So, with that:
p. B374: An active defense is a deliberate attempt to avoid a particular attack. It’s only possible if the defender is aware of the possibility of an attack from his assailant and is free to react . . . by moving out of the way of the attack (a dodge), deflecting the attack with a weapon or empty hand (a parry), or interposing a shield (a block).
p. B391: Against an attack that comes from your back hex, you cannot defend at all unless you have Peripheral Vision (which lets you defend at -2) or 360° Vision (which lets you defend at no penalty).
p. B391 – Runaround Attacks: A fast-moving fighter can sometimes start in front of a foe and run behind him to strike from his back hex. Against a true attack from behind, no active defense is possible, because the victim did not know the attack was coming. If the attacker starts in front and runs behind, outmaneuvering his victim through sheer speed, the victim does know he’s being attacked.
Technical Grappling, p. 11 – Rear Arc: When grappled, you know where your foe is! You defend against his attacks from your rear arc at -2 (as if from the side) if standing, crouching, or kneeling, or at -4 otherwise.
To me, it’s going to be pretty simple. If your cannot perceive the attack in any way, you may not defend.
In my article Dodge This, from Pyramid #3/57, I spent some time on the Vision part of Perception rolls. I didn’t spend that much time – only a throwaway line – on other senses. Still, one big one is going to be hearing, as many people and critters will growl or give off some warning when they attack. Hearing is more of a 360-degree sense, and if you make a Hearing-2 roll against an invisible foe, you may parry or block at -4 . . . at least from the front. You may dodge at -4 if you’re aware you’re being attacked (again, this doesn’t specify arc, but I’ll assume mostly from the front). This is all covered on p. B394.
The way I read the rules, especially the quickie table on p. B549, is as follows
1. If an attack is from behind (excepting runaround attacks and pre-existing grapples), you don’t get to defend.
2. If an attack is not from behind, you may only defend if you can succeed in some sort of Perception check. Penalties for Vision rolls are given in various places, and Hearing at -2 is a good option even against invisible foes. You still get plugged in the back if the attack starts there, though.
There are always exceptions, and one I might make would be that if you’re being attacked from behind, you might be able to make a Hearing-based Melee or Unarmed Combat Skill roll, penalized, to hear your foe’s footsteps, breath, rustle of clothing or jingle of mail.
The only defense you can make is a Dodge, and you must either Retreat by stepping away from your foe, or if you feel compelled to stay in your own hex, Change Posture to kneeling or lower.
For penalties, I’d make it -2 generically for ‘can’t see attacker,’ -4 if you’re wearing a helmet that covers the ears, -4 if you’re actively fighting someone else, and likely another -4 for rear, rather than side or front arc.
That’s basically Hearing-6 if you’re just standing around, Hearing-10 if you’ve got open ears but are fighting someone in front of you, or have Hard of Hearing because you’ve got a helm on your ears, or a whopping Hearing-14 if all apply. If your Hearing roll drops below 3, you can’t rolll.
The simple rule, attacks from the rear hex get no defense, is what we use in +Nathan Joy‘s DF game. It’s accepted by all, and we use it to our own advantage as much as the bad guys use it to theirs. The easiest way to deal with a foe in your rear hex is to either (a) have friends who can watch out for your, or (b) turn a bit to expose your peripheral vision to various arcs. But by and large, the reason you just get nailed if an attack comes from your rear hex is that it works in play.