On Sun, 19 Mar 2023 at 12:25, Nathan Wagner <nw@xxxxxxxxxxxxxxx> wrote:
> I'm trying to compile openssh with openssl 3.1 on a linux machine with
> kernel 4.15.10. I seem to get stuck at:
>
> configure: error: OpenSSH has no source of random numbers. Please
> configure OpenSSL with an entropy source or re-run configure using one
> of the --with-prngd-port or --with-prngd-socket options
>
> I haven't done anything special in configuring openssl. If I have read
> the configuration for openssl correctly, with will default to the 'os'
> source, which I think then is getrandom(2).
There's also the getentropy interface that was added glibc relatively recently.
Does the OpenSSL self-test ("make tests") pass? Does its basic RNG
function work (eg "openssl rand -base64 9")? And if "openssl rand"
doesn't work, if you strace it what is it trying to do?
> I think the check in openssh for this is a call to RAND_status(), which
> is apparently returning a failure.
>
> I can't compile without openssl, because I need to allow RSA keys.
>
> Any work arounds? Ideas follow.
>
> Compile openssh with /dev/urandom as the prngd-socket?
No, the prngd socket interface works differently to /dev/random.
> Edit the configure script to force a success where RAND_status()
> is called?
Probably not. When it's built with OpenSSL, it just defers to OpenSSL
for random seeding.
> Call whatever openssl needs to initialize the random seed somewhere
> early in openssh startup? It may already do this, I see calls to
> RAND_seed() in sshd.c.
>
> More generally, would it make sense (on linux at least) to use
> getrandom() if available, or /dev/urandom otherwise regardless
> of whether or not openssl is used?
Probably not. RNG seeding is surprisingly convoluted due to the
varying standards over time (see entropy.c,
openbsd-compat/arc4random.c and openbsd-compat/bsd-getentropy.c) which
is why when it's built with OpenSSL, it just defers to OpenSSL.
You might be able to get this to compile, but if the RNG seeding in
your OpenSSL build is broken I would be concerned about what else
might be broken in it, possibly in some subtle way. I would be
looking at fixing your OpenSSL.
--
Darren Tucker (dtucker at dtucker.net)
GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860 37F4 9357 ECEF 11EA A6FA
Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.
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