Port 513 rlogin / who · TCP and UDP

What runs on port 513

rlogin (TCP) and who (UDP): the BSD "r" commands, which trusted the client's claimed username and host. Replaced by SSH in the 1990s; appears today only on very old Unix systems and in nmap output.

Software that uses it: rlogind (historical)

Should port 513 be open?

Disable it. This is a legacy or insecure protocol. The recommendation is not to firewall it but to turn the service off and use the modern replacement.

• Anything still using the r-commands should be migrated to SSH.

Check port 513

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :513'
ss -lunp 'sport = :513'
sudo lsof -i :513

# from another machine: is it reachable?
nc -zv host 513
nmap -p 513 host
nc -zuv host 513
sudo nmap -sU -p 513 host
curl -v telnet://host:513      # 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 513

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 513 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 513 proto udp
sudo ufw deny 513/tcp
sudo ufw deny 513/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 513 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 513 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 513 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 513 -j DROP
iptables -A INPUT -p udp --dport 513 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 513 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=513 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 513" with no source is just "open".

Related ports

• 514 syslog — Remote logging (tcp 514 is rsh/rlogin historically)
• 22 SSH — SSH, SCP, SFTP, git over ssh
• 23 Telnet — Plaintext remote shell; should be off

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