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iptables

Reference manpage: man iptables

Deactivate firewall: Clear iptables rules and accept all traffic.

iptables-restore << EOF
> *raw
:PREROUTING ACCEPT [461477:823976242]
:OUTPUT ACCEPT [0:0]
COMMIT
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
COMMIT
*nat
:PREROUTING ACCEPT [104281:12249471]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [3109:273082]
:POSTROUTING ACCEPT [3080:270727]
COMMIT
EOF

Make router from linux machine

Assuming empty or rules allowing routed traffic to get trough iptables.
Allow IPv4 forwarding and masquerade transiting packets.

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING ! -d 127.0.0.1 -j MASQUERADE