Unable to connect via adb on a firewalled Android device

Question

Connecting from Linux, using an older adb (downloaded at tried 1.0.41 just to troubleshoot this - no difference, except that the messages now do include "Connection refused"), and the devices runs Android 9.

This has worked fine literally for years, but all of a sudden, I can no longer connect to a device via TCP/IP. I can't think of anything that has changed.

The device has a static IP (and I usually connect and disconnect multiple times per day), and I do run a firewall on it (AFWall+), which has never caused a problem in the past.

$ ADB_TRACE=1 adb connect 192.168.0.2
            main:  1915:140003988076352 | Handling commandline()
       adb_query:  1915:140003988076352 | adb_query: host:connect:192.168.0.2
    _adb_connect:  1915:140003988076352 | _adb_connect: host:version
  WriteFdExactly:  1915:140003988076352 | writex: fd=3 len=4: 30303063 000c
  WriteFdExactly:  1915:140003988076352 | writex: fd=3 len=12: 686f73743a76657273696f6e host:version
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4 got=4
4f4b4159 OKAY
    _adb_connect:  1915:140003988076352 | _adb_connect: return fd 3
     adb_connect:  1915:140003988076352 | adb_connect: service host:connect:192.168.0.2
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4 got=4
30303034 0004
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4 got=4
30303230 0020
    _adb_connect:  1915:140003988076352 | _adb_connect: host:connect:192.168.0.2
  WriteFdExactly:  1915:140003988076352 | writex: fd=3 len=4: 30303161 001a
  WriteFdExactly:  1915:140003988076352 | writex: fd=3 len=26: 686f73743a636f6e6e6563743a313732 host:connect:192
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4 got=4
4f4b4159 OKAY
    _adb_connect:  1915:140003988076352 | _adb_connect: return fd 3
     adb_connect:  1915:140003988076352 | adb_connect: return fd 3
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=4 got=4
30303237 0027
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=39
   ReadFdExactly:  1915:140003988076352 | readx: fd=3 wanted=39 got=39
756e61626c6520746f20636f6e6e6563 unable to connec
unable to connect to 192.168.0.2:5555

Any ideas what might cause this?

Answer 1

So (especially with the "Connection refused" message) I turned the firewall off, and I immediately could connect. I turned the firewall right back on a few seconds later..., and I could still connect (and disconnect) as I always could before.

I have no idea what happened (I even rebooted the device before posting as part of my troubleshooting efforts).

Since I no longer have the problem, I will have to change this question to answered, but would still appreciate any additional thoughts about what happened.