How can openssl verify the server certificates' chain without Root CA certificate

Question

1) Here openssl verifies the www.google.com certificate, telling me everything is fine, see last line from the openssl return output:

Verify return code: 0 (ok)

2) But the actual root CA certificate is not sent, here the last intermediate CA certificate referring to the Root CA certificate:

2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
  i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority

3) And the openssl command executed has no CAfile specified, which would include the Root CA certificate to complete the chain:

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

So how can openssl verify the whole server certificates' chain up to the root CA?

Here is the whole output:

# openssl  s_client -connect www.google.com:443 -showcerts
CONNECTED(00000003)
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
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
 1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2
   i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
-----BEGIN CERTIFICATE-----
MIIEKDCCAxCgAwIBAgIQAQAhJYiw+lmnd+8Fe2Yn3zANBgkqhkiG9w0BAQsFADBC
MQswCQYDVQQGEwJVUzEWMBQGA1UEChMNR2VvVHJ1c3QgSW5jLjEbMBkGA1UEAxMS
R2VvVHJ1c3QgR2xvYmFsIENBMB4XDTE3MDUyMjExMzIzN1oXDTE4MTIzMTIzNTk1
OVowSTELMAkGA1UEBhMCVVMxEzARBgNVBAoTCkdvb2dsZSBJbmMxJTAjBgNVBAMT
HEdvb2dsZSBJbnRlcm5ldCBBdXRob3JpdHkgRzIwggEiMA0GCSqGSIb3DQEBAQUA
A4IBDwAwggEKAoIBAQCcKgR3XNhQkToGo4Lg2FBIvIk/8RlwGohGfuCPxfGJziHu
Wv5hDbcyRImgdAtTT1WkzoJile7rWV/G4QWAEsRelD+8W0g49FP3JOb7kekVxM/0
Uw30SvyfVN59vqBrb4fA0FAfKDADQNoIc1Fsf/86PKc3Bo69SxEE630k3ub5/DFx
+5TVYPMuSq9C0svqxGoassxT3RVLix/IGWEfzZ2oPmMrhDVpZYTIGcVGIvhTlb7j
gEoQxirsupcgEcc5mRAEoPBhepUljE5SdeK27QjKFPzOImqzTs9GA5eXA37Asd57
r0Uzz7o+cbfe9CUlwg01iZ2d+w4ReYkeN8WvjnJpAgMBAAGjggERMIIBDTAfBgNV
HSMEGDAWgBTAephojYn7qwVkDBF9qn1luMrMTjAdBgNVHQ4EFgQUSt0GFhu89mi1
dvWBtrtiGrpagS8wDgYDVR0PAQH/BAQDAgEGMC4GCCsGAQUFBwEBBCIwIDAeBggr
BgEFBQcwAYYSaHR0cDovL2cuc3ltY2QuY29tMBIGA1UdEwEB/wQIMAYBAf8CAQAw
NQYDVR0fBC4wLDAqoCigJoYkaHR0cDovL2cuc3ltY2IuY29tL2NybHMvZ3RnbG9i
YWwuY3JsMCEGA1UdIAQaMBgwDAYKKwYBBAHWeQIFATAIBgZngQwBAgIwHQYDVR0l
BBYwFAYIKwYBBQUHAwEGCCsGAQUFBwMCMA0GCSqGSIb3DQEBCwUAA4IBAQDKSeWs
