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c/apt# pip2 install -U pip
Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f74f8bb9050>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/pip/
Retrying (Retry(total=3, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f74f8bb9190>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/pip/
Retrying (Retry(total=2, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f74f8bb92d0>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/pip/
Retrying (Retry(total=1, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f74f8bb9410>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/pip/
Retrying (Retry(total=0, connect=None, read=None, redirect=None)) after connection broken by 'NewConnectionError('<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f74f8bb9550>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/pip/
以上问题发现原来是DNS配置错误，域名服务器地址是本机IP地址，127.0.0.1，所以修改域名服务器
sudo gedit /etc/resolv.conf
nameserver 8.8.8.8
nameserver 114.114.114.114