(White Walkers) What are their weaknesses, again???

So, after watching last episode of Game of Thrones, my dad asked me two simple questions:

• Who put the chains on the dragon, if white walkers can't swim?? 
• And who forged those huge chains if they are weak to fire??

and I must add:

• Is it possible to forge such chains in a frozen wasteland?

As a GoT lore master in my family, I really don't know how to answer these questions.. Please help me guys!

Let me know what you think in the comments.

Cheers!

p.s Just for Westeros sake, I hope they are at least weak against dragon glass.