Port 1813 RADIUS accounting · UDP

What runs on port 1813

RADIUS accounting: session start/stop/interim records (who connected, for how long, how many bytes). Used for billing and for logging in NAC and Wi-Fi deployments. Same security properties as 1812; old servers used 1646.

Software that uses it: FreeRADIUS · Microsoft NPS · Cisco ISE

Should port 1813 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.

• Accounting-Interim-Interval on the NAS controls how often you get updates; too short floods the server.

Check port 1813

# on the host: is anything listening, and which process?
ss -lunp 'sport = :1813'
sudo lsof -i :1813

# from another machine: is it reachable?
nc -zuv host 1813
sudo nmap -sU -p 1813 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 1813

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1813 proto udp
sudo ufw deny 1813/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 1813 accept
nft add rule inet filter input udp dport 1813 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 1813 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 1813 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1813 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 1813" with no source is just "open".

Related ports

• 1812 RADIUS auth
• 49 TACACS+

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