Apache conf with several crt files - apache

Question

I bought Comodo PositiveSSL and got 4 crt files:
AddTrustExternalCARoot.crt
COMODORSAAddTrustCA.crt
COMODORSADomainValidationSecureServerCA.crt
domain.com.crt
And I have this config:
<VirtualHost *:443>
ServerName domain.com
ServerAlias www.domain.com
SSLEngine on
SSLCertificateFile /var/www/domain.com/domain.com.crt
SSLCertificateKeyFile /var/www/domain.com/domain.com.key
ServerAdmin webmaster#localhost
DocumentRoot /var/www/domain.com/html
<Directory /var/www/domain.com/html>
Options Indexes FollowSymLinks MultiViews
AllowOverride All
Order allow,deny
allow from all
</Directory>
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
How do I use these 3 files:
AddTrustExternalCARoot.crt
COMODORSAAddTrustCA.crt
COMODORSADomainValidationSecureServerCA.crt
https connection works fines, but it seems that browsers don't see my signature. I think the problem is to add all 4 files to apache config, but I don't know how to do it.

Answer 1

These files are the certificate chain. There's a root domain certificate, there are intermediate certificates, and there's your own certificate.
Your own certificate is already referenced with the SSLCertificateFile. The root certificate is usually installed in the user's browser (that's what you pay for … the fact that they paid the browser vendor to include their root certificate).
But your certificate is not directly derived from the root certificate, but there are these intermediate certificates.
Because you do not have a certificate that is directly derived from one of the root certificates in the browser, you must deliver the entire certificate chain to the user. (Yes, the root cert, too, to have a complete chain.)
It is usually done by putting all of the three files into one cert file (let's say intermediate.comodo.crt) and referencing them in the Apache config, too. It would look like this:
…
SSLEngine on
SSLCertificateFile /var/www/domain.com/domain.com.crt
SSLCertificateChainFile /var/www/domain.com/intermediate.comodo.crt
SSLCertificateKeyFile /var/www/domain.com/domain.com.key
…
The certificates in this file must be in the right order … root on top, and then down the chain (IIRC, but you may need to try different orders).
If your server is public, use the SSLlabs service to test your setup: https://www.ssllabs.com/ssltest/ (Note, when testing multiple times with different configurations, you must clear their cache after each change. Otherwise you'll instantly get the results from their last test of your server.)