Port / UDP
1813RADIUS accounting
RADIUS accounting records session start, stop, and usage data for network access.
If it is internet-facing: Port 1813 is routine where it is expected. Its risk lives in the service configuration behind it, not the port itself.
Exposure
Trusted network only
Transport
Encryption optional
Risk if public
Low
Exposure verdict
Port 1813 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Encryption is available but negotiated or configured rather than guaranteed. A client that does not insist on it will silently fall back to plaintext.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other directory & auth ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Accounting records identify users and their session metadata, so keep them on the management network.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 1813 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 1813 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 1813 -j DROPIf you do need it open to everyone
sudo ufw allow 1813/udpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -ulpn 'sport = :1813'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:1813macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 1813 <host>A version probe confirms whether RADIUS accounting is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 1813The quick yes/no when you only need to know whether a path exists through the firewall.