Trailcap

DNSTrail uses iframes as screenshots. Trailcap pre-processes them for us.

To make this fast and secure, there are a few things we need to do:

• The iframe HTML shouldn't have any sensitive information left over from when we captured it, like hidden form elements holding sessions IDs.
• The iframe HTML file should be completely self-contained (inline styles, images).
• The iframe HTML should not include any javascript (and it's displayed with the sandbox attribute anyway)
• the iframe should be as small as possible.

Trailcap brute-force minimizes an HTML file. It uses puppeteer to render screenshots while it tries to remove:

• Each DOM element
• Each attribute
• Each member of a "class" attribute.

It only considers a modification a success if the result of a render is a pixel-perfect match for the original file.

Basically, it trys to reduce an HTML file to the minimum required to maintain its visual presentation.

Trailcap is slow, by design.

This is really unavoidable. Every modification triggers a Blink render, PNG generation, and an image compare. On my maxed-out 2017 iMac, the Cloudflare admin page (~90K) takes about 5 minutes to process, and gets the page down to 38K.

The CNN homepage (1.2MB) takes much longer (1h), and is reduced to 80K.

Workflow

1. Load the page you'd like to capture in Chrome.
2. Use Devtools Inspector to edit visual things you'd like (example.com, user@example.com). Obscure API keys, any domain and usernames, etc.
3. Export the HTML file with the "SinglePage" Chrome extension. This inlines images, styles, and removes Javascript.
4. Run that file through Trailcap. Wait 15 to 20 hours.
5. Use DNSTrail's interactive editor to mark-up your PageCap.

Usage

Note, I haven't converted this to an installable executable yet, it's just index.js, but the rest of the command line usage should be accurate.

trailcap [--verbose] [--dump] [--diff] [--show] [--phase phase0] inputfile.html

• --verbose (or -V) prints activity to STDERR while it operates.

• --dump (or -d) will write snap-%d.png images every time a modification is attempted, and before it's compared to the pristine initial page. pristine.png is also generated.

• --diff (or -D) will write a diff-%d.png image for each modification it makes that does not match the pristine page.

• --show (or -s) Will take puppeteer out of headless mode so you can see what its doing. This can lead to false-positives, as if you interact with that instance of Chrome, it'll be caught by puppeteer's screenshot (including scrolling)

• --phase PHASE (or -p) selects the phases trailcap will use. PHASE must be one of denode, deattribute, declass, purgecss or minify.

  The --phase option may be given multiple times, and defaults to --phase denode --phase deattribute.

  Note that the declass phase is particularly slow, but normally very useful. purgecss and minify are pretty fast, but mangle the output enough to make it hard to reason with.