Continuing the discussion from Raspbian Stretch:
I have been trying to install Mosquitto on a fresh DietPi Stretch image and hitting the problems Ian referred to. I am surprised there have not been more issues. I found this on the problem which is exactly what I am seeing.
I’ve not used DietPi, but there were early problems with Stretch because the Raspbian repo had not updated it’s sources.
For that reason, I suggested in the Stretch thread, to always update before trying to install Mosquitto. ie.
sudo apt update && sudo apt upgrade -y
I don’t know if the same applies with DietPi?
To avoid confusion, it would be great if could you add DietPi to the post title, as this is a DietPi issue, and not Raspbian.
Paul
More fiddling revealed that DietPi picks up the mosquitto.org repo. Using the Debain repo is fine 
@Paul - one thing though, I’m now running mosquitto version 1.4.10 which has been pulled off the Raspbian Stretch repo. I think 3.1.1 is the MQTT version it supports Debian -- Details of package mosquitto in stretch.
The Mosquitto.org repo is at version 1.4.14 so Debian etc is behind which is presumably why DietPi try and pull off there.
The Debian repo’s are always behind due to extensive testing etc. Some time ago when the Debian repo’s were trailing with 3.1 and 3.1.1 was the actual version everyone wanted, many images (including the emonSD) started using the Mosquitto.org repo’s directly and I suspect, simply haven’t switched back.
There will always be leading edge releases for almost all softwares that take time to filter through to the “stable” branch of the Debian repo’s, so unless there is a specific post 1.4.10 feature/fix you desire or you just like flying by the seat of your pants (with potentially unstable code), you will probably be better off with the Debian “stable” (aka Stretch) repo to avoid any discrepancies in dependencies etc.
If you do want everything cutting edge (not just Mosquitto) you could try running “Buster” which is the “testing” branch, that is using version 1.4.14 of Mosquitto.
I’m currently testing upgrading Jessie to Stretch. I’ve heard mosquitto could cause issues therefore I removed it and the related sources before starting the upgrade. What’s the current recommended version of Mosquitto with Stretch? Is it recommended to install the version from the Debian repos? What version are you successfully running with Stretch?
I’m running a mosquitto broker v1.4.14 on a DietPi Stretch setup. I suspect the problem was as above. DietPi picked up 1.4.15 from mosquitto.org but Debian is still (or was) at 1.4.14 (which works fine). 1.4.15 did not install due to a dependency conflict. I’ve not tried it since, but I think DietPi may have reverted to the Debian Repo.
As discussed elsewhere, the actual startup of the service may cause an issue if the mqtt_input service starts before mosquitto. I’m not seeing that, as my broker runs on a different machine.