I am currently using golang pq library to connect to postgres database. I am successfully connecting using kerberos principal, but i can't figure out where can i specify keytab file to use to. In the source code it kinda happens magically, using some third-party library. It actually works, but i need to know for sure how does it know where my keytab is stored, so it can request initial ticket.
Usually Kerberos clients do not directly use a keytab; they expect the initial ticket to be already acquired and present in the environment. That is, you're expected to kinit before running the program, and afterwards the client's GSSAPI library looks for the KRB5CCNAME environment variable, which points at a file containing the ticket cache left by kinit.
(Normally with MIT Kerberos or Heimdal it could be many other things besides a file... but the 'pq' library uses a very minimal pure-Go Kerberos implementation which only accepts a traditional file-based ccache. So be careful if Krb5 on your distro was set up to use 'DIR' or 'KEYRING' or 'KCM' cache types, those aren't going to work here.)
If the initial ticket isn't present, the MIT Krb5 implementation will in fact automatically use a keytab to acquire the ticket if the KRB5_CLIENT_KTNAME environment variable is pointing to one. Unfortunately, the 'pq' library doesn't use the system Kerberos library, so that won't work here either. (But it also wouldn't work if your OS was using Heimdal Kerberos; it's a MIT-specific extension.)
So the approach that will always work is to set KRB5CCNAME to a temporary path, then use either kinit or k5start to acquire a ticket from the keytab, before running your program. (The k5start tool will also keep automatically renewing or re-acquiring the ticket before it expires, without needing to use cron.)
Really, the whole krb_unix.go file is disappointing. If they can call the native SSPI on Windows, surely they could call the native GSSAPI on Linux...