您好，我正在尝试从ubuntu 18.04中的源代码构建桌面linphone应用程序。当我克隆源代码时，请使用以下命令：
git clone git://git.linphone.org/linphone-desktop.git --recursive
但是在linphone-sdk上进行克隆时存在问题。本节中出现的错误是：
Submodule 'linphone-sdk' (git://public/linphone-sdk.git) registered for path 'linphone-sdk'
Cloning into '/home/ubuntu/Documents/linphone-desktop/linphone-sdk'...
fatal: Unable to look up public (port 9418) (Name or service not known)
fatal: clone of 'git://public/linphone-sdk.git' into submodule path '/home/ubuntu/Documents/linphone-desktop/linphone-sdk' failed
Failed to clone 'linphone-sdk'. Retry scheduled
Cloning into '/home/ubuntu/Documents/linphone-desktop/linphone-sdk'...
fatal: Unable to look up public (port 9418) (Name or service not known)
fatal: clone of 'git://public/linphone-sdk.git' into submodule path '/home/ubuntu/Documents/linphone-desktop/linphone-sdk' failed
Failed to clone 'linphone-sdk' a second time, aborting
有人可以帮忙吗？我非常感谢您提供的协助。谢谢