12Rkd1u+cfrP9B4jx5ppY1Rf60zWGSgjZGaOHMeHgGRfBIsmr5jfCnC8vBk97nsz
qX+99AXUcLsFJnnqmseYuQcZZTTMPOk/xQH6bwx+23pwXEz+LQDwyr4tjrSogPsB
E4jLnD/lu3fKOmc2887VJwJyQ6C9bgLxRwVxPgFZ6RGeGvOED4Cmong1L7bHon8X
fOGLVq7uZ4hRJzBgpWJSwzfVO+qFKgE4h6LPcK2kesnE58rF2rwjMvL+GMJ74N87
L9TQEOaWTPtEtyFkDbkAlDASJodYmDkFOA/MgkgMCkdm7r+0X8T/cKjhf4t5K7hl
MqO5tzHpCvX2HzLc
-----END CERTIFICATE-----
 2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
   i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
---
Server certificate
subject=/C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
issuer=/C=US/O=Google Inc/CN=Google Internet Authority G2
---
No client certificate CA names sent
Peer signing digest: SHA256
Server Temp Key: ECDH, P-256, 256 bits
---
SSL handshake has read 3832 bytes and written 434 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256
    Session-ID: 7D57BFBABDC7BA509D54CDDAFEA198BD74EAE81735FA9F00F899A709215A0594
    Session-ID-ctx:
    Master-Key: 54C91529C5098B4C19D58E560EAF17EC48F46B9FA3AC8814DEEB5356C5C0CD80731B4D8811022DFD77AB7FA9C8049964
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket lifetime hint: 100799 (seconds)
    TLS session ticket:
    0000 - 00 5b 6a fd 5b 84 91 88-da 5f e6 d2 51 35 e3 db   .[j.[...._..Q5..
    0010 - 95 61 bd e3 9c 7a 09 1a-60 ca 1a 49 ad fd 30 19   .a...z..`..I..0.
    0020 - 98 14 54 b6 f4 23 93 35-09 f3 44 6a f0 fe 39 5c   ..T..#.5..Dj..9\
    0030 - 39 58 a5 9a 59 58 d6 04-49 5e bf 24 15 90 3d cd   9X..YX..I^.$..=.
    0040 - 88 47 09 1e bd a9 af 76-fa 68 ba ba bd 53 e4 68   .G.....v.h...S.h
    0050 - de bb 33 81 59 48 ae 5f-8a fa 62 7b 9d dc 69 db   ..3.YH._..b{..i.
    0060 - 5b fd 1e 80 11 fb 62 b0-af 1b 9e bc 40 84 fb 74   [[email protected]
    0070 - 6d 79 47 54 fa 08 a6 2d-6b 46 a9 c0 80 99 36 76   myGT...-kF....6v
    0080 - 58 53 bf 99 8a a4 53 48-18 93 7a b0 38 f5 dd 87   XS....SH..z.8...
    0090 - b7 e7 ca a4 3e d1 79 7f-9c 84 d1 16 e0 41 dc e3   ....>.y......A..
    00a0 - a6 a8 cb 4d 34 48 f5 d5-75 40 4c 9c 4d ae 61 f6   [email protected].
    00b0 - e8 23 1e 27 bb e2 d6 5c-9e bb b7 a1 96 e6 c4 ef   .#.'...\........
    00c0 - 4d c6 9b 29 af be d1 fd-70 69 7a 11 f6 b6 59 a4   M..)....piz...Y.
    00d0 - e0 c0 db 7f e9                                    .....

