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From:   Pradeep <pradeepthomas@gmail.com>
Date:   Tue, 23 Mar 2021 14:56:36 -0700
Message-ID: <CAD8zhTB-ie445UwCJnd9qW242EzcFX9SuWJxvaU3KnB1h-dFyg@mail.gmail.com>
Subject: NFSv4 referrals with FQDN.
To:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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Hello,

While testing NFSv4 referrals, I noticed that if the server name in FS
locations does not have IPv4 mapping (server name has AAAA record for
IPv6; but no A record in DNS), the referral mount fails. With debug
enabled, I get something like this:

 nfs_follow_referral: referral at /nfs_export_1
 nfs4_path: path server-1.domain.com:/nfs_export_1 from nfs_path
 nfs4_path: path component /nfs_export_1
 nfs4_validate_fspath: comparing path /nfs_export_1 with fsroot /nfs_export_1
 ==> dns_query((null),server-2.domain.com,19,(null))
 call request_key(,server-2.domain.com,)
 <== dns_query() = -126
 nfs_follow_referral: done
 nfs_do_refmount: done
 RPC:       shutting down nfs client for server-1.domain.com
 RPC:       rpc_release_client(ffff97fdf170c600)
 RPC:       destroying nfs client for server-1.domain.com
 <-- nfs_d_automount(): error -2

It looks like NFS client does an upcall to "/sbin/key.dns_resolver".
"/sbin/key.dns_resolver" works if callout info is set to 'ipv6'.
Otherwise it fails too.

Does this mean setups with only IPv6 records (AAAA records in DNS),
NFSv4 referrals won't work if server returns FQDN in referral? If
anyone has tried this and made it work, please let me know.

Thanks,
Pradeep