I have two usb sound cards. The first one is for the playback and the second one is for audio stream capture. I was able to redirect the streams with ecasound:
ecasound -B:rtlowlatency -b:256 -f:s32_le,2,48000 -i:alsahw,1,0 -o:alsahw,0,0
But it requires me to start it manually all the time. Is it possible to implement it with alsa, in particularly in asoundrc file?
(Virtual) devices defined in the .asoundrc file are used only when an application opens such a device. So it is not possible to get loopback without running some application (such as alsaloop or ecasound).