Port 1812 RADIUS auth · UDP

What runs on port 1812

RADIUS authentication: the protocol behind WPA2/WPA3-Enterprise Wi-Fi (802.1X), VPN logins, switch port authentication and ISP subscriber authentication. Only the password is obfuscated with the shared secret; everything else is plaintext, so RADIUS traffic stays on the management network or inside RadSec (TLS, port 2083). Old servers used 1645.

Software that uses it: FreeRADIUS · Microsoft NPS · Cisco ISE · Aruba ClearPass · Okta/JumpCloud RADIUS

Should port 1812 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.

• radtest user pass server 0 secret is the quickest end-to-end check.
• A wrong shared secret shows up as "Received packet from X with invalid Message-Authenticator" on the server.

Check port 1812

# on the host: is anything listening, and which process?
ss -lunp 'sport = :1812'
sudo lsof -i :1812

# from another machine: is it reachable?
nc -zuv host 1812
sudo nmap -sU -p 1812 host

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 1812

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1812 proto udp
sudo ufw deny 1812/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 1812 accept
nft add rule inet filter input udp dport 1812 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 1812 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 1812 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1812 protocol=udp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 1812" with no source is just "open".

Related ports

• 1813 RADIUS accounting
• 49 TACACS+
• 2083 cPanel (TLS)

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