Rust Spot on Water Pipe (Cast Iron or Black Iron maybe?)
My guess is if you have a water meter their is a valve just before it. IF not when you shut off the water put one in same time,,,better yet,put one on both sides of the meter. If you take the meter out you dont have water back thru the pipes. Do you have a curb stop?? Thats out in the lawn or sidewalk area in front of the house,,,maybe,,depending which way your water comes in. No plumber friends?? They all have curb stop keys. OR call and see how much they charge for a turn off,,,and on?? Otherwise if you cant afford that,,stop by your local grocery and see if they have dry ice,,,pack the line where it comes in and it stops the flow. Best to have all leaks and faucets off, a contiuous trickle will drive you up the wall!!
But as has been said,,,if that pipe is rusted thru and leaking,just a matter of time for more,,,best to replace as much as possibble but a short repair will work for temp too. Is it leaking much?? Can wrap it with couple wraps of inner tube and put a hose clamp over it for temp fix,,,live!!