reverseVNC.sh

#!/bin/bash

# Reverse VNC tunnel setup
# Run as root on the remote host, after reverseSSH.sh has completed.
# Establishes a persistent reverse tunnel for VNC (port 5900) back to the Target,
# using the second port allocated by findopenport.sh.
#
# Usage: reverseVNC.sh [--systemd] <user> <target-host>
#
#   --systemd   Install a systemd unit instead of a cron @reboot entry.
#               Creates /etc/autossh-vnc.conf and autossh-vnc.service.
#
# Requires:
#   - A VNC server running on this host (e.g. tigervnc, x11vnc) on port 5900
#   - /home/<user>/install/openportlist.txt written by reverseSSH.sh
#     (second line is used as the VNC tunnel port)
#   - SSH key exchange already completed by reverseSSH.sh

USE_SYSTEMD=false
if [[ "$1" == "--systemd" ]]; then
    USE_SYSTEMD=true
    shift
fi

if [[ $# -lt 2 ]]; then
    echo "Usage: $0 [--systemd] <user> <target-host>"
    exit 1
fi

USER="$1"
TARGET="$2"
PORT_FILE="/home/$USER/install/openportlist.txt"

if [[ ! -f "$PORT_FILE" ]]; then
    echo "Port list not found at $PORT_FILE. Run reverseSSH.sh first."
    exit 1
fi

readarray -t remote_port < "$PORT_FILE"
VNC_PORT="${remote_port[1]}"

if [[ -z "$VNC_PORT" ]]; then
    echo "No second port found in $PORT_FILE. Ensure findopenport.sh returned 2 ports."
    exit 1
fi

VNC_AUTOSSH_OPTS='-M 0 -o "ServerAliveInterval 10" -o "ServerAliveCountMax 3"'

echo "Setting up reverse VNC tunnel on port $VNC_PORT -> $TARGET:5900"

if $USE_SYSTEMD; then
    cat > /etc/autossh-vnc.conf <<EOF
TUNNEL_USER=$USER
TUNNEL_TARGET=$TARGET
TUNNEL_PORT=$VNC_PORT
VNC_PORT=5900
SSH_KEY=/home/$USER/.ssh/id_rsa
EOF
    chmod 600 /etc/autossh-vnc.conf

    cat > /etc/systemd/system/autossh-vnc.service <<EOF
[Unit]
Description=AutoSSH reverse VNC tunnel on port $VNC_PORT
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
EnvironmentFile=/etc/autossh-vnc.conf
Environment="AUTOSSH_GATETIME=0"
ExecStart=/usr/bin/autossh \\
    -M 0 \\
    -o "ServerAliveInterval 10" \\
    -o "ServerAliveCountMax 3" \\
    -o "ExitOnForwardFailure yes" \\
    -NR \${TUNNEL_PORT}:localhost:\${VNC_PORT} \\
    -i \${SSH_KEY} \\
    \${TUNNEL_USER}@\${TUNNEL_TARGET}
Restart=always
RestartSec=30
User=${USER}
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target
EOF

    systemctl daemon-reload
    systemctl enable autossh-vnc.service
    systemctl start autossh-vnc.service
    echo "Systemd unit autossh-vnc.service installed and started."

else
    CRON_ENTRY="@reboot nohup /usr/bin/autossh -M 0 -o 'ServerAliveInterval 10' -o 'ServerAliveCountMax 3' -NR ${VNC_PORT}:localhost:5900 $USER@$TARGET &"
    CRONTAB_FILE="/var/spool/cron/crontabs/$USER"

    if ! grep -qF "NR ${VNC_PORT}:localhost:5900" "$CRONTAB_FILE" 2>/dev/null; then
        echo "$CRON_ENTRY" >> "$CRONTAB_FILE"
    fi

    nohup /usr/bin/autossh -M 0 \
        -o "ServerAliveInterval 10" \
        -o "ServerAliveCountMax 3" \
        -NR ${VNC_PORT}:localhost:5900 \
        $USER@$TARGET &
fi

## Create VNC connection shortcut on Target

VNC_SHORTCUT="/home/$USER/install/vnc-${VNC_PORT}.sh"
cat > "$VNC_SHORTCUT" <<EOF
#!/bin/bash
# Connect to VNC on remote host via reverse tunnel
# Requires a VNC viewer — adjust the command for your viewer of choice:
#   TigerVNC:  vncviewer localhost:${VNC_PORT}
#   Remmina:   remmina -c vnc://localhost:${VNC_PORT}
#   macOS:     open vnc://localhost:${VNC_PORT}
vncviewer localhost:${VNC_PORT}
EOF
chmod +x "$VNC_SHORTCUT"
scp -i /home/$USER/install/$USER.pem \
    "$VNC_SHORTCUT" \
    $USER@$TARGET:/home/$USER/

echo "VNC tunnel started. Port on $TARGET: $VNC_PORT"
echo "On $TARGET run: ./vnc-${VNC_PORT}.sh"