Cos'è iptables?

iptables permette di configurare un parafuoco e NAT.

Per esempio, nel terminal Debian/Linux, questo permette di usare l'internet di un interfaccia wifi verso un interfaccia a cavio :

sudo vim /etc/sysctl.conf
i
net.ipv4.ip_forward=1
:wq
sudo sysctl -p
sudo vim /etc/network/interfaces
:set paste
i
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source /etc/network/interfaces.d/*
auto enp0s25
iface enp0s25 inet static
    address 192.168.91.1
    netmask 255.255.255.0
    network 192.168.91.0
    broadcast 192.168.91.255
:wq
sudo apt install iptables
sudo apt install iptables-persistent
sudo iptables -t nat -A POSTROUTING -o wlp3s0 -j MASQUERADE
sudo iptables -A FORWARD -i enp0s25 -o wlp3s0 -j ACCEPT
sudo iptables -A FORWARD -i wlp3s0 -o enp0s25 -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo netfilter-persistent save
sudo systemctl restart networking
sudo apt install isc-dhcp-server
sudo vim /etc/dhcp/dhcpd.conf
i
subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.91.100 192.168.91.200;
    option routers 192.168.91.1;
    option domain-name-servers 8.8.8.8, 8.8.4.4;
}
:wq
sudo vim /etc/default/isc-dhcp-server
sudo systemctl restart isc-dhcp-server
i
# Defaults for isc-dhcp-server (sourced by /etc/init.d/isc-dhcp-server)

# Path to dhcpd's config file (default: /etc/dhcp/dhcpd.conf).
#DHCPDv4_CONF=/etc/dhcp/dhcpd.conf
#DHCPDv6_CONF=/etc/dhcp/dhcpd6.conf

# Path to dhcpd's PID file (default: /var/run/dhcpd.pid).
#DHCPDv4_PID=/var/run/dhcpd.pid
#DHCPDv6_PID=/var/run/dhcpd6.pid

# Additional options to start dhcpd with.
#       Don't use options -cf or -pf here; use DHCPD_CONF/ DHCPD_PID instead
#OPTIONS=""

# On what interfaces should the DHCP server (dhcpd) serve DHCP requests?
#       Separate multiple interfaces with spaces, e.g. "eth0 eth1".
INTERFACESv4="enp0s25"
INTERFACESv6=""
:wq
systemctl status isc-dhcp-server.service
sudo systemctl enable isc-dhcp-server
sudo systemctl restart isc-dhcp-server
less /var/log/journal/7b1aa93bc247436a93808d8f80b9e6e5/
sudo journalctl
sudo dmesg

Per rimuovere la configurazione precedente:

sudo systemctl disable isc-dhcp-server
sudo apt purge isc-dhcp-server
sudo iptables -D FORWARD 1
sudo iptables -D FORWARD 1
sudo iptables -L -v
sudo iptables -t nat -D POSTROUTING 1
sudo iptables -t nat -L
sudo netfilter-persistent save
sudo vim /etc/network/interfaces
d99d
:set paste
i
# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source /etc/network/interfaces.d/*
auto enp0s25
#iface enp0s25 inet static
#    address 192.168.91.1
#    netmask 255.255.255.0
#    network 192.168.91.0
#    broadcast 192.168.91.255
:wq
sudo vim /etc/sysctl.conf
/net.ipv4.ip_forward
i
#
:wq
sudo sysctl -p
sudo systemctl restart networking
sudo reboot

Nota 1:

isc-dhcp-server.service is not a native service, redirecting to systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install disable isc-dhcp-server

Nota 2:

sudo systemctl restart networking
Job for networking.service failed because the control process exited with error code.
See "systemctl status networking.service" and "journalctl -xeu networking.service" for details.