https://classic.gazebosim.org/tutorials?tut=install_from_source&cat=install ，this page have error,

Dependencies managed by OSRF Gazebo development is tightly linked to the development of a few other libraries:

SDFormat ignition-math ignition-transport ignition-msgs

http://ignitionrobotics.org/libraries/math http://ignitionrobotics.org/libraries/transport http://ignitionrobotics.org/libraries/messages

these webs can not visit.