Mudbus is a full featured Modbus TCP Slave for Arduino
Mudbus was started by Dee Wykoff, and ported to Arduino 1.0 by Martin Petterson
https://code.google.com/archive/p/mudbus/#!
Then came Siamect, origionally on gitorious, who turned it into a well featured client. At some point, it moved to github.
https://github.com/siamect/mudbus
While it was on gitorious I added error checking and proper complient fault codes.
And I just went looking for it, and hence re-forked it from siamect.