REDU format ruling
Machina Sniper
Nearest-era rulings (Tengu, Sept 2011) — no REDU-specific archive exists.
Card effect (current official text)Monsters cannot attack "Machina" monsters, except "Machina Sniper".
Rulings
If you control "Machina Sniper" and both players have 'Machina' monsters on their fields, neither player can attack 'Machina' monsters except "Machina Sniper".
If a face-down 'Machina' monster is attacked while "Machina Sniper" is on the field, the attack will proceed as normal because the monster is not flipped face-up until the Damage Step.
If a face-down 'Machina' monster is attacked while "Machina Sniper" is on the field, but the monster is flipped face-up by "Ceasefire" before the Damage Step, the attack is canceled because of the effect of "Machina Sniper".
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the REDU forum — sourced change requests get reviewed.