nixpulvis - "Borrowing" a Better WiFi Signal

"Borrowing" a Better WiFi Signal 911 words drafted on February 15, 2018.

Let me start by saying that “stealing” WiFi (or anything for that matter) is wrong, and bad. Please don’t do it. But if you happen to find yourself “borrowing” someone elses WiFi with permission, i.e. they gave you the password, then the next step is clearly improving the link quality.

Attenuate like Hell

I’m currently living in the mountains of Colorado, in a place that doesn’t have it’s own ISP hookup. One of our neighbors is nice enough to let us borrow thier WiFi though, so all is not lost. If I had to guess I’d say that I’m about 100ft from the access point (AP), and about 6 or so walls stand in the way. It’s not an ideal RF setup by any means, but it works.

The AP is broadcasting at 2412MHz (channel 1 of the 2.4GHz band), the bandwidth is 20MHz. TODO: Other link stats. TODO: wavemon screenshot.

However, it’s nowhere near good enough. Enter the world of fancy antennas. In the RF antenna world there exists a device named “Yagi”, after one of it’s creators. A Yagi is a directional antenna which operates with a single driven element, a reflecting element, and many directing elements. These are the same design as many household TV antennas from the old days.

I’m not going to get into the design of these things, because frankly that’s above my pay-grade at the moment, but there are a few key things to know about these antennas:

They are directional, TODO: angles?
The antenna should be arranged to match the polarization of the AP.
The frequency range (bandwidth) is very narrow, so there’s no way a 2.4GHz antenna will work on the 5GHz band.

So I bought one, and a USB WiFi adapter to go along with it. I chose the adapter to meet the following requirments:

802.11n, 802.11ac isn’t needed because we’re only dealing with the 2.4GHz band.
Drivers must be included with Linux by default.
External antenna connection (typically SMA).

I After ordering the wrong kind of SMA adapter I finnaly got the right one, and we’re in business. Simply moving from my laptop’s built-in WiFi to this new setup improved signal level by TODO, and link quality by TODO.

TODO: another wavemon screenshot.

It’s one thing to have access to a nice, fast network on a single computer, but I want my roomates to be able to benifit from this high quality link as well. Enter the NAT.

Building a NAT device isn’t too hard. I’ve got an extra Rasperry Pi (v1) lying around running [ArchARM], so we’ll work with that, but these techiques should work for any modern Linux machine.

TODO: more words, and links to GitHub sources.

Manually Configure

First let’s connect to the WiFi network, and make sure it’s working.

netctl start <profile>
…
ping 8.8.8.8

We need to tell the kernal to route packets accross network interfaces. For more information about kernal ip forwarding start with this Stack Overflow post.

sysctl net.ipv4.ip_forward=1

Once we’ve enabled packet forwarding, we need to setup the NAT between the wireless interface, and the Pi’s ethernet interface.

iptables -t nat -A POSTROUTING -o $from -j MASQUERADE
iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i $to -o $from -j ACCEPT

Finally, we enable DHCP to allow clients to negociate for an IP address on the new local network.

# /etc/dhcpd.conf
option domain-name-servers 8.8.8.8, 8.8.4.4;
option subnet-mask 255.255.255.0;
option routers 192.168.0.1;
subnet 192.168.0.0 netmask 255.255.255.0 {
  range 192.168.0.150 192.168.0.250;
}

systemctl start dhcpd4@$to.service

With that all said and done we should be able to connect a computer via ethernet and access the internet. Woot!

Systemd Unit

Ideally we should be able to simply power up the pi (or whatever machine) and have all the needed networking configuration happen automatically. This is the domain of systemd services and targets.

TODO: Actually figure this all out…

Setup our own Wireless Network

All that’s left is to connect the ethernet cable from the pi to a functional wireless router, and we’ve got our own WiFi network with good connectivity.

The observent reader will notice that we now have two NATs, the NAT between our WiFi borrower, and the NAT for our personal WiFi hotspot. It’s NATs all the way down!

Going Futher (literally…)

One thing that I’m now dying to test is how far a 2.4GHz signal can go, with modern, FCC complient hardware like I have. If I were to buy another one of these antennas and point them at each other, how many meters, or kilometers could it go.

Another factor in all this is the TX (sending) power. TODO: a bit of info on this.