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NAT+
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tropf
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quick introduction into forwarding your connection from one interface to another
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.DA
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This guide explains how you can share your internet connection with other devices using NAT and setup a DHCP server so connected devices are automatically configured.
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.NH 1
Preface
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Goal
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Afterwards you can connect to the device via e.g. ethernet and you automatically get an IP address via DHCP and all traffic will be tunneled and forwarded through the device.
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.NH 2
Limitations
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This guide will not cover:
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connecting to an existing network
.CW "wpa_supplicant" ")" "(see: "
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creating a wifi hotspot
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Conventions & Preconditions
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The following names will be used throughout the guide and can be changed according to your setup.
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nospaces center tab(|);
lB lB
fC l.
Name        | Meaning
wlan0       | connected to an internet uplink
eth0        | device that the connection will be forwarded to
10.0.0.1    | IP address of \fCeth0
10.0.0.0/24 | subnet of \fCeth0\fR
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Indexing
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This section contains alternative titles and topics of this article that can be used to build a search index.
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search tags:
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raspberry pi raspberrypi dnsmasq dhcp static
share sharing bridge bridging reverse router
nat wifi wlan
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alternative titles:
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how to set up a router
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how to turn wifi into ethernet
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sharing a network connection
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Setting a Static IP Address
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.CW eth0
needs a static IP Address, because there is no DHCP server to get an address from.
(If there is, skip this step.)
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Setting the Actual Address
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Create a file named
.CW /etc/network/interfaces.d/eth0_static
with this content:
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auto eth0
allow-hotplug eth0
iface eth0 inet static
        address 10.0.0.1
        netmask 255.255.255.0
        network 10.0.0.0
        broadcast 10.0.0.255
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Preventing DHCP From Interfering
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By default the DHCP client will overwrite these settings.
Tell them to not care about
.CW eth0 .
Insert this line into
.CW /etc/dhcpcd.conf :
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.CB
denyinterfaces eth0
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DHCP Server
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There is no DHCP server behind
.CW eth0 .
We have to create one.
(If there is, skip this step.)

Install
.CW dnsmasq .
Create a file named
.CW /etc/dnsmasq.d/dhcp_server.conf
with this content:
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.CB
interface=eth0              # use interface eth0
listen-address=10.0.0.1     # explicitly specify the address to listen on
bind-interfaces             # Bind to the interface to make sure we aren't sending things elsewhere
server=46.182.19.48         # the DNS server to be used. this one is run by digitalcourage e.v.; feel free to change it
domain-needed               # Don't forward short names
bogus-priv                  # Never forward addresses in the non-routed address spaces.

# Assign IP addresses between 10.0.0.50 and 10.0.0.150 with a 12 hour lease time
dhcp-range=10.0.0.50,10.0.0.150,12

dhcp-option=3,10.0.0.1      # gateway which is connected to the internet -- this computer
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Forwarding
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All incoming traffic on
.CW eth0
should be forwarded and NATed to the uplink
.CW wlan0 .
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Enable Forwarding
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Forwarding has to be enabled in the kernel.

Uncomment or insert this line into
.CW /etc/sysctl.conf :
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net.ipv4.ip_forward=1
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This will only be loaded on boot.
Reload the file now using:
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sysctl -p
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Setup iptables
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Execute the following commands:
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sudo iptables -t filter -A FORWARD -i eth0 -j ACCEPT
sudo iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
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.B "These rules are not persistent by default" ","
so install
.CW iptables-persistent
and then execute:
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iptables-save > /etc/iptables/rules.v4
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Troubleshooting
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Things i do when things go wrong.
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reboot
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.CW "sudo systemctl restart networking"
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check config files
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.CW "ip a"
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rewrite config files, srsly
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.CW "ip r"
(especially check the default route)
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.CW "apropos [problem]" ","
.CW "man [program]"
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get upset
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.CW dmesg
(look for
.CW "link is not ready"
without
.CW "link becomes ready"
afterwards)
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.CW "sudo ss -tulpn"
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