At the quantity the OP might use, buying by the gallon might make more sense - having a look to Amazon, the popular concentrations in gallon+ sizes are 70% and 99.9% (about the same price, $25 USD/gal) - it probably makes more logistical sense to go with 70% here to reduce evaporation and increase usable liquid on these tall, thin objects (so let’s say “sloppy use” of oddly shaped hard to handle glass).

I’ll leave my update at 70% concentration as the more economical choice - I’d presume based on their comment a soak in ZAP ($18 USD/gal) first is needed, then followed by the iso method… so it’s a little expensive no matter what for something they might not care about that much.

There are ways to clean glass passively, it sounds like your residue is organic.

• acetone, the pure kind you buy in a tin can at the hardware store. it will require some form of sealed container to put the glass in (acetone evaporates quickly and eats almost all organic matter) - finding a container big enough for your glass might be the hard part of this but it works (soak for days, and do not touch acetone with hands or use organic gloves - internet search for proper gloves)
• ZAP heavy duty citrus cleaner, comes in a gallon jug. soak the glass in it for days or longer, doesn’t need a sealed container. This is the same stuff you can use to clean your sink drain and is pretty safe to handle but still, wear basic gloves just in case.
• high-purity (like say 70%) iso alcohol with table salt as an abrasive (standard grocery store things). This is more of for the inside, where you can put in alcohol + salt and seal with your hand and vigorously shake to let the salt scrub the residue and the alcohol to eat it. Uses a lot of alcohol due to it’s evaporation, so buy a bigger jug.
• specialty products found on 420-friendly websites or your local 420-friendly store; weed residue is a thing for bongs, bubblers, pipes and any other sort of smoking apparatus and they need cleaned and are hard to get inside; products are made to soak the glassware in to try and get the junk out. generally expensive and hit or miss on quality but they exist

Hope this helps. (edit: acetate -> acetone, oops) (edit2: 90% -> 70% alcohol per comment)

In addition to the other comments which more directly address your question, DNS has been / can be used to exfiltrate data from “secure” networks. Search “dns data exfiltration” in your favourite search engine and you’ll get several high quality articles. Typical mitigations might be to limit which DNS servers your network can contact, restrict packet sizes to the bare minimum which valid use would have and so forth.