Smelly kitchen sink

You need to remove the trap under the sink and clean it out. If you have never done this before you might need to get a friend who has.

If the trap is full of crap it might be just that it hasn't be cleaned in a long time, or the fall on the out going pipes might be wrong. Post a picture here and people can advise.

IF they outgoing pipes are not correct, no amount of chemical cleaning will do the job. Many sinks are installed with the wrong fall on the waste pipe, leading to the traps getting backed up and stinking. Once the fall is sorted on the waste pipes the problem doesn't come back again.

In addition to checking the above, if that doesn’t work you may also find that your outside drain is blocked- so essentially the smell is emanating back the pipe and up into your kitchen - no harm even checking that first- its usually the cause in my own experience

leave a piece of kitchen roll flat under the u bend to check for leaks.

If you find you need to clean this out every 6 -8 months, there is probably an issue with the fall on the outgoing pipes.

+1 for outside drain, especially if it's a drain for other pipes as well.