When using –negotiate with curl, is a keytab file required?

Being a once-in-a-while-contributor to curl in that area. Here is what you need to know:

curl(1) itself knows nothing about Kerberos and will not interact neither with your credential cache nor your keytab file. It will delegate all calls to a GSS-API implementation which will do the magic for you. What magic depends on the library, Heimdal and MIT Kerberos.

Based on your question, I assume that you have little knowledge about Kerberos and want simply automate API calls to a REST endpoints secured by SPNEGO.

Here is what you need to do:

1. Have a Unix-like OS
2. Install at least MIT Kerberos 1.11
3. Install at least curl 7.38.0 against MIT Kerberos
4. Verify this with curl --version mentioning GSS-API and SPNEGO and with ldd linked against your MIT Kerberos version.
5. Create a client keytab for the service principal with ktutil or mskutil
6. Try to obtain a TGT with that client keytab by kinit -k -t <path-to-keytab> <principal-from-keytab>
7. Verify with klist that you have a ticket cache

Environment is now ready to go:

8. Export KRB5CCNAME=<some-non-default-path>
9. Export KRB5_CLIENT_KTNAME=<path-to-keytab>
10. Invoke curl --negotiate -u : <URL>

MIT Kerberos will detect that both environment variables are set, inspect them, automatically obtain a TGT with your keytab, request a service ticket and pass to curl. You are done.

Note: this will not work with Heimdal.