在Debian Stretch上，
我尝试升级libss，但失败：
# apt-get upgrade libssl1.1
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libssl1.1 : Depends: libc6 (>= 2.25) but 2.24-11+deb9u4 is to be installed
E: Broken packages
uname给 Linux vpsXXXXXX 4.9.0-8-amd64＃1 SMP Debian 4.9.144-3.1（2019-02-19）x86_64 GNU / Linux
实际上，当我尝试续订letencrypt证书时，就会出现此问题：
# /opt/letsencrypt/letsencrypt-auto renew
Bootstrapping dependencies for Debian-based OSes... (you can skip this with --no-bootstrap)
Hit:1 http://security.debian.org stretch/updates InRelease
Ign:2 http://deb.debian.org/debian stretch InRelease
Hit:3 http://deb.debian.org/debian stretch-updates InRelease
Get:4 http://ppa.launchpad.net/ondrej/php/ubuntu disco InRelease [20.8 kB]
Hit:5 http://deb.debian.org/debian stretch Release
Get:6 https://packages.sury.org/php stretch InRelease [6,760 B]
Ign:4 http://ppa.launchpad.net/ondrej/php/ubuntu disco InRelease
Err:6 https://packages.sury.org/php stretch InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
Fetched 27.6 kB in 0s (50.6 kB/s)
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/ondrej/php/ubuntu disco InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4F4EA0AAE5267A6C
W: The repository 'http://ppa.launchpad.net/ondrej/php/ubuntu disco InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/php stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
W: Failed to fetch https://packages.sury.org/php/dists/stretch/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
W: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists... Done
Building dependency tree
Reading state information... Done
augeas-lenses is already the newest version (1.8.0-1+deb9u1).
libaugeas0 is already the newest version (1.8.0-1+deb9u1).
gcc is already the newest version (4:6.3.0-4).
libffi-dev is already the newest version (3.2.1-6).
python is already the newest version (2.7.13-2).
python-dev is already the newest version (2.7.13-2).
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libssl-dev : Depends: libssl1.1 (= 1.1.1c-1+ubuntu19.04.1+deb.sury.org+1) but 1.1.0k-1~deb9u1 is to be installed
openssl : Depends: libssl1.1 (>= 1.1.1) but 1.1.0k-1~deb9u1 is to be installed
E: Unable to correct problems, you have held broken packages.
任何帮助都将受到欢迎...