修复 Ubuntu 无法进行版本更新的错误

将 Ubuntu 服务器从 18.04 升级到 20.04 ，在执行 do-release-upgrade 时提示无法检查新版本，完整的错误信息如下：

Checking for a new Ubuntu release
Failed to connect to https://changelogs.ubuntu.com/meta-release-lts. Check your Internet connection or proxy settings
There is no development version of an LTS available.
To upgrade to the latest non-LTS develoment release
set Prompt=normal in /etc/update-manager/release-upgrades.

于是先用 ping 命令测试网络连接性：

ping -c 4 changelogs.ubuntu.com

PING changelogs.ubuntu.com (91.189.88.185) 56(84) bytes of data.
64 bytes from 91.189.88.185 (91.189.88.185): icmp_seq=1 ttl=46 time=235 ms
64 bytes from 91.189.88.185 (91.189.88.185): icmp_seq=2 ttl=46 time=237 ms
64 bytes from 91.189.88.185 (91.189.88.185): icmp_seq=3 ttl=46 time=235 ms
64 bytes from 91.189.88.185 (91.189.88.185): icmp_seq=4 ttl=46 time=235 ms

--- changelogs.ubuntu.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3001ms
rtt min/avg/max/mdev = 234.613/235.262/237.087/1.054 ms

可以 ping 通，网络是没问题的。用 curl 测试一下 https://changelogs.ubuntu.com/meta-release-lts ，看是不是 Ubuntu 的 changelogs 服务器问题

curl https://changelogs.ubuntu.com/meta-release-lts

Dist: dapper
Name: Dapper Drake
Version: 6.06 LTS
Date: Thu, 01 Jun 2006 9:00:00 UTC
Supported: 0
Description: This is the Dapper Drake release
Release-File: http://old-releases.ubuntu.com/ubuntu/dists/dapper/Release
ReleaseNotes: http://changelogs.ubuntu.com/EOLReleaseAnnouncement
UpgradeTool: http://old-releases.ubuntu.com/ubuntu/dists/dapper/main/dist-upgrader-all/current/dapper.tar.gz
UpgradeToolSignature: http://old-releases.ubuntu.com/ubuntu/dists/dapper/main/dist-upgrader-all/current/dapper.tar.gz.gpg

可以用 curl 获取到内容， changelogs 服务器看来也是没问题的。

因为是用 https 访问 changelogs 服务器，突然想到会不会是因为 https 证书没有同步的导致的呢？于是再尝试刷新一下服务器上的证书：

sudo update-ca-certificates --verbose --fresh
export SSL_CERT_DIR=/etc/ssl/certs

刷新完 https 证书之后，再次运行 do-release-upgrade ，还是提示同样的错误。

经过一番搜索，发现修改 MetaRelease.py 文件可以忽略证书的错误

sudo nano /usr/lib/python3/dist-packages/UpdateManager/Core/MetaRelease.py

修改内容如下：

from __future__ import absolute_import, print_function

+ import ssl
+ ssl._create_default_https_context = ssl._create_unverified_context

import apt
import apt_pkg

保存修改过后的 MetaRelease.py ，再次运行 do-release-upgrade ，终于可以进行更新了。

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