I think that by 5 years your daughter is old enough that you can tell her that its fine to talk to people whom she has met with you or when you are around but she should ask you before getting very friendly with any unknown person.

You should give her examples how you talk to people you know or you meet with family members.

Its not very unusual. Some kids are very jolly and they get friendly fast and as you said you have been asking your kid to be friendly. Now that she is 5 yrs old you can try telling her where to set her limits.
Like you can tell her to talk to people with whom you interact or whom she has been meeting frequently with you. And when she gets comfortable with strangers you can softly let her know that she should keep some distance. I am sure slowly your child will know who are strangers.

I think u should not force anything on him. Slowly he will understand with time only thing u need to do is keep telling him the right things.

I am sure you child is not going to anyone and everyone. Maybe he is attracted by some person's smile or maybe someone is offering him something to eat or play with. I am telling this because my son used to do the same. Infact now also he does that once in a while.

What I did was first let my son know that if he likes something to eat he should ask his parents or if he is out with us he should eat if we do. I did let him know the reason that sometimes food items offered by other people may not be good for his health. Another aspect of getting something to play with for my son it was totally a big NO. He can share and play with his friend's toys but never take any toys from any unknown person.

Now comes being attracted to a particular person one cannot predict this but I tried to tell my child that if he meets some new person and he wants to talk to him or play with him he should first come to me or his Dad and ask us. If the person is the right person we will definitely let him talk/play with.