With SNI

If the remote server is using SNI (that is, sharing multiple SSL hosts on a single IP address) you will need to send the correct hostname in order to get the right certificate.

openssl s_client -showcerts -servername www.example.com -connect www.example.com:443 </dev/null

Without SNI

If the remote server is not using SNI, then you can skip -servername parameter:

openssl s_client -showcerts -connect www.example.com:443 </dev/null

To view the full details of a site's cert you can use this chain of commands as well:

$ echo | \
    openssl s_client -servername www.example.com -connect www.example.com:443 2>/dev/null | \
    openssl x509 -text

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  Hmm. I still get the same error when trying that command. I noticed my Openssl version is 'OpenSSL 0.9.8g 19 Oct 2007'. Do you have any ideas? – nasty pasty Oct 26 '11 at 2:07
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  Useful: echo "" | openssl s_client -connect server:port -prexit 2>/dev/null | sed -n -e '/BEGIN\ CERTIFICATE/,/END\ CERTIFICATE/ p' stackoverflow.com/a/12918442/843000 – mbrownnyc Jan 9 '13 at 20:31
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  Alternative useful script, from madboa.com: echo | openssl s_client -connect server:port 2>&1 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > cert.pem – rmeakins Aug 5 '13 at 5:44
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  To make this a bit more concise, you can replace the sed with openssl x509, and read it in using a sub-shell: openssl x509 -in <(openssl s_client -connect server:port -prexit 2>/dev/null) – Gabe Martin-Dempesy Aug 14 '13 at 17:28
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  Also echo | openssl s_client -connect google.com:443 2>/dev/null | openssl x509 – MattSizzle Jan 15 '14 at 3:35

While I agree with Ari's answer (and upvoted it :), I needed to do an extra step to get it to work with Java on Windows (where it needed to be deployed):

openssl s_client -showcerts -connect www.example.com:443 < /dev/null | openssl x509 -outform DER > derp.der

Before adding the openssl x509 -outform DER conversion, I was getting an error from keytool on Windows complaining about the certificate's format. Importing the .der file worked fine.

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  Odd. I've been using PEM certificates with keytool on Windows since Java 6 and never faced an issue. – imgx64 Nov 1 '17 at 5:19

It turns out there is more complexity here: I needed to provide many more details to get this rolling. I think its something to do with the fact that its a connection that needs client authentication, and the hankshake needed more info to continue to the stage where the certificates were dumped.

Here is my working command:

openssl s_client -connect host:port -key our_private_key.pem -showcerts \
                 -cert our_server-signed_cert.pem

Hopefully this is a nudge in the right direction for anyone who could do with some more info.

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  I am sorry, but your answer doesn't make much sense. You needed to pass the certificate to the server in order to get the certificate? – Ari Maniatis Nov 14 '11 at 22:12
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  Yep. Client authentication AFAIK. – nasty pasty Nov 15 '11 at 2:34
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  It turns out '-prexit' will return that data as well. E.g.; openssl s_client -connect host:port -prexit – Robert Sep 20 '12 at 12:54

The easiest command line for this, which includes the PEM output to add it to the keystore, as well as a human readable output and also supports SNI, which is important if you are working with an HTTP server is:

openssl s_client -servername example.com -connect example.com:443 \
    </dev/null 2>/dev/null | openssl x509 -text

The -servername option is to enable SNI support and the openssl x509 -text prints the certificate in human readable format.

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  You may add to your -servername your subdomain, for instance ws.example.com instead of example.com (apply this to the -connect parameter too). – russellhoff Jan 26 '17 at 11:26

To get the certificate of remote server you can use openssl tool and you can find it between BEGIN CERTIFICATE and END CERTIFICATE which you need to copy and paste into your certificate file (CRT).

Here is the command demonstrating it:

ex +'/BEGIN CERTIFICATE/,/END CERTIFICATE/p' <(echo | openssl s_client -showcerts -connect example.com:443) -scq > file.crt

To return all certificates from the chain, just add g (global) like:

ex +'g/BEGIN CERTIFICATE/,/END CERTIFICATE/p' <(echo | openssl s_client -showcerts -connect example.com:443) -scq

Then you can simply import your certificate file (file.crt) into your keychain and make it trusted, so Java shouldn't complain.

On OS X you can double-click on the file or drag and drop in your Keychain Access, so it'll appear in login/Certificates. Then double-click on the imported certificated and make it Always Trust for SSL.

On CentOS 5 you can append them into /etc/pki/tls/certs/ca-bundle.crt file (and run: sudo update-ca-trust force-enable), or in CentOS 6 copy them into /etc/pki/ca-trust/source/anchors/ and run sudo update-ca-trust extract.

In Ubuntu, copy them into /usr/local/share/ca-certificates and run sudo update-ca-certificates.

You can get and store the server root certificate using next bash script:

CERTS=$(echo -n | openssl s_client -connect $HOST_NAME:$PORT -showcerts | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p')
echo "$CERTS" | awk -v RS="-----BEGIN CERTIFICATE-----" 'NR > 1 { printf RS $0 > "'$SERVER_ROOT_CERTIFICATE'"; close("'$SERVER_ROOT_CERTIFICATE'") }'

Just overwrite required variables.

HOST=gmail-pop.l.google.com
PORT=995

openssl s_client -servername $HOST -connect $HOST:$PORT < /dev/null 2>/dev/null | openssl x509 -outform pem

to print only the certificate chain and not the server's certificate:

# MYHOST=myhost.com
# MYPORT=443
# openssl s_client -connect ${MYHOST}:${MYPORT} -showcerts 2>/dev/null </dev/null | awk '/^.*'"${MYHOST}"'/,/-----END CERTIFICATE-----/{next;}/-----BEGIN/,/-----END CERTIFICATE-----/{print}'

to update CA trust on CentOS/RHEL 6/7 :

# update-ca-trust enable
# openssl s_client -connect ${MYHOST}:${MYPORT} -showcerts 2>/dev/null </dev/null | awk '/^.*'"${MYHOST}"'/,/-----END CERTIFICATE-----/{next;}/-----BEGIN/,/-----END CERTIFICATE-----/{print}' >/etc/pki/ca-trust/source/anchors/myca.cert
# update-ca-trust extract

on CentOS/RHEL 5:

# openssl s_client -connect ${MYHOST}:${MYPORT} -showcerts 2>/dev/null </dev/null | awk '/^.*'"${MYHOST}"'/,/-----END CERTIFICATE-----/{next;}/-----BEGIN/,/-----END CERTIFICATE-----/{print}' >>/etc/pki/tls/certs/ca-bundle.crt

For the benefit of others like me who tried to follow the good advice here when accessing AWS CloudFront but failed, the trick is to add -servername domain.name...

Source: https://serverfault.com/a/780450/8972