Kerberos NFS マウントを実行すると、rpc.gssd が 'No credentials found for connection to server' とログに記録する理由

Issue

nfsv4/kerberos マウントの試行が失敗しています。以下の設定後、

SECURE_NFS="yes"

あるいは、

RPCGSSDARGS="-v -v -v"

/etc/sysconfig/nfs には次の出力が表示されます。

#  mount -t nfs4 -o sec=krb5 nfsserver:/homedir /mnt
[..]
nfsclient rpc.gssd[4653]: handling gssd upcall (/var/lib/nfs/rpc_pipefs/nfs/clnt41)
nfsclient rpc.gssd[4653]: handle_gssd_upcall: 'mech=krb5 uid=0 enctypes=18,17,16,23,3,1,2 '
nfsclient rpc.gssd[4653]: handling krb5 upcall (/var/lib/nfs/rpc_pipefs/nfs/clnt41)
nfsclient rpc.gssd[4653]: process_krb5_upcall: service is '<null>'
nfsclient rpc.gssd[4653]: Full nfsclient for 'nfsserver.example.org' is 'nfsserver.example.org'
nfsclient rpc.gssd[4653]: Full nfsclient for 'nfsclient.example.org' is 'nfsclient.example.org'
nfsclient rpc.gssd[4653]: No key table entry found for NFSCLIENT.EXAMPLE.ORG$@EXAMPLE.ORG while getting keytab entry for 'NFSCLIENT.EXAMPLE.ORG$@EXAMPLE.ORG'
nfsclient rpc.gssd[4653]: No key table entry found for root/nfsclient.example.org@EXAMPLE.ORG while getting keytab entry for 'root/nfsclient.example.org@EXAMPLE.ORG'
nfsclient rpc.gssd[4653]: Success getting keytab entry for 'nfs/nfsclient.example.org@EXAMPLE.ORG'
nfsclient rpc.gssd[4653]: WARNING: Client 'nfs/nfsclient.example.org@EXAMPLE.ORG' not found in Kerberos database while getting initial ticket for principal 'nfs/nfsclient.example.org@EXAMPLE.ORG' using keytab 'WRFILE:/etc/krb5.keytab'
nfsclient rpc.gssd[4653]: ERROR: No credentials found for connection to server nfsserver.example.org
nfsclient rpc.gssd[4653]: doing error downcall
[..]
mount.nfs4: access denied by server while mounting nfsserver:/exportdir
[..]
nfsclient rpc.gssd[4653]: destroying client /var/lib/nfs/rpc_pipefs/nfs/clnt42
nfsclient rpc.gssd[4653]: destroying client /var/lib/nfs/rpc_pipefs/nfs/clnt41