unable to install build-essential in 6.9.1

This issue was caused from the `20defaultrelease` file configured to use Debian Jessie.

/etc/apt/apt.conf.d/20defaultrelease 

It is not recommended to configure other debian repos to have higher primary. Please be careful when following websites that provide debian guides and may provide conflicts with os libraries, and make sure to keep the readynas repo with higher priority.

Here were some findings for other that might accedently run into this:

To help find a sample package to look at:

# dpkg -l | grep libc 
ii  libc-bin                       2.19-18+deb8u10.netgear1      armel        GNU C Library: Binaries
ii  libc-dev-bin                   2.19-18+deb8u10               armel        GNU C Library: Development binaries
ii  libc6:armel                    2.19-18+deb8u10.netgear1      armel        GNU C Library: Shared libraries
...

That is the list of packages that you have and the libc-dev-bin is a good example to use for the priority checking and seeing why we are not able to install the parent packaging.

Output from the lab unit:

# dpkg -l | grep libc
ii  libc-bin                       2.19-18+deb8u10.netgear1   armel        GNU C Library: Binaries
ii  libc-dev-bin                   2.19-18+deb8u10.netgear1   armel        GNU C Library: Development binaries
ii  libc6:armel                    2.19-18+deb8u10.netgear1   armel        GNU C Library: Shared libraries
ii  libc6-dev:armel                2.19-18+deb8u10.netgear1   armel        GNU C Library: Development Libraries and Header Files
...

The version is showing a newer version that is posted in the readynas repo.

Policy where the issue would start surfacing:

# apt-cache policy  libc-dev-bin
libc-dev-bin:
  Installed: 2.19-18+deb8u10
  Candidate: 2.19-18+deb8u10
  Version table:
     2.19-18+deb8u10.netgear1 900
        900 http://apt.readynas.com/packages/readynasos 6.9.2/main armel Packages
 *** 2.19-18+deb8u10 990
        990 http://mirrors.kernel.org/debian jessie/main armel Packages
        990 http://security.debian.org jessie/updates/main armel Packages
        990 http://http.us.debian.org/debian jessie/main armel Packages
        100 /var/lib/dpkg/status

It shows that it installed the older version that is not coming from the readynas repo. It also highlights the preferred repo to update from.

Policy from the lab unit:

# apt-cache policy libc-dev-bin
libc-dev-bin:
  Installed: 2.19-18+deb8u10.netgear1
  Candidate: 2.19-18+deb8u10.netgear1
  Version table:
 *** 2.19-18+deb8u10.netgear1 900
        900 http://apt.readynas.com/packages/readynasos 6.9.2/main armel Packages
        100 /var/lib/dpkg/status
     2.19-18+deb8u10 500
        500 http://mirrors.kernel.org/debian jessie/main armel Packages
        500 http://security.debian.org jessie/updates/main armel Packages

It shows that it prefers and installs the readynas repo version.

In this case, I remove the `20defaultrelease` config, and ran the following commands to check:

# apt clean
# apt update
# apt install build-essential
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  g++ g++-4.9 libc-dev-bin libc6-dev libstdc++-4.9-dev
Suggested packages:
  gcc-4.9-doc libstdc++6-4.9-dbg glibc-doc manpages-dev libstdc++-4.9-doc
Recommended packages:
  manpages-dev
The following NEW packages will be installed:
  build-essential g++ g++-4.9 libc6-dev libstdc++-4.9-dev
The following packages will be upgraded:
  libc-dev-bin
1 upgraded, 5 newly installed, 0 to remove and 61 not upgraded.
Need to get 11.7 MB of archives.
After this operation, 39.4 MB of additional disk space will be used.
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