Port 13 daytime · TCP and UDP

What runs on port 13

Daytime returns the current date and time as a human-readable line. It predates NTP and has no accuracy guarantees; it is still enabled on some network gear and old Unix systems, and is used by scanners to fingerprint them.

Software that uses it: inetd/xinetd · Cisco IOS (service tcp-small-servers)

Should port 13 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.

• Use NTP (UDP 123) for time; disable the small servers on routers.

Check port 13

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :13'
ss -lunp 'sport = :13'
sudo lsof -i :13

# from another machine: is it reachable?
nc -zv host 13
nmap -p 13 host
nc -zuv host 13
sudo nmap -sU -p 13 host
curl -v telnet://host:13      # 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 13

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 13 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 13 proto udp
sudo ufw deny 13/tcp
sudo ufw deny 13/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 13 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 13 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 13 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 13 -j DROP
iptables -A INPUT -p udp --dport 13 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 13 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=13 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 13" with no source is just "open".

Related ports

• 37 time
• 123 NTP — Time synchronisation

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