master_kdc tag in krb5.conf

[Use Nas]

I am trying to kinit using keytab file and it fails saying that 'not able to
contact KDC for REALM'. As per the error, it is looking for master_kdc tag
and fails.
When i tried it without keytab (prompting for password), it works fine.

So, I want to understand the use of master_kdc tag in krb.conf file.   What
are the possible cases where the code will look for master_kdc tag in
krb5.conf
Also in gic_keytab.c file, when do we set use_master variable.

Thanks in advance.