Why doesn't openssl s_client correctly validate google.com:443?

Question

I try

$ openssl s_client -connect www.google.com:443

but it openssl complains that the cert chain is invalid:

$ openssl s_client -connect www.google.com:443
CONNECTED(00000003)
depth=2 C = US, O = GeoTrust Inc., CN = GeoTrust Global CA
verify error:num=20:unable to get local issuer certificate
verify return:0

Certificate chain
 0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
   i:/C=US/O=Google Inc/CN=Google Internet Authority G2
 1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2
   i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
 2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
   i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority

Why is this happening? The Equifax root cert is in my /etc/ssl/certs directory, and if I download the chain and use verify, openssl will verify the chain.

$ uname -a
Linux moxie 3.13.0-74-generic #118-Ubuntu SMP Thu Dec 17 22:52:02 UTC 2015 i686 i686 i686 GNU/Linux
$ openssl version
OpenSSL 1.0.1f 6 Jan 2014
$ openssl version -d
OPENSSLDIR: "/usr/lib/ssl"

Answer 1

On OpenSSL 1.0.1f

$ openssl version
OpenSSL 1.0.1f 6 Jan 2014

$ openssl s_client -connect www.google.com:443
CONNECTED(00000003)
depth=2 C = US, O = GeoTrust Inc., CN = GeoTrust Global CA
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
 0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
   i:/C=US/O=Google Inc/CN=Google Internet Authority G2
 1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2
   i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
 2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
   i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
---

On OpenSSL 1.0.2e

$ openssl version
OpenSSL 1.0.2e 3 Dec 2015

$ openssl s_client -connect www.google.com:443
CONNECTED(00000003)
depth=3 C = US, O = Equifax, OU = Equifax Secure Certificate Authority
verify return:1
depth=2 C = US, O = GeoTrust Inc., CN = GeoTrust Global CA
verify return:1
depth=1 C = US, O = Google Inc, CN = Google Internet Authority G2
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google Inc, CN = www.google.com
verify return:1
---
Certificate chain
 0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
   i:/C=US/O=Google Inc/CN=Google Internet Authority G2
 1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2
   i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
 2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
   i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
---

So looks like there is a change in behaviour from the version which you have. You could either specify the CApath explicitly or upgrade to a newer version.

Answer 2

You need to explicitly specify the CA-path (at least on Ubuntu):

openssl s_client -connect www.google.com:443 -CApath /etc/ssl/certs/

Answer 3

You need to read your error message, there's an answer :

unable to get local issuer certificate

the magic word is local. Point them to the folder where your root SSL certs are living, like that :

i# openssl s_client -verify on -CApath /etc/ssl/certs -connect  www.google.com:443
verify depth is 0
CONNECTED(00000003)
depth=3 C = US, O = Equifax, OU = Equifax Secure Certificate Authority
verify return:1
depth=2 C = US, O = GeoTrust Inc., CN = GeoTrust Global CA
verify return:1
depth=1 C = US, O = Google Inc, CN = Google Internet Authority G2
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google Inc, CN = google.com
---- snip ---   
verify return:1
Start Time: 1458160977
Timeout   : 300 (sec)
Verify return code: 0 (ok)
---
^C