linux-kernel - What's a good default TTL for DNS keys in the kernel

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Date:   Tue, 14 Apr 2020 15:20:10 +0100
From:   David Howells <dhowells@...hat.com>
To:     linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        linux-afs@...ts.infradead.org, ceph-devel@...r.kernel.org
cc:     dhowells@...hat.com, keyrings@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        fweimer@...hat.com
Subject: What's a good default TTL for DNS keys in the kernel

Since key.dns_resolver isn't given a TTL for the address information obtained
for getaddrinfo(), no expiry is set on dns_resolver keys in the kernel for
NFS, CIFS or Ceph.  AFS gets one if it looks up a cell SRV or AFSDB record
because that is looked up in the DNS directly, but it doesn't look up A or
AAAA records, so doesn't get an expiry for the addresses themselves.

I've previously asked the libc folks if there's a way to get this information
exposed in struct addrinfo, but I don't think that ended up going anywhere -
and, in any case, would take a few years to work through the system.

For the moment, I think I should put a default on any dns_resolver keys and
have it applied either by the kernel (configurable with a /proc/sys/ setting)
or by the key.dnf_resolver program (configurable with an /etc file).

Any suggestion as to the preferred default TTL?  10 minutes?

David