{"id":1298,"date":"2023-01-22T17:21:02","date_gmt":"2023-01-22T23:21:02","guid":{"rendered":"https:\/\/www.nathanhunstad.com\/blog\/?p=1298"},"modified":"2023-01-22T17:24:08","modified_gmt":"2023-01-22T23:24:08","slug":"adding-a-san-to-a-certificate-using-openssl","status":"publish","type":"post","link":"https:\/\/www.nathanhunstad.com\/blog\/2023\/01\/adding-a-san-to-a-certificate-using-openssl\/","title":{"rendered":"Adding a SAN to a certificate using OpenSSL"},"content":{"rendered":"\n

A long time ago, I set up an internal PKI<\/a> so I could create my own TLS certificates to add to internal devices and HTTPS servers. I used this primarily for my EdgeOS router since that was the main device I would log into that would give me a warning about untrusted certificates.<\/p>\n\n\n\n

That was all the way back in 2016. Since then, I’ve kind of kept on top of things, renewing the certificate when it expired a few years ago, but it had expired yet again so I was faced with that dreaded warning when connecting to my router. This time, though, creating a new cert using the old commands wasn’t good enough, because it was missing a Subject Alternate Name (SAN), and that prompted a slightly different warning with the same result: no trust in the browser.<\/p>\n\n\n\n

What happened? Using a SAN instead of the Common Name for certificate validation has been required by browsers for a while (since 2017 in Chrome<\/a> for example). This is because the Common Name is ambiguous, whereas the SAN can specify a domain, IP, or URI; if you want the gnarly details, see RFC2818<\/a> and RFC6125<\/a>.<\/p>\n\n\n\n

This wasn’t a problem in 2016 since browsers weren’t requiring this, but it’s been an issue for a while now. Unfortunately, it’s a bit harder to create a certificate-signing request (CSR) and sign a certificate in such a way that a SAN is included. There are ways you can do this using OpenSSL configuration and extension files, but I was looking for one-liners like I could do before.<\/p>\n\n\n\n

Eventually, thanks to some searching, I was able to find a way to add a DNS SAN to a certificate and still make it a one-liner for the most part. To start by generating the CSR, I ran the following command (replaced FQDN with the FQDN of the target server):<\/p>\n\n\n\n

openssl req -new -sha256 -subj \"\/C=US\/ST=Minnesota\/L=Minneapolis\/O=MyOrg\/CN=FQDN\" -addext \"subjectAltName = DNS:FQDN\" -key private.key -out req.csr<\/code><\/pre>\n\n\n\n
That generated a CSR with the right extension, as you can see when looking at the text dump via OpenSSL:<\/p>\n\n\n\n
 Attributes:\n            Requested Extensions:\n                X509v3 Subject Alternative Name:\n                    DNS:FQDN\n<\/code><\/pre>\n\n\n\n
I thought that was it and signing it the old way would be all that I’d need, but nope, it didn’t include the SAN (a fact that a user calls out explicitly<\/a> in the page where I found this. So I needed to find out how to sign it with the extension as well. After a bit more digging, I found that this would do the trick<\/p>\n\n\n\n
openssl x509 -sha256 -req  -days 1096 -in req.csr -CA ia.crt -CAkey ia.key -extensions SAN -extfile <(cat \/etc\/ssl\/openssl.cnf <(printf \"\\n[SAN]\\nsubjectAltName=DNS:FQDN\")) -set_serial 4 -out cert.pem<\/code><\/pre>\n\n\n\n
This essentially creates the proper configuration on the fly, and as a result the certificate that is generated has the proper SAN. After adding to the router and restarting the lighttpd daemon, we are trusted again!<\/p>\n\n\n\n
\"\"<\/a>
Success!<\/figcaption><\/figure>\n\n\n\n
If there was a need to add another SAN such as an IP address, the same method could be used.<\/p>\n\n\n\n
That’s it!<\/p>\n","protected":false},"excerpt":{"rendered":"
A long time ago, I set up an internal PKI so I could create my own TLS certificates to add to internal devices and HTTPS servers. I used this primarily for my EdgeOS router since that was the main device I would log into that would give me a warning about untrusted certificates. That was… Continue reading Adding a SAN to a certificate using OpenSSL<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[1],"tags":[286,285,287],"class_list":["post-1298","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-openssl","tag-security","tag-tls","entry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t