在一台新的Linux（CentOS 7.7）服务器上使用pip安装python包时遇到下面错误和告警，如下所示：
# pip install pymssql
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8f866f3860>: Failed to establish a new connection: [Errno 113] No route to host',))': /simple/pymssql/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8f8670d2b0>: Failed to establish a new connection: [Errno 113] No route to host',))': /simple/pymssql/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8f8670d320>: Failed to establish a new connection: [Errno 113] No route to host',))': /simple/pymssql/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8f866f3710>: Failed to establish a new connection: [Errno 113] No route to host',))': /simple/pymssql/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8f866f3518>: Failed to establish a new connection: [Errno 113] No route to host',))': /simple/pymssql/
ERROR: Could not find a version that satisfies the requirement pymssql (from versions: none)
ERROR: No matching distribution found for pymssql
最后检查发现是因为代理弄错了。注意：下面敏感信息做了混淆，用xxx.xxx.xxx.xxx代替了具体IP。
export http_proxy='http://xxx.xxx.xxx.xxx:3128'
export https_proxy='http://xxx.xxx.xxx.xxx:3128'
修改/etc/profile，更改设置，如下所示
export http_proxy='http://xxx.xxx.xxx.xxx:3128'
export https_proxy='https://xxx.xxx.xxx.xxx:3128'
执行source /etc/profile使之生效，问题解决。