sysctcl -w net.ipv4.ip_forward=1

# grep net.ipv4.ip_forward /etc/sysctl.conf
net.ipv4.ip_forward=1

下記の状態で、eth1側から192.168.100.0/24からのアクセスだけIPマスカレードする。

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A POSTROUTING -o enp1s0 -j MASQUERADE

iptables -P FORWARD DROP
iptables -A FORWARD -i enp6s0 -s 192.168.100.0/24 -j ACCEPT 

下記で再起動後も反映するように設定しておく
03 Ubuntu iptables

net.ipv4.ip_forward設定までは同じ

# ufw enable
Command may disrupt existing ssh connections. Proceed with operation (y|n)? y
Firewall is active and enabled on system startup

現状の接続が切れるとSSHできなくなるので、すぐにSSHをEnableへ

# ufw allow ssh
Rule added
Rule added (v6)

# vi /etc/ufw/before.rules

# FORWARD
*filter
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i enp6s0 -s 192.168.100.0/24 -j ACCEPT
COMMIT

# NAT
*nat
-F
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o enp1s0 -j MASQUERADE
COMMIT

ufw reload

# ufw reload
Firewall reloaded

# ufw status 
Status: active

To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       XXX.XXX.XXX.XXX