It took me a while to figure out why many of my self-hosted services were intermittently failing to connect on my phone after updating to Android 17. I eventually figured out there's a new ACCESS_LOCAL_NETWORK permission which means the "Nearby devices" permission is now required to access devices on the same network subnet. For an avid self-hoster, this can be quite a bit.
I had my DNS configured so my public-facing server resolved to a local IP address when on my LAN. This meant that my web, immich, xmpp, NTP, jellyfin and DoH servers all resolved to a local IP address on wifi. On Android 17, it all broke without warning. No error messages. No asking for extra permissions. Just silent packet dropping.
I've solved it by configuring my public-facing services to resolve to my external (static) IP address, even when inside the network. I couldn't make any internal services resolve to the external address (SMB, CUPS etc) because they are (obviously) not bound to my WAN interface.
I get why the change was made. A lot of apps were snooping around people's networks to gather intel. A potentially massive privacy violation.
Has anyone else had this issue? Is this the cleanest solution?
Ran into this issue a few days ago, exactly the same way you described.
I am on GrapheneOS and figured it was something to do with a a setting where it's blocking network access for my specific browser app. After reading your post I see it's a android 17 feature.
Checked all the settings in my browsers related to DNS at first and was stumped for a bit, until I flipped on the nearby device setting and everything worked.
I also looked into the DNS settings under Network & Internet > Private DNS. Make sure your phone is using your local networks DNS server. Just make sure to understand the implications of changing this (especially when you disconnect from you're home networks DNS). Also making sure your home DNS is not defaulting to your ISP DNS.
Two websites that help me sanity check myself
I self-host a DoH/DoT server and my local DHCP advertises it to local clients. It's kinda cool because android phones on my wifi will use DoT when private DNS is set to "automatic".
Private DNS sort-of broke while I was tinkering with internal/external IP addresses for nearby devices. It wouldn't always connect when I specified my external IP (despite being available externally). I made it IPv6-only too. It wouldn't be the first time something couldn't handle it.