1.apt update

新的虚拟机，安装git发现apt不能正常使用，报错如下

Err:1 http://linux.mellanox.com/public/repo/mlnx_ofed/5.1-2.5.8.0/ubuntu18.04/amd64 ./ InRelease
  Temporary failure resolving 'linux.mellanox.com'
Err:2 https://mirrors.aliyun.com/ubuntu focal InRelease
  Temporary failure resolving 'mirrors.aliyun.com'
Err:3 https://mirrors.aliyun.com/ubuntu focal-security InRelease
  Temporary failure resolving 'mirrors.aliyun.com'
Err:4 https://mirrors.aliyun.com/ubuntu focal-updates InRelease
  Temporary failure resolving 'mirrors.aliyun.com'
Err:5 https://mirrors.aliyun.com/ubuntu focal-backports InRelease
  Temporary failure resolving 'mirrors.aliyun.com'
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
W: Failed to fetch https://mirrors.aliyun.com/ubuntu/dists/focal/InRelease  Temporary failure resolving 'mirrors.aliyun.com'
W: Failed to fetch https://mirrors.aliyun.com/ubuntu/dists/focal-security/InRelease  Temporary failure resolving 'mirrors.aliyun.com'
W: Failed to fetch https://mirrors.aliyun.com/ubuntu/dists/focal-updates/InRelease  Temporary failure resolving 'mirrors.aliyun.com'
W: Failed to fetch https://mirrors.aliyun.com/ubuntu/dists/focal-backports/InRelease  Temporary failure resolving 'mirrors.aliyun.com'
W: Failed to fetch http://linux.mellanox.com/public/repo/mlnx_ofed/5.1-2.5.8.0/ubuntu18.04/amd64/./InRelease  Temporary failure resolving 'linux.mellanox.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.

第一反应是考虑到apt源配置可能有问题，用vi /etc/apt/source.list打开源发现没有问题，用ping测试网络，ping百度报错unkown host，ping 8.8.8.8可以通，应该是域名解析错误，vi /etc/resolv.conf更改nameserver，输入两个8.8.8.8和8.8.4.4，然后apt update可以成功了。

2.git clone 失败

git clone https://github.com/NVIDIA/nccl.git
Cloning into 'nccl'...
fatal: unable to access 'https://github.com/NVIDIA/nccl.git/': Failed to connect to github.com port 443: No route to host

原因：服务器没挂梯子，可能的解决办法：

更改DNS

vi /etc/resolv.conf
#输入以下内容 :wq退出保存
nameserver 114.114.114.114

用其他机器上传到国内仓库

#在其他机器上下载并上传到国内仓库
git clone https://github.com/NVIDIA/nccl-tests.git
cd nccl-tests
git remote set-url origin https://gitee.com/username/nccl-tests.git
git add .
git commit -m "initial commit"
git push -u origin master