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The task

Let's suppose the following:

your server's ip address is 1.2.3.4,
and you wish to route for the subnet 192.168.1.0/24
Your primary ethernet adapter is enp2s0
and the secondary is enp7s0
this document does not contain info on setting up DHCP for the subnet. (e.g. use fix IP addresses)

Add a new netplan file

 network:
     version: 2
     renderer: networkd
     ethernets:
        enp7s0:
           dhcp4: no
           addresses: [192.168.1.1/24]
           gateway4: 1.2.3.4
           nameservers:
              addresses: [8.8.8.8]
              routes:
                 - to: 192.168.1.0/24
                   via: 192.168.1.1
                   metric: 100

then run:

netplan generate
netplan apply

Enable packet forwarding

/etc/sysctl.conf

Uncomment the next line to enable packet forwarding for IPv4

net.ipv4.ip_forward=1

then run:

sysctl -p

Set up a NAT rule in ufw

add the following in /etc/ufw/before.rules BEFORE the *filter rules

# NAT table rules
 *nat
 :POSTROUTING ACCEPT [0:0]
 -A POSTROUTING -s 192.168.34.0/24 -o enp2s0 -j MASQUERADE
 COMMIT

Note, that each section must be between:

*tablename
...
COMMIT

Firewall checks

Make sure your firewall allows the traffic needed.

For example ufw:

~$ sudo ufw status verbose

Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing), deny (routed)
New profiles: skip
...