`openssl`: Subject Alternative Name

This might not work under every circumstance, but try

openssl s_client -connect google.com:443 2>&1 | openssl x509 -text | grep DNS

What @stuart-p-bentley wrote got me thinking and I came up with this way of getting a comma delimited list of "Subject Alternative Names" using openssl, awk and tr. The sed line in his answer does not work on FreeBSD per example.

openssl s_client -connect google.com:443 2>&1 | openssl x509 -text | awk '/X509v3 Subject Alternative Name/ {getline;gsub(/ /, "", $0); print}' | tr -d "DNS:"

Here is what you get with google.com

*.google.com,*.android.com,*.appengine.google.com,*.cloud.google.com,*.google-analytics.com,*.google.ca,*.google.cl,*.google.co.in,*.google.co.jp,*.google.co.uk,*.google.com.ar,*.google.com.au,*.google.com.br,*.google.com.co,*.google.com.mx,*.google.com.tr,*.google.com.vn,*.google.de,*.google.es,*.google.fr,*.google.hu,*.google.it,*.google.nl,*.google.pl,*.google.pt,*.googleadapis.com,*.googleapis.cn,*.googlecommerce.com,*.googlevideo.com,*.gstatic.cn,*.gstatic.com,*.gvt1.com,*.gvt2.com,*.urchin.com,*.url.google.com,*.youtube-nocookie.com,*.youtube.com,*.youtubeeducation.com,*.ytimg.com,android.com,g.co,goo.gl,google-analytics.com,google.com,googlecommerce.com,urchin.com,youtu.be,youtube.com,youtubeeducation.com

Here's a version that will work in every circumstance (and strips leading space):

openssl s_client -connect google.com:443 2>&1 | openssl x509 -text |
  sed -nr '/^ {12}X509v3 Subject Alternative Name/{n;s/^ *//p}'