Add easy binding protocol

[svpcom]

Optional easy binding protocol proposal

Prereqs:

• Add special profiles like drone_bind and gs_bind with only IP tunnel enabled
• Use some predefined channel for bind (for example 165)
• Use some predefined link_domain for binding
• Use some predefined keypair (will be valid for binding only)

Bind protocol:

1. Switch drone and gs to bind mode by some event (key press, mavlink command, etc). This is out of scope and defined by vendor.
2. Stop all standard wfb-ng profiles
3. Start drone_bind profile on drone and gs_bind on gs
4. Start bind protocol server on drone and bind protocol client on gs
5. GS will initiate binding process and send config files (as base64 encoded tar.gz archive) via IP tunnel using TCP socket
6. Drone will receive it, check for correctness and apply changes. Then return binding status to GS
7. After binding finished then both drone and gs will switch to normal state

Implementation notes:

• Use special IP-addresses for drone and gs tunnel endpoints during binding. This will avoid security risks due to using default keys
• Always stop default wfb-ng profiles to disallow using bind mode for normal operation
• Use simple text protocol for communication via TCP socket.
• No executable scripts will be transferred from gs to drone due to security reasons
• Drone must check file checksums before any actions

Protocol description

• Each command or response are always single line of text
• Command has a format: command_name \t [arg1 [arg2] ... ] where \t is tabulation
• Response has a format: status \t [optional message]. Predefined statuses OK and ERR

Commands:

• VERSION Should return drone version
• BIND message Where message is tar.gz archive base64 encoded

Bind archive contents:

• checksum.txt Contains sha1 checksum of all files inside of archive (use sha1sum command to generate)
• wifibroadcast.cfg (optional). WFG-ng config file for drone.
• bind.yaml (optional). Simple machine readable config. Will include basic link parameters.
• drone.key (optional). Encryption key.

Bind archive must not be empty.

bind.yaml description

It should include:

• link_domain (string)
• link_id (int - hash from link_domain)
• channel number

Also it may include any additional data needed by vendor

Reference implementation

Drone bind protocol server: socat TCP4-LISTEN:5555,bind=10.5.99.2,reuseaddr,crlf EXEC:/usr/bin/bind-drone.sh
GS bind protocol client: socat TCP4:10.5.99.2:5555,crlf,retry=10,interval=1 EXEC:/usr/bin/bind-gs.sh

bind-drone.sh:

#!/bin/bash
set -e
IFS=$'\t'

show_version()
{
    echo $'OK\twfb-ng_25.1.25.81795'
}

report_err()
{
    echo $'ERR\tInternal error'
}

do_bind()
{
    set -e
    trap report_err ERR

    tmpdir=$(mktemp -d)
    echo "$1" | base64 -d | tar xz -C $tmpdir

    cd $tmpdir

    if ! [ -f checksum.txt ] || ! sha1sum --quiet --status --strict -c checksum.txt
    then
        echo $'ERR\tChecksum failed'
        exit 0
    fi

    for i in wifibroadcast.cfg drone.key bind.yaml
    do
        if [ -f $i ]
        then
            cp $i /etc/
        fi
    done

    rm -r $tmpdir
    echo "OK"
}

while read cmd arg
do
    case $cmd in
        "VERSION")
            show_version
            ;;
        "BIND")
            do_bind "$arg"
            ;;
    esac
done

bind-gs.sh

#!/bin/bash
set -e

IFS=$'\t'

echo "VERSION"
read status version

if [ "$status" != "OK" ]
then
    echo "Unable to fetch version" 1>&2
    exit 1
fi

echo "Drone version: $version" 1>&2

tmpdir=$(mktemp -d)

for i in /etc/wifibroadcast.cfg /etc/drone.key bind.yaml
do
    if [ -f $i ]
    then
        cp $i ${tmpdir}/
        (cd $tmpdir && sha1sum $(basename $i) >> checksum.txt)
    fi
done

echo $'BIND\t'"$(tar czS -C $tmpdir . | base64 -w0)"

rm -r $tmpdir

read status msg

if [ "$status" != "OK" ]
then
    echo "Bind failed: $msg" 1>&2
    exit 1
fi

echo "Bind succeeded" 1>&2

[svpcom]

Reference implementation: https://github.com/svpcom/wfb-ng/tree/auto-provisioning
After installing drivers and wfb-ng deb packet you just need to run:

1. sudo init_drone.sh on the drone
2. sudo init_gs.sh on the gs

It will create (if doesn't exists) on the GS:

• /etc/wifibroadcast.cfg
• /etc/bind.key
• /etc/gs.key
• /etc/drone.key

and /etc/bind.key on the drone

After this it will transfer /etc/wifibroadcast.cfg and /etc/drone.key to the drone and restart wifibroadcast services both on drone and GS.

[snokvist]

Hello! Consider the scenario of a many to many (drones/groundstations) scenario which is normal in openipc-fpv. A user may have aviateur (windows), android, radxa, custom x86 synced to many drones. There is a need to be able to manage binds on any one of the groundstations without breaking the others and having to rely on keypair file syncing. For example local keypair generation using a passphrase like ELRS.

[svpcom]

Merged into master branch