Consider localhost-based addresses (127.0.0.0/8, ::1) as either LAN or separate localhost firewall group in iptables firewall about invizible

[Gedsh]

Are you using InviZible in Root mode? Do you have any specific issue when enabling the firewall in InviZible? Does disabling the firewall solve the problem?

[zer0def]

Yes, using root mode (iptables), no issues when enabling the firewall, just apps unable to communicate with one another across 127.0.0.0/8 when any functionality is enabled or kill-switch is in effect and disabling all functionality along with the kill-switch fixes the issue.

[Gedsh]

disabling all functionality along with the kill-switch fixes the issue

This means that the problem is not in the mentioned code. It only applies when the firewall is enabled.

Do you have "Bypass LAN addresses" enabled in the Fast Settings?

The problem could also be with DNS. InviZible redirects all DNS traffic to DNSCrypt or Tor, since it is not possible to separate DNS traffic of different applications on android.

[zer0def]

"Bypass LAN addresses" doesn't affect outcomes in any way.

I have to insist on disagreeing, as per original post, enabling any external firewall zone (for example WiFi, but not WiFi-LAN) on the listening app's end (in this case, XSDL) successfully allows Termux to connect. That would make sense, because at that point, both apps' UIDs are allowed to communicate virtually freely, even though the destination address is localhost/LAN-scoped.

However, if the listening app only has either no allowed zones or only LAN allowed (which omits localhost-based addressing, per code reference in original post), other apps are unable to connect to it's listening socket.

This becomes much more apparent once you look at iptables -t filter -S.