Dual monitors using a laptop as a second display. This guide assumes that you are using a Debian based distro (X11 only). Both devices must be connected to the same network.

1.  Define an output to be a used as a dummy monitor

a.  The simple method: select an unused existing graphics port

# find an unused output
find -L /sys/class/drm -maxdepth 1 -type d -name 'card0-*' -exec sh -c 'if [ $(cat $0/status) = "disconnected" ]; then echo $0; fi' {} \;|cut -c22-|sort
#on my system DP-1 is available
OUTPUT=DP-1
DIR=/sys/kernel/debug/dri/0/$OUTPUT

b.  The other way: use the evdi kernel module to add an output
In case you can't use an existing graphics port

# typically the evdi module must be installed first
sudo apt install -y evdi-dkms

# load the module, requesting a dummy device
sudo modprobe evdi initial_device_count=1

# find the new output
DIR=$(sudo find /sys/kernel/debug/dri/1/ -maxdepth 1 -name 'DVI*'|head -1)

2.  Force activate the output

# override EDID (pretend to be a 1080P monitor with lots of resolution options)
EDID_BASE64=AP///////wAebdVZ+KEJAAQXAQNoNR546jMxpFdRoCYQUFSna4CzAIGAlQBxT6nAgQCBwJBAAjqAGHE4LUBYLEUACSUhAAAeAAAA/QA4Sx5TDwAKICAgICAgAAAA/AAyNEVOMzMKICAgICAgAAAA/wAzMDRORFBISksyODgKAJ8=
sudo bash -c "base64 -d <<<$EDID_BASE64 > $DIR/edid_override"

# force output on
sudo sh -c "echo on > $DIR/force"

# add an additional resolution to the dummy display (optional)
# check the xrandr output name, it may be different than the kernel name
xrandr|grep ' connected'
OUTPUT=DP-1
RES="1920 1200"
MODELINE=$(cvt $RES|awk '/Modeline/{print substr($0,10)}')
MODE=$(sed 's/^"\([^"]\+\).*/\1/' <<<$MODELINE)
sh -c "xrandr --newmode $MODELINE"
xrandr --addmode $OUTPUT $MODE

# Use the standard display configuration utility to configure the dummy display.
# Match the resolution of your device and position the display.

If the display fails to activate or 1920x1080 doesn't show up, then reboot and select a different output in step one.

3.  Serve the dummy monitor with FreeRDP

sudo apt install -y freerdp2-shadow-x11
freerdp-shadow-cli -auth -may-interact /monitors:1

4.  Connect FreeRDP client

sudo apt install -y freerdp2-x11
xfreerdp /v:YOUR_HOST_IP /cert-ignore /floatbar:sticky:off,show:always -gfx /rfx /f

Notes:
freerdp-shadow-cli is somewhat buggy in this setup, even the development version, but the settings here work well.
-may-interact is required for the cursor to show up on the client.  Alternately the local cursor can be used on each device.
Disabling GFX is required for reliable operation, but we still have rfx
GFX works in the development version of the client, but no cursor
aFreeRDP works on android, but no cursor
wfreerdp works on windows, but no cursor
Does not work reliably with other RDP clients
In my testing, VNC is not a suitable replacement for FreeRDP in this setup--it's too slow.
Thank you to all of the FreeRDP developers for this awesome software.

Discussion:
https://news.ycombinator.com/item?id=34010970

Zwap is a Linux kernel module that will attempt to compress memory pages and keep them in ram before swapping to disk.  This is helpful on low memory systems and will result in a noticeable performance improvement in most cases.  It is required that a swap device exists.  Here are some commands to enable it on Linux.  Tested on Debian and Ubuntu.

sudo perl -pi -e 's/^(GRUB_CMDLINE_LINUX_DEFAULT.*)"$/$1 zswap.enabled=1 zswap.compressor=lz4 zswap.zpool=z3fold zswap.max_pool_percent=25"/' /etc/default/grub
sudo update-grub
sudo echo -e 'lz4\nlz4_compress\nz3fold' >> /etc/initramfs-tools/modules
sudo update-initramfs -u
sudo reboot

This will update grub to instruct linux to load the required modules, set sensible parameters, and add the modules to initramfs so they are able to be loaded early in the boot process.

Explanation of parameters:

zswap.enabled=1 #enable zswap
zswap.compressor=lz4 #use lz4 compression, the best balance of speed and performance
zswap.zpool=z3fold #the z3fold allocator stores up to three compressed pages per physical page, allowing for a higher compression ratio
zswap.max_pool_percent=25 #max percent of ram that may be used for compressed pages.  Default is 20, I like to bump it up just a little.  Be careful not to go too high or there won't be any memory left for cache and it will hurt performance instead of helping it.

This is especially helpful on low end systems with <= 4GB RAM.  It also works well those t2.nano instances and other low memory VPS instances.

Here are some commands to check if it is working:

cat /sys/module/zswap/parameters/enabled
should return Y

examine all debug parameters for the module
sudo grep -R . /sys/kernel/debug/zswap/

view current compression ratio
sudo bash -c 'echo "scale=2; " $(</sys/kernel/debug/zswap/stored_pages) " * 4096 /" $(</sys/kernel/debug/zswap/pool_total_size) | bc'

Connect to XFINITY hotspots from Android and Linux without an app.

Android 11
EAP method: TTLS
Identity: your-username
Password: your-password
CA certificate: Use system certificates
Online certificate status: Don't validate
Domain: secure.aaa.wifi.comcast.com
Phase 2 authentication: PAP

The others require a certificate
Get the certificate from FireFox:
/usr/share/ca-certificates/mozilla/COMODO_RSA_Certification_Authority.crt
Or download it here

Android 10
EAP method: TTLS
Phase 2 authentication: PAP
Identity: your-username
Password: your-password
CA certificate: (install and use cert file)

Linux (network manager)
Security WPA & WPA2 Enterprise
Authentication: Tunneled TLS
CA certificate: (use file)
Inner authentication: PAP
username: your-username
password: your-password

Linux (wpa_supplicant)
network={
ssid="XFINITY"
scan_ssid=1
key_mgmt=WPA-EAP
eap=TTLS
phase2="auth=PAP"
ca_cert="/root/COMODO_RSA_Certification_Authority.crt"
identity="your-username"
password="your-password"
}