Port 37 time · TCP and UDP

What runs on port 37

The time protocol (RFC 868) returns the time as a 32-bit count of seconds since 1900. It was replaced by NTP decades ago but is still implemented in rdate and some embedded devices.

Software that uses it: rdate · inetd/xinetd

Should port 37 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 or chrony; the 32-bit counter in this protocol wraps in 2036.

Check port 37

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :37'
ss -lunp 'sport = :37'
sudo lsof -i :37

# from another machine: is it reachable?
nc -zv host 37
nmap -p 37 host
nc -zuv host 37
sudo nmap -sU -p 37 host
curl -v telnet://host:37      # 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 37

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 37 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 37 proto udp
sudo ufw deny 37/tcp
sudo ufw deny 37/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 37 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 37 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 37 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 37 -j DROP
iptables -A INPUT -p udp --dport 37 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 37 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=37 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 37" with no source is just "open".

Related ports

• 13 daytime
• 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