A dragon has an almost inpenetrable armour of scales. The only thing that can pierce the hide of a dragon is a dragon's claws or teeth as they are the only thing sharp enough to do even moderate damage. Magic can also damage the dragon without interference by their scales but any human sword will be made blunt by the scales' toughness. Even their eyes had an extra eyelid that they could see through that is alsmost as inpenetrable as the rest of the body. All the places you would think there would be a weakness THERE ISN'T! You can by pass the armour by killing the dragon from the inside out with poisin or acid. This is considered to be an easy way to dispose of a annoying dragons because acid leaves no evidence for vengeful friend dragons to find. 

A way that some people think that we humans were able to start hunting dragons is becuase of eroded dragon corpses and 'EXPERT' dragon hunters bringing home dragon pelts. Also a popular armour style was invented, usually called "fish scale" armour. It was thought that it originally was made out of ACTUAL dragon scales. DISGUSTING! It is absolutely nausiating.

If you look closely at the scales of a dragon you can see that they overlap each other, because of this the part that is being covered is lighter than the rest of the scale from lack of sunlight.

Armour Values
White Black Soul-Silver Green Darkness Wyvern
Chest    490    605    245    125    185    86
Back    680    885    450    275    400    115
Forearm    375    710    117    96    250    35
Claw    205    756    95    75    155    23
Wings    25    350    85    35    67    13
Head    345    999    119    79    185    79
Tail    315    875    128    117    208    100

The above armour values represent how much force one scale can withstand before any damage is dealt, with 1 being 10000 pascals. This is enough for even Wyvern wings to withstand crushing attacks and for nearly every part on every dragon to withstand piercing attacks. Although if killing a dragon is all your after[revenge]then corrosive acid will quickly eliminate a dragon.^_^