Apt is a wonderful tool, but it does have its quirks. One of those is that it likes to ask you interactive questions during package installation. This makes total sense when a human is doing apt-get install foobar, but it causes all sorts of consternation when you want to automatically install packages. (You do this all the time with Puppet, which I’ll talk more about at the end of this post.)

The answer to this problem is to pre-seed debconf with the correct answers to it’s questions. To do this, first you need to install the package by hand:

    
$ apt-get install libnss-ldap
    

You’ll need to provide answers to the questions it asks, which we’re going to re-use later as the basis for our seed file. Next, make sure you have debconf-utils installed, and grab the answers to your questions:

    
$ apt-get install debconf-utils # Only if you need it
$ debconf-get-selections | grep libnss-ldap
libnss-ldap     libnss-ldap/rootbindpw  password
libnss-ldap     libnss-ldap/bindpw      password
libnss-ldap     libnss-ldap/dblogin     boolean false
# Automatically update libnss-ldap's configuration file?
libnss-ldap     libnss-ldap/override    boolean false
libnss-ldap     shared/ldapns/base-dn   string   dc=example,dc=com
libnss-ldap     libnss-ldap/rootbinddn  string   cn=admin,dc=example,dc=com
libnss-ldap     shared/ldapns/ldap_version      select   3
libnss-ldap     libnss-ldap/binddn      string   cn=proxyuser,dc=example,dc=net
libnss-ldap     shared/ldapns/ldap-server       string   ldapi:///
libnss-ldap     libnss-ldap/nsswitch    note
libnss-ldap     libnss-ldap/confperm    boolean false
libnss-ldap     libnss-ldap/dbrootlogin boolean true
    

Take the output of that and stick it in a file, say libnss-ldap.seed. You can now use that file to pre-seed a new system with those answers using debconf-set-selections:

    
$ debconf-get-selections | grep libnss-ldap > /tmp/libnss-ldap.seed
$ debconf-set-selections /tmp/libnss-ldap.seed
    

Or use ssh to pre-seed a new box:

    
$ cat /tmp/libnss-ldap.seed | ssh somehost debconf-set-selections
    

If you are using Puppet (and if you’re not, you should be) to manage your systems, you can use a recipe like this to automatically install packages that require interaction:

    
define seed_package($ensure = latest) {
  $seedpath = "/var/cache/local/preseeding" 
  file { "$seedpath/$name.seed":
    source => "puppet://$puppet_server/seeds/$lsbdistcodename/$name.seed",
    mode => 0600,
    owner => root,
    group => root
  }
  package { $name:
    ensure => $ensure,
    responsefile => "$seedpath/$name.seed",
    require => File["$seedpath/$name.seed"],
  }
}

class foobar {
  seed_package { "libnss-ldap":
    ensure => latest
  }
}
    

Some other resources on ways to use pre-seeding to great effect:

• Automating new Debian installations with preseeding at Debian Administration
• Preseed entry at the Debian Wiki

Happy unattended package installation! :)