    Start Time: 1509720918
    Timeout   : 300 (sec)
    **Verify return code: 0 (ok)**
---

Answer 1

And the openssl command executed has no CAfile specified, which would include the Root CA certificate to complete the chain

Absent a directive telling it what should be trusted, it uses the default system trust list. By convention that's /etc/ssl/certs/* (and/or multiple values from /etc/ssl/certs/ca-certificates.crt`), but the specific directory (and/or multi-certificate file) are build-time options and vary by distro (though frequently they leave symbolic links behind to allow convention-based programs to still work).

In this particular case, for my machine, the chain doesn't actually end at Equifax Secure Certificate Authority, because I don't trust that one. But I do happen to trust GeoTrust Global CA, so OpenSSL builds the chain using the 0 and 1 certs from the TLS connection and ignores the 2 cert as belonging to a chain that it doesn't care about, using the one from the trusted store instead.

$ openssl x509 -in /etc/ssl/certs/2c543cd1.0 -text -noout
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 144470 (0x23456)
    Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=US, O=GeoTrust Inc., CN=GeoTrust Global CA
        Validity
            Not Before: May 21 04:00:00 2002 GMT
            Not After : May 21 04:00:00 2022 GMT
        Subject: C=US, O=GeoTrust Inc., CN=GeoTrust Global CA
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:da:cc:18:63:30:fd:f4:17:23:1a:56:7e:5b:df:
                    3c:6c:38:e4:71:b7:78:91:d4:bc:a1:d8:4c:f8:a8:
                    43:b6:03:e9:4d:21:07:08:88:da:58:2f:66:39:29:
                    bd:05:78:8b:9d:38:e8:05:b7:6a:7e:71:a4:e6:c4:
                    60:a6:b0:ef:80:e4:89:28:0f:9e:25:d6:ed:83:f3:
                    ad:a6:91:c7:98:c9:42:18:35:14:9d:ad:98:46:92:
                    2e:4f:ca:f1:87:43:c1:16:95:57:2d:50:ef:89:2d:
                    80:7a:57:ad:f2:ee:5f:6b:d2:00:8d:b9:14:f8:14:
                    15:35:d9:c0:46:a3:7b:72:c8:91:bf:c9:55:2b:cd:
                    d0:97:3e:9c:26:64:cc:df:ce:83:19:71:ca:4e:e6:
                    d4:d5:7b:a9:19:cd:55:de:c8:ec:d2:5e:38:53:e5:
                    5c:4f:8c:2d:fe:50:23:36:fc:66:e6:cb:8e:a4:39:
                    19:00:b7:95:02:39:91:0b:0e:fe:38:2e:d1:1d:05:
                    9a:f6:4d:3e:6f:0f:07:1d:af:2c:1e:8f:60:39:e2:
                    fa:36:53:13:39:d4:5e:26:2b:db:3d:a8:14:bd:32:
                    eb:18:03:28:52:04:71:e5:ab:33:3d:e1:38:bb:07:
                    36:84:62:9c:79:ea:16:30:f4:5f:c0:2b:e8:71:6b:
                    e4:f9
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Subject Key Identifier:
                C0:7A:98:68:8D:89:FB:AB:05:64:0C:11:7D:AA:7D:65:B8:CA:CC:4E
            X509v3 Authority Key Identifier:
                keyid:C0:7A:98:68:8D:89:FB:AB:05:64:0C:11:7D:AA:7D:65:B8:CA:CC:4E

    Signature Algorithm: sha1WithRSAEncryption
         35:e3:29:6a:e5:2f:5d:54:8e:29:50:94:9f:99:1a:14:e4:8f:
         78:2a:62:94:a2:27:67:9e:d0:cf:1a:5e:47:e9:c1:b2:a4:cf:
         dd:41:1a:05:4e:9b:4b:ee:4a:6f:55:52:b3:24:a1:37:0a:eb:
         64:76:2a:2e:2c:f3:fd:3b:75:90:bf:fa:71:d8:c7:3d:37:d2:
         b5:05:95:62:b9:a6:de:89:3d:36:7b:38:77:48:97:ac:a6:20:
         8f:2e:a6:c9:0c:c2:b2:99:45:00:c7:ce:11:51:22:22:e0:a5:
         ea:b6:15:48:09:64:ea:5e:4f:74:f7:05:3e:c7:8a:52:0c:db:
         15:b4:bd:6d:9b:e5:c6:b1:54:68:a9:e3:69:90:b6:9a:a5:0f:
         b8:b9:3f:20:7d:ae:4a:b5:b8:9c:e4:1d:b6:ab:e6:94:a5:c1:
         c7:83:ad:db:f5:27:87:0e:04:6c:d5:ff:dd:a0:5d:ed:87:52:
         b7:2b:15:02:ae:39:a6:6a:74:e9:da:c4:e7:bc:4d:34:1e:a9:
         5c:4d:33:5f:92:09:2f:88:66:5d:77:97:c7:1d:76:13:a9:d5:
         e5:f1:16:09:11:35:d5:ac:db:24:71:70:2c:98:56:0b:d9:17:
         b4:d1:e3:51:2b:5e:75:e8:d5:d0:dc:4f:34:ed:c2:05:66:80:
         a1:cb:e6:33