Redistribute HL1 with mod freely?

If I plan to release my HL1 mod and allow anyone to play it, what legal things should I know before I release it with a copy of the HL1 engine.

- av3rs1on

[taz-ed]

i think you might find unless the ppl that want to play your mod own HL1 the install of steam will tell them the dont have to the right to play that game and thus stop them from playing the mod to

EDIT
All so you cannot Redistribute HL1 because it is downloaded though steam to players that have bought the game and atm is not under a freeware licence so if you was to give away copies of hl1 you would have to pay for each copy and make up steam accounts to go with each copy you gave away