openssl how to check server name indication (SNI)

Question

I'm trying to verify whether a TLS client checks for server name indication (SNI).

I'm trying at first to reproduce the steps using openssl.

I tried to connect to google with this openssl command

openssl version
openSSL 1.1.1b  26 Feb 2019

openssl s_client -connect google.com:443 -servername "ibm.com"

Purposely I set the TLS extension -servername to a wrong domain, and I expect that openssl refuses connection, but it proceeds.

I also tried specifiying a CA certificate but nothing happened.

What should I expect when choosing a wrong SNI?

Log

openssl s_client -connect www.google.com:443 -servername "ibm.com"
CONNECTED(000001BC)
depth=1 C = US, O = Google Trust Services, CN = GTS CA 1O1
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 C = US, ST = California, L = Mountain View, O = Google LLC, CN = www.google.com
verify return:1
---
Certificate chain
 0 s:C = US, ST = California, L = Mountain View, O = Google LLC, CN = www.google.com
   i:C = US, O = Google Trust Services, CN = GTS CA 1O1
 1 s:C = US, O = Google Trust Services, CN = GTS CA 1O1
   i:OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=C = US, ST = California, L = Mountain View, O = Google LLC, CN = www.google.com

issuer=C = US, O = Google Trust Services, CN = GTS CA 1O1

---
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: RSA-PSS
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 3018 bytes and written 389 bytes
Verification error: unable to get local issuer certificate
---
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
Server public key is 2048 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 20 (unable to get local issuer certificate)
---

Approbate

Answer 1

The certificate you got back is not yours, it's Google's.

Google implements SNI. Your request states that it wants to establish a connection with the hostname "ibm.com" that Google does not know about. So it decides to answer with its own certificate.

If this was a browser, you would get a name mismatch warning of some kind.

So to answer your question, to test an invalid SNI, look for the hostname in the output. Here is a command I use:

echo -n | openssl s_client -connect google.com:443 -servername ibm.com | openssl x509 -noout -text | grep ibm.com

You won't get any hits, that means that either SNI is not supported or the server you are connecting to does not recognize it.

Daria

Answer 2

I am checking on a Mac and I had to make a modification by adding the -connect directive.

echo -n | openssl s_client -connect google.com:443 -servername ibm.com | openssl x509 -noout -text | grep ibm.com

Since we don't need to input anything, I'd update the command to close the openssl command by forcing input from /dev/null:

echo -n | openssl s_client -connect google.com:443 -servername ibm.com </dev/null | openssl x509 -noout -text | grep ibm.com

Mirilla

Using /dev/null is a good idea that would save the creation of a process but you should remove the first echo -n. – Daria