
curl -v telnet://host:1433      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 1433

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1433 proto tcp
sudo ufw deny 1433/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 1433 accept
nft add rule inet filter input tcp dport 1433 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 1433 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 1433 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1433 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 1433" with no source is just "open".

Related ports

• 1434 SQL Server Browser — Instance → port lookup; SQL Slammer
• 3306 MySQL / MariaDB
• 5432 PostgreSQL
• 1521 Oracle DB

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions