如果要实现验证码，需要从NuGet引用System.Drawing.Common这个包（最新版的已经集成引用了libgdiplus.so，不需要在做ln链接了）。
执行一下操作，安装包。
- 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.”则执行下面的操作，安装所需环境。
1、下载 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
安装完Nginx后，记得运行 sudo setsebool -P httpd_can_network_connect 1 否则Nginx无法正确工作（浏览器访问不了）
