如果要實現驗證碼，需要從NuGet引用System.Drawing.Common這個包（最新版的已經集成引用了libgdiplus.so，不需要在做ln鏈接了）。
執行一下操作，安裝包。
- yum install autoconf automake libtool
- yum install freetype-devel fontconfig libXft-devel
- yum install libjpeg-turbo-devel libpng-devel giflib-devel libtiff-devel libexif-devel
- yum install glib2-devel cairo-devel
- git clone https://github.com/mono/libgdiplus
- cd libgdiplus
- ./autogen.sh
- make
- make install
- cd /usr/lib64/
- ln -s /usr/local/lib/libgdiplus.so gdiplus.dll
如果提示“The type initializer for 'System.DrawingCore.GDIPlus' threw an exception. ---> System.DllNotFoundException: Unable to load shared library 'gdiplus' or one of its dependencies.”則執行下面的操作，安裝所需環境。
1、下載 epel-release-7-11.noarch.rpm
wget http://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-7-11.noarch.rpm
2、rpm -Uvh epel-release*rpm
3、sudo yum install libgdiplus
安裝完Nginx後，記得運行 sudo setsebool -P httpd_can_network_connect 1 否則Nginx無法正確工作（瀏覽器訪問不了）