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---
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title: "RaspberryPi LTE-Failover Router With DNS Caching"
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date: 2021-10-06
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draft: false
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tags: ["RaspberryPi", "Arch", "linux", "router", "tethering", "android", "winblows"]
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summaryimage: PXL_20211006_142214161_672x504.png
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summaryimagew: 672
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summaryimageh: 504
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authors: ["trent"]
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post: 23
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---
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date: 2021-10-06
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## Introduction
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Apparently Windows has a problem resolving hosts when you tether from Mobile HotSpot.
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The solution is to build a DNS-Caching router that tethers off the smartphone. This takes
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advantage of Android's ability so transparently fail-over to LTE when residential
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internet service goes down.
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This solution also takes advantage of the RaspberryPi's incredibly low price,
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the fact that LineageOS will run on cheap old phones that are no longer supported by the mfgr,
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and the fact that GoogleFi will ship you a data-sim for free.
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For instance, I just bought a brand-new, open-box Pixel phone for $85, and presumable the
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MotoX4 can also be had for next to nothing.
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## Materials
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* [RaspberryPi](https://www.amazon.com/CanaKit-Raspberry-Starter-Premium-Black/dp/B07BCC8PK7){target="_blank"}
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running [Arch Linux Arm](https://archlinuxarm.org/platforms/armv8/broadcom/raspberry-pi-3){target="_blank"}
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* Old Android Phone Running [LineageOS](https://wiki.lineageos.org/devices/){target="_blank"}
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* [Free Data Sim Card From GoogleFi](https://support.google.com/fi/answer/6330195?hl=en#zippy=%2Corder-your-data-only-sim){target="_blank"}
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<figure>
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<img src=../../photos/PXL_20211006_142214161_672x504.png width="100%" />
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<figcaption>RaspberryPi Router tethered off MotoX4 (running LineageOS)</figcaption>
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</figure>
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## Setup
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Personally I would
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* [secure the ssh server](https://wiki.archlinux.org/title/OpenSSH#Force_public_key_authentication){target="_blank"}
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* [generate and configure the locale](https://wiki.archlinux.org/title/locale){target="_blank"}
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* [remove the default root password, and default user](https://wiki.archlinux.org/title/users_and_groups){target="_blank"}
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* write your preferred hostname in `/etc/hostname`
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* configure your preferred timezone:
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```shell
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ln -sf /usr/share/zoneinfo/<Zone>/<SubZone> /etc/localtime
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```
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Additionally, the router won't be accessible for administrative tasks when it
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is behind the _Android Tether_ ; for this I would use a
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[wireguard vpn](https://wiki.archlinux.org/title/WireGuard){target="_blank"}.
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## Configure The Router.
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The entire configuration of the router consists of two `systemd-networkd`
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interface definitions, as well as `/etc/resolv.conf`, and `/etc/dnsmasq.conf`.
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### resolvconf
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`systemd-resolved` is no use to us because it only listens on localhost.
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```shell
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# disable systemd-resolved
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systemctl stop systemd-resolved
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systemctl disable systemd-resolved
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unlink /etc/resolv.conf
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```
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After unlinking the symlinked version of `/etc/resolv.conf`,
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write your nameservers and options in a real `/etc/resolv.conf`.
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```cfg
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# the default timeout of 5 seconds is too slow
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options timeout:1
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# nameserver when connected to lan
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nameserver 192.168.1.1
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# nameserver when connected to mobile network
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nameserver 8.8.8.8
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```
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### Interface Definitions For `systemd-networkd`
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I believe the usb interfaces are numbered 1-4,
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so either be careful which one you use, or maybe a wildcard name
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will work, i.e. `Name=usb*`
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```cfg
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# uplink
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# /etc/systemd/network/usb0.network
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[Match]
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Name=usb0
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[Network]
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DHCP=yes
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DNSSEC=no
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IPForward=yes
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```
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```cfg
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# downlink, ethernet cable
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# /etc/systemd/network/eth0.network
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[Match]
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Name=eth0
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[Network]
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Address=10.12.34.1/24
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DHCPServer=yes
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IPForward=yes
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IPMasquerade=both
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```
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### Configuration For `dnsmasq`
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Install [dnsmasq](https://wiki.archlinux.org/title/dnsmasq){target="_blank"},
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and enable it `systemctl enable dnsmasq`.
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```cfg
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# /etc/dnsmasq.conf
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resolv-file=/etc/resolv.conf
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interface=eth0
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no-dhcp-interface=eth0
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```
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## Reboot
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Plug in the Android Phone, reboot the RaspberryPi, and when it comes back up
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toggle on the USB tether on the Android Phone.
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Plug ethernet cable into Windows Computer, open **CMD** prompt and type
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`ping google.com` to test connectivity and name resolution. Or on a
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Linux computer type `ping -c 3 google.com`.
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## Alternate DHCP Service
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You can use `dnsmasq` for DHCP Service instead of `systemd-networkd`.
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```cfg
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# downlink, ethernet cable
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# /etc/systemd/network/eth0.network
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[Match]
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Name=eth0
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[Network]
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Address=10.12.34.1/24
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# DHCPServer=yes
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IPForward=yes
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IPMasquerade=both
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```
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```cfg
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# /etc/dnsmasq.conf
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resolv-file=/etc/resolv.conf
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interface=eth0
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# no-dhcp-interface=eth0
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dhcp-range=10.12.34.50,10.12.34.150
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```
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## Reference For `systemd-networkd`
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* examples in `/usr/lib/systemd/network/`
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* [Man Page](https://www.freedesktop.org/software/systemd/man/systemd.network.html){target="_blank"}
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## Use With Multiple Computers
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Just add an
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[unmanaged switch](https://www.amazon.com/Ethernet-Splitter-Optimization-Unmanaged-TL-SG108/dp/B00A121WN6){target="_blank"}.
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## Wifi Instead of Ethernet
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Use downlink definition for `wlan0` instead of `eth0`,
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and install `hostapd`
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```cfg
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# /etc/hostapd/hostapd.conf
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interface=wlan0
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hw_mode=g
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channel=7
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wmm_enabled=0
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macaddr_acl=0
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auth_algs=1
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ignore_broadcast_ssid=0
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wpa=2
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wpa_key_mgmt=WPA-PSK
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wpa_pairwise=TKIP
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rsn_pairwise=CCMP
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ssid=NETWORK
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wpa_passphrase=PASSWORD
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```
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