You are trying to guess a 4-nybble sequence. Each nybble will have either 1 or 2 bits set, and the highest bit will never be set. The game server will tell you how many nybbles in each guess were correct, and how many had one correct bit. It does not tell you which positions.
The client connects to the Cow server running on the IPv6 address provided in argument 1. The client reads a guess in the form of 4 ASCII numerals, and prints the number of correct nybbles followed by the number of nybbles with one correct bit.
Here is an example of a session:
1111 12 2222 10 4444 02 4244 12 1244 22 1255 moo:xylep-radar-nanox
You can use a program like socat to connect the client to a logic program you write:
socat EXEC:"./cowcli fd84:b410:3441::ec6" EXEC:./mysolution
This will allow your program to read and write from stdio of the client program.
cowd runs on port 3782 on fd84:b410:3441::ec6.
The client always sends 6 octets. To request a new session, it sends all zeroes. Otherwise it sends the 4-octet game identifier provided by the server, concatenated with a 2-octet guess.
The server will respond with a new game identifier (4 octets) to a new game request or if the game requested is too old. If a guess is incorrect, the server will respond with either a 1-octet score in which the high nybble is the number of correct nybbles in the guess, and the low nybble is the number of nybbles in the guess with one correct bit. If a guess is correct, the server will respond with a token of length 5 octets or more.
There are multiple tokens, one per number of guesses used, up to some maximum number of guesses defined per server instance.