我在运行 apt-get update 时遇到了一些问题：
W: Failed to fetch http://br.archive.ubuntu.com/ubuntu/dists/raring/Release.gpg
Something wicked happened resolving 'br.archive.ubuntu.com:http' (-11 - System error)
E: Some index files failed to download.
They have been ignored, or old ones used instead.
此外，当我尝试将某些内容推送到 GitHub 时，我收到此错误：
error: Couldn't resolve host 'github.com'
while accessing https://github.com/lapl/HardL1.git/info/refs?service=git-receive-pack
fatal: HTTP request failed
当我在大学时，它工作正常。有什么帮助解决这个问题吗？