Is there any floor drains by the front door ? Have you gone in the attic above that door way to see is a vent pipe for the plumbing system is broken causing this smell? Also check under that door way in the basement or crawl space, their might be a broken pipe giving off sewer gas. The other thing is that maybe a mouse or bird or something like that has died in the attic or under the front stoup giving off a bad smell.
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