FakeMCServer Minimal Edition (requires 3 syscall: read(), write(), close())

fakemcserver-minimal.c

#define MC_MTU 1500
#define MC_COLOR_PREFIX "\xc2\247"
#define MC_MOTD MC_COLOR_PREFIX "b" MC_COLOR_PREFIX "l" "FakeMCServer Test"
#define MC_KICK_MESSAGE "You are not white-listed on this server!"

typedef int type_debug_func (const char*, ...);

size_t mc_varintSize(unsigned char* varint) {
    int i;
    for(i=0;i<4;i++) if((varint[i] & 0x80) == 0) break;
    return i+1;
}

int mc_varintToint(unsigned char* varint) {
    int value = 0;
    char currentByte;

    for(int i=0;i<4;i++) {
        currentByte = varint[i];
        value |= (currentByte & 0x7f) << i*7;

        if ((currentByte & 0x80) == 0) break;
    }

    return value;
}

size_t mc_intTovarint(int data, unsigned char* varint) {
    int i;
    for (i=0; i<4; i++) {
        if ((data & ~0x7f) == 0) {
            varint[i] = data;
            break;
        }

        varint[i] = (data & 0x7f) | 0x80;

        data >>= 7;
    }
    return i+1;
}

size_t mc_appendLengthvarint(char* string, size_t length, char* mcstring) {
    unsigned char len[4];
    int i;
    size_t headerlen;
    
    headerlen = mc_intTovarint(length, len);
    for (i=0; i<headerlen; i++) mcstring[i] = len[i];
    for (i=0; i<length; i++) mcstring[i+headerlen] = string[i];
    
    return headerlen+length;
}

size_t mc_strlen(char* string) {
    size_t cnt;
    for(cnt=0;(cnt<MC_MTU && string[cnt] != '\0'); cnt++) {}
    return cnt;
}

void mc_handle_session(int fd, type_debug_func debug_func) {
        int i, n, dn, expected_packet_size, mode, yebiopt, protocol;
        unsigned char buf[MC_MTU], returnbuf[MC_MTU], yebibuf[MC_MTU], packedJSON[MC_MTU], packetID, payloadsize_varint[4];
        char *json, *motd_json, *kick_json;
        size_t payloadsize, lengthsize, final_size, sz_i;

        motd_json = "{\"version\": {\"name\": \"ldmsys 1.12.2\", \"protocol\":340},\"players\":{\"max\":99999,\"online\": 0,\"sample\":[]},\"description\":{\"text\": \"" MC_MOTD "\"}}";
        kick_json = "{\"text\": \"" MC_KICK_MESSAGE "\"}";
        
        i = n = expected_packet_size = mode = yebiopt = protocol = 0;
         
        (!!debug_func) && debug_func("One connected!\n");

        while (1) {
            if (yebiopt) {
                (!!debug_func) && debug_func("Restoring from yebi! %d\n", yebiopt);
                n = yebiopt;
                for (i=0; i<yebiopt; i++) buf[i] = yebibuf[i];
                yebiopt = 0;
            } else {
                (!!debug_func) && debug_func("Reading... %d\n", mode);
                dn = read(fd, buf+n, 1500-n);
                if (dn <= 0) {
                    close(fd);
                    break;
                }
                n += dn;
                (!!debug_func) && debug_func("Read! +%d=%d\n", dn,n);
            }
            
            if(expected_packet_size == 0) {
                expected_packet_size = mc_varintToint(buf);
                (!!debug_func) && debug_func("Expected packet size: %d\n", expected_packet_size);
            }
            if(expected_packet_size == 0) continue;
            
            if(n < expected_packet_size + mc_varintSize(buf)) {
                (!!debug_func) && debug_func("Johnbeo! %d < %d\n", n, expected_packet_size + mc_varintSize(buf));
                continue;
            } else if(n > expected_packet_size + mc_varintSize(buf)) {
                (!!debug_func) && debug_func("Moving! %d > %d\n", n, expected_packet_size + mc_varintSize(buf));
                for (i=0; i<n-expected_packet_size; i++)
                    yebibuf[i] = *(buf+expected_packet_size+mc_varintSize(buf)+i);
                yebiopt = n-expected_packet_size-mc_varintSize(buf);
                n = expected_packet_size + mc_varintSize(buf);
            }

            // Complete Packet Received
            packetID = buf[mc_varintSize(buf)];
            (!!debug_func) && debug_func("Packet done! %x %x %x %x %x (%d)\n", buf[0], buf[1], buf[2], buf[3], buf[4], mc_varintSize(buf));
            if(mode == 0) { // pre-handshaking
                protocol = mc_varintToint(buf+mc_varintSize(buf)+1);
                mode = buf[expected_packet_size]; // new mode
                (!!debug_func) && debug_func("Handshaking! %d\n", mode);
                if(packetID != 0 || mode > 2 || mode <= 0) {
                    // Undefined behavior
                    (!!debug_func) && debug_func("Undefined behavior! %d %d\n", packetID, mode);
                    close(fd); break;
                }
            } else if(mode == 1 || mode == 2) {
                (!!debug_func) && debug_func("Respond Mode! %d\n", packetID);
                if (packetID == 0) { // MOTD Request
                    // json should vary on mode == 1 or mode == 2, ping and kick respectively.
                    json = (mode - 1) ? kick_json : motd_json;

                    payloadsize = mc_appendLengthvarint(json, mc_strlen(json), (void*)packedJSON);
                    lengthsize = mc_intTovarint(payloadsize+1, payloadsize_varint);
                    for (sz_i=0; sz_i<lengthsize; sz_i++) returnbuf[sz_i] = payloadsize_varint[sz_i];
                    returnbuf[lengthsize] = 0x00;
                    for (sz_i=0; sz_i<payloadsize; sz_i++) returnbuf[lengthsize+1+sz_i] = packedJSON[sz_i];
                    (!!debug_func) && debug_func("Sent JSON! 1 %d %d %d\n", payloadsize, lengthsize, 1 + payloadsize + lengthsize);
                    write(fd, returnbuf, 1 + payloadsize + lengthsize);
                } else if(packetID == 1) { // Ping Request
                    if (mode == 1)
                        write(fd, buf, expected_packet_size+mc_varintSize(buf)); // Send back the full packet
                    else {
                        close(fd); // Undefined behavior
                        break;
                    }
                }
            }

            n = expected_packet_size = 0;
            for (i=0; i<MC_MTU; i++) buf[i] = 0;
        }
    (!!debug_func) && debug_func("Got out of loop!\n");
    close(fd);
}