I have an installation with an MCU with proprietary (unfortunately) and immutable firmware, that provides a serial interface to connect a PC or laptop to. Planning to make it accessible via the network, I was looking into creating a WLAN bridge with a ESP8266 module, like the ESP-12. Espressif provides the AT firmware for doing this kind of thing, however, researching the documentation, I gained the impression that with that firmware it is not possible for the device just to sit there and wait for an incoming connection. This is not made explicit in the documentation, unfortunately, hence my question.
Since I cannot make the aforementioned MCU aware of the bridge, the bridge needs to work in pass-through mode. It seems this mode is only available in TCP-client mode, where it would flood the network with connection attempts if the laptop is not present. Listening to incoming connections seems to require creating a server, which only works in multi-connection mode, which precludes pass-through mode, if my understanding is right.
Is it possible, with the AT firmware, to have the ESP8266 listen for an incoming connection, providing pass-through mode once connected?
Edit: My main objection to the TCP-client mode of course is that the peer needs to fixed in the ESP's configuration, i.e. it would need to be always the same machine. The network being flooded with connection attempts is not nice, but secondary.