Debian | PPTP server

Install the PPTP server
sudo apt install pptpd

Open pptpd-options, find and comment out "require-mppe-128"
sudo vi /etc/ppp/pptpd-options

...
# Encryption
# (There have been multiple versions of PPP with encryption support,
# choose with of the following sections you will use.)


# BSD licensed ppp-2.4.2 upstream with MPPE only, kernel module ppp_mppe.o
# {{{
...
# require-mppe-128
# }}}
...

Open chap-secrets and create a client secret
sudo vi /etc/ppp/chap-secrets

# Secrets for authentication using CHAP
# client        server  secret                  IP addresses
client1         pptpd   supersecure             *

Open pptpd.conf and define a local and remote IP
sudo vi /etc/pptpd.conf

...
localip 172.16.10.1
# Supports one remote client, if you want more, you can define a range e.g. remoteip 172.16.10.2-4
remoteip 172.16.10.2

Open sysctl.conf and uncomment net.ipv4.ip_forward=1 to enable IP routing.
sudo vi /etc/sysctl.conf

...
# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1

Activate the change using the following command
sudo sysctl -p

Enable masquerading on eth0
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE 

Install iptables-persistent and select yes on saving current rules
sudo apt install iptables-persistent

Enable and start the PPTPD server
sudo systemctl enable --now

You should now be able to connect to the PPTP server using the username and password you created in the chap-secrets file.