Leak from toilet waste pipe from floor

[Mperry]

I’d be grateful for some guidance on the best way to fix a leak from my toilet. The leak doesn’t come from the toilet so presuming it’s the seal next to the floor and involves the 4inch waste pipe. I am replacing the toilet anyway and presume I need to replace some kind of flange at the base, just not sure what to buy / best way to go about it. I’ve included a picture to help. The old sealant is brittle like crumbling mortar. Many thanks for any advice.

 

[rpm]

Think I can see a water stain from the pan connection at the pan end, water could then down to the floor joint you mention.

 

[Mperry]

Thanks. Yes I’ve tried feeling for a leak all around the pipe near the pan (used some tissue) but seems dry there.