We have an old cast iron drain pipe (maybe 100 years?) going from the
main vent stack to the city drainage system.

I just noticed a damp spot underneath an area of the pipe and a
corresponding light wetness along the bottomside of the pipe. Looking
more closely, there was an irregular hairline crack in the pipe,
extending perhaps 2-3 inches.

The discharge is (currently) clear and odorless and is truly minimal
-- i.e. you can't even see drops dripping it is so slow.

So, short of replacing the pipe, what can i do to fix it?

Thanks