SPNEGO uses wrong KRBTGT principal name - linux

Question

I am trying to enable Kerberos authentication for our website - The idea is to have users logged into a Windows AD domain get automatic login (and initial account creation)
Before I tackle the Windows side of things, I wanted to get it work locally.
So I made a test KDC/KADMIN container using git#github.com:ist-dsi/docker-kerberos.git
Thee webserver is in a local docker container with nginx and the spnego module compiled in.
The KDC/KADMIN container is at 172.17.0.2 and accessible from my webserver container.
Here is my local krb.conf:
default_realm = SERVER.LOCAL
[realms]
SERVER.LOCAL = {
kdc_ports = 88,750
kadmind_port = 749
kdc = 172.17.0.2:88
admin_server = 172.17.0.2:749
}
[domain_realms]
.server.local = SERVER.LOCAL
server.local = SERVER.LOCAL
and the krb.conf on the webserver container
[libdefaults]
default_realm = SERVER.LOCAL
default_keytab_name = FILE:/etc/krb5.keytab
ticket_lifetime = 24h
kdc_timesync = 1
ccache_type = 4
forwardable = false
proxiable = false
[realms]
LOCALHOST.LOCAL = {
kdc_ports = 88,750
kadmind_port = 749
kdc = 172.17.0.2:88
admin_server = 172.17.0.2:749
}
[domain_realms]
.server.local = SERVER.LOCAL
server.local = SERVER.LOCAL
Here is the principals and keytab config (keytab is copied to the web container under /etc/krb5.keytab)
rep ~/project * rep_krb_test $ kadmin -p kadmin/admin#SERVER.LOCAL -w hunter2
Authenticating as principal kadmin/admin#SERVER.LOCAL with password.
kadmin: list_principals
K/M#SERVER.LOCAL
kadmin/99caf4af9dc5#SERVER.LOCAL
kadmin/admin#SERVER.LOCAL
kadmin/changepw#SERVER.LOCAL
krbtgt/SERVER.LOCAL#SERVER.LOCAL
noPermissions#SERVER.LOCAL
rep_movsd#SERVER.LOCAL
kadmin: q
rep ~/project * rep_krb_test $ ktutil
ktutil: addent -password -p rep_movsd#SERVER.LOCAL -k 1 -f
Password for rep_movsd#SERVER.LOCAL:
ktutil: wkt krb5.keytab
ktutil: q
rep ~/project * rep_krb_test $ kinit -C -p rep_movsd#SERVER.LOCAL
Password for rep_movsd#SERVER.LOCAL:
rep ~/project * rep_krb_test $ klist
Ticket cache: FILE:/tmp/krb5cc_1000
Default principal: rep_movsd#SERVER.LOCAL
Valid starting Expires Service principal
02/07/20 04:27:44 03/07/20 04:27:38 krbtgt/SERVER.LOCAL#SERVER.LOCAL
The relevant nginx config:
server {
location / {
uwsgi_pass django;
include /usr/lib/proj/lib/wsgi/uwsgi_params;
auth_gss on;
auth_gss_realm SERVER.LOCAL;
auth_gss_service_name HTTP;
}
}
Finally etc/hosts has
# use alternate local IP address
127.0.0.2 server.local server
Now I try to access this with curl:
* Trying 127.0.0.2:80...
* Connected to server.local (127.0.0.2) port 80 (#0)
* gss_init_sec_context() failed: Server krbtgt/LOCAL#SERVER.LOCAL not found in Kerberos database.
* Server auth using Negotiate with user ''
> GET / HTTP/1.1
> Host: server.local
> User-Agent: curl/7.71.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
....
As you can see it is trying to use the SPN "krbtgt/LOCAL#SERVER.LOCAL" whereas kinit has "krbtgt/SERVER.LOCAL#SERVER.LOCAL" as the SPN
How do I get this to work?
Thanks in advance..

Answer 1

So it turns out that I needed
auth_gss_service_name HTTP/server.local;
Some other tips for issues encountered:
Make sure the keytab file is readable by the web server process with user www-data or whatever user
Make sure the keytab principals are in the correct order
Use export KRB5_TRACE=/dev/stderr and curl to test - kerberos gives a very detailed log of what it's doing and why it fails