If someone addressed me in the street in this way I would be most offended. However, I imagine male teenagers might think it perfectly ok to address each other this way! You'd have to ask a teenager! There is, of course, the other meaning of dude: "He's a bit of a dude." (a bit of a dandy/city boy). You get dude ranches in the USA where city folk can play at being cowboys. But I imagine dudes do not like being called dudes even if they are dudes!

Dude is an American expression meaning something like 'bro' as you thought, to speak to friends in an informal way and is especially common among younger people .e.g. you could say, 'Hey dude' to a friend or 'Hey dudes' (to a group, instead of saying 'hey guys'). You wouldn't hear it very often, if at all in the UK although we know what it means from American film and TV. I guess its sort of similar to 'mate' in the UK. Whether it would be weird to say it to strangers, I don't really know, someone from the US probably needs to clarify. I would feel strange if someone I didn't know addressed me as dude, or bro, but then that could be because it would be odd to hear someone in the UK use those words. For example I wouldn't consider it strange if someone in the street who wanted to ask me for directions said 'Excuse me mate...'. You can use 'mate' like that, so maybe you can for 'dude' too. Maybe one day it will change and people in the UK will say 'dude'. Nowadays you hear people say 'Hey man' all the time in this country, which was originally an American expression but now it is really common to hear.

I don't think is very appropriate...but maybe it's better to get more opinions ;)

I wouldn't use "dude" to address anyone over the age of 30, as it might be considered rude and make them uncomfortable. However, feel free to use it with younger men (15-25, perhaps), it's usually acceptable. As far as with strangers...It really depends on the person. For some people, even younger men, it can be too informal and may make the conversation awkward. Personally, I would avoid using it with total strangers.

Its important to note that this is very informal and can only be used among younger people of the same age and people you have an informal relationship with. Dont try to use it with strangers as they will think it is very rude.