Certificate chain checker

How a certificate chain works

Your site’s certificate is signed by an intermediate certificate, which is signed by a root certificate. Browsers and operating systems ship with a list of trusted roots. To trust your certificate, a client has to follow the signatures from it, through the intermediates, to one of those roots.

The server’s job is to send its own certificate followed by the intermediates. The root stays on the client. If an intermediate is missing, the client can’t make the link and the connection fails, unless the client fills the gap itself.

How to read the result

1. Certificate 1 is the site certificate: it should list your domain and be within its dates.
2. The certificates after it are the intermediates the server sent, each issued by the next one down.
3. The last one is the root, taken from the trust store when the chain leads to one. If it doesn’t, the result marks the root as untrusted, or shows a torn ticket where the missing intermediate should be.

Common chain errors and how to fix them

Missing intermediate

The server sends only its own certificate. Fix it by serving the full chain file your CA or ACME client provides. With certbot and nginx, that means fullchain.pem, not cert.pem:

ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

Hosting panels often have a separate “CA bundle” or “chain” field for the intermediates.

Untrusted root or self-signed certificate

The chain ends in a root that isn’t in the public trust stores: a self-signed certificate, a company’s private CA, or a test setup. For a public site, replace it with a certificate from a public CA. Let’s Encrypt is free.

Expired intermediate

The site certificate is current but an intermediate in the chain has expired. This usually means an old bundle saved on the server. Download the current chain from your CA and serve that.

Hostname mismatch

Not a chain problem, but often reported alongside one: the certificate doesn’t list the domain you visited. Issue one that does, or check that the domain points at the right server.

Check a chain from the command line

This prints every certificate the server sends, with its subject and issuer:

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

Each certificate appears as a block, starting with “s:” (subject) and “i:” (issuer). The issuer of each should be the subject of the next. More in Fix “unable to get local issuer certificate”.