During upgrade, apt-get may order some of packages in upgrade list to be removed.
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server1# apt-get -o Debug::pkgProblemResolver=true dist-upgrade
Reading Package Lists... Done
Collecting File Provides... Done
Building Dependency Tree... Done
Calculating Upgrade... Starting
Starting 2
Done
Done
The following packages will be upgraded
  bind bind-utils 
The following packages will be REPLACED:
  caching-nameserver (by bind)  
The following NEW packages will be installed:
  libdns8 libisc4 libisccc0 libisccfg0 liblwres1 
The following packages have been kept back
  gnupg 
2 packages upgraded, 5 newly installed, 1 replaced, 0 removed and 1 not upgraded.
Need to get 0B/985kB of archives.
After unpacking 886kB disk space will be freed.
Do you want to continue? [Y/n] 
Executing RPM (/bin/rpm -e --fancypercent --nodeps)...
Executing RPM (/bin/rpm -Uvh --fancypercent --oldpackage)...
Preparing...                 [100%]
1: libdns8                   [ 14%]
2: libisc4                   [ 28%]
3: libisccc0                 [ 42%]
4: libisccfg0                [ 57%]
5: liblwres1                 [ 71%]
6: bind                      [ 85%]
warning: /var/lib/named/etc/named.conf saved as /var/lib/named/etc/named.conf.rpmsave
7: bind-utils                [100%]

removed package is bind-utils-0:8.3.4-alt1

server2# apt-get dist-upgrade
Reading Package Lists... Done
Collecting File Provides... Done
Building Dependency Tree... Done
Calculating Upgrade... Starting
Starting 2
Done
Done
The following packages will be upgraded
  bind bind-doc bind-utils 
The following packages will be REPLACED:
  caching-nameserver (by bind)  
The following NEW packages will be installed:
  libdns8 libisc4 libisccc0 libisccfg0 liblwres1 
The following packages have been kept back
  gnupg 
3 packages upgraded, 5 newly installed, 1 replaced, 0 removed and 1 not upgraded.
Need to get 0B/2347kB of archives.
After unpacking 3698kB disk space will be freed.
Do you want to continue? [Y/n] 
Executing RPM (/bin/rpm -e --fancypercent --nodeps)...
Shutting down named: [  OK  ]
Executing RPM (/bin/rpm -Uvh --fancypercent --oldpackage)...
Preparing...                 [100%]
1: libdns8                   [ 12%]
2: libisc4                   [ 25%]
3: libisccc0                 [ 37%]
4: libisccfg0                [ 50%]
5: liblwres1                 [ 62%]
6: bind-utils                [ 75%]
7: bind                      [ 87%]
warning: /var/lib/named/etc/named.conf saved as /var/lib/named/etc/named.conf.rpmsave
8: bind-doc                  [100%]

removed package is bind-0:8.3.4-alt1
---