Port 1723 PPTP · TCP

What runs on port 1723

PPTP control channel, with the tunnel itself in GRE (IP protocol 47). PPTP's MS-CHAPv2 authentication has been practically broken since 2012; the only reason to see it is a router or Windows server that was never migrated. Replace with WireGuard, OpenVPN or IKEv2.

Software that uses it: Windows RRAS (legacy) · pptpd · old routers

Should port 1723 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.

• If a VPN needs "GRE passthrough" on the firewall, it is PPTP, and it is time to replace it.

Check port 1723

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :1723'
sudo lsof -i :1723

# from another machine: is it reachable?
nc -zv host 1723
nmap -p 1723 host
curl -v telnet://host:1723      # 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 1723

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1723 proto tcp
sudo ufw deny 1723/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 1723 accept
nft add rule inet filter input tcp dport 1723 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 1723 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 1723 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1723 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 1723" with no source is just "open".

Related ports

• 500 IKE — IPsec key exchange
• 1194 OpenVPN
• 51820 WireGuard

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