Is there a chat program that works offline and online and has mobile apps.
I am wondering if there is a chat program that works locally when the internet is out but still be connected to from the internet. I know this will be something that is self hosted.
Bonus points if I can set it up on the raspberry pi that is running Home Assistant.
I think you should check out Briar.
I have read a bit about the Matrix protocol and once tried running the Synapse home server and Element front end which is also available as Android/iOS apps.
Matrix is a federated chat protocol in the same way that ActivityPub is a federated social networking protocol. You can host your own Matrix compatible server and chat with people on other servers.
Recently I watched a video about their recent progress in which they showed a chat working peer to peer over bluetooth low energy when devices did not have access to the internet. Writing this, I just watched another P2P demo video where they show off this functionality once again.
This means you can use your own chat server as usual when
Take all of this with a grain of salt as I am not completely sure I understand how the Matrix services work. I hope this helps.
I did some looking into matrix but couldn't find if it would still work when the internet is out.
Not really. You need to use a domain (correct me, if I'm wrong, but I think IP adresses don't work) to connect to the homeserver.
In theory you could setup a DNS server in your nezwork, which resolves this domain with the local adress and then it might work, but I'm not sure if the homeserver would like that.
Also I want to add, that I use TeamSpeak, which works perfectly globally and locally, but it's for voice chat.
[This comment has been deleted by an automated system]