Using letsencrypt certificates with DANE

If, like me, you run your own cloud server, at some point you need TLS certificates to export secure services.  Like a lot of people I object to paying a so called X.509 authority a yearly fee just to get a certificate, so I’ve been using a free startcom one for a while.  With web browsers delisting startcom, I’m unable to get a new usable certificate from them, so I’ve been investigating letsencrypt instead (if you’re in to fun ironies, by the way, you can observe that currently the letsencrypt site isn’t using a letsencrypt certificate, perhaps indicating the administrative difficulty of doing so).

The problem with letsencrypt certificates is that they currently have a 90 day expiry, which means you really need to use an automated tool to keep your TLS certificate current.  Fortunately the EFF has developed such a tool: certbot (they use a letsencrypt certificate for their site, indicating you can have trust that they do know what they’re doing).  However, one of the problems with certbot is that, by default, it generates a new key each time the certificate is renewed.  This isn’t a problem for most people, but if you use DANE records, it causes significant issues.

Why use both DANE and letsencrypt?

The beauty of DANE, as I’ve written before, is that it gives you a much more secure way of identifying your TLS certificate (provided you run DNSSEC).  People verifying your certificate may use DANE as the only verification mechanism (perhaps because they also distrust the X.509 authorities) which means the best practice is to publish a DANE TLSA record for each service and also use an X.509 authority rooted certificate.  That way your site just works for everyone.

The problem here is that being DNS based, DANE records can be cached for a while, so it can take a few days for DANE certificate updates to propagate through the DNS infrastructure. DANE records have an answer for this: they have a mode where the record identifies only the hash of the public key used by the website, not the certificate itself, which means you can change your certificate as much as you want provided you keep the same public/private key pair.  And here’s the rub: if certbot is going to try to give you a new key on each renewal, this isn’t going to work.

The internet society also has posts about this.

Making certbot work with DANE

Fortunately, there is a solution: the certbot manual mode (certonly) takes a –csr flag which allows you to construct your own certificate request to send to letsencrypt, meaning you can keep a fixed key … at the cost of not using most of the certbot automation.  So, how do you construct a correct csr for letsencrypt?  Like most free certificates, letsencrypt will only allow you to specify the certificate commonName, which must be a DNS pointer to the actual website.  If you need a certificate that covers multiple sites, all the other sites must be enumerated in the x509 v3 extensions field subjectAltName.  Let’s look at how openssl can generate such a certificate request.  One of the slight problems is that openssl, being a cranky tool, does not allow you to specify a subjectAltName on the command line, so you have to construct a special configuration file for it.  I called mine letsencrypt.conf

[req]
prompt = no
distinguished_name = req_dn
req_extensions = req_ext

[req_dn]
commonName = bedivere.hansenpartnership.com

[req_ext]
subjectAltName=@alt_names

[alt_names]
DNS.1=bedivere.hansenpartnership.com
DNS.2=www.hansenpartnership.com
DNS.3=hansenpartnership.com
DNS.4=blog.hansenpartnership.com

As you can see, I’ve given my canonical server (bedivere) as the common name and then four other subject alt names.  Once you have this config file tailored to your needs, you merely run

openssl req -new -key <mykey.key> -config letsencrypt.conf -out letsencrypt.csr

Where mykey.key is the path to your private key (you need a private key because even though the CSR only contains the public key, it is also signed).  However, once you’ve produced this letsencrypt.csr, you no longer need the private key and, because it’s undated, it will now work forever, meaning the infrastructure you put into place with certbot doesn’t need to be privileged enough to access your private key.  Once this is done, you make sure you have TLSA 3 1 1 records pointing to the hash of your public key (here’s a handy website to generate them for you) and you never need to alter your DANE record again.  Note, by the way that letsencrypt certificates can be used for non-web purposes (I use mine for encrypted SMTP as well), so you’ll need one DANE record for every service you use them for.

Putting it all together

Now that you have your certificate request, depending on what version of certbot you have, you may need it in DER format

openssl req -in letsencrypt.csr -out letsencrypt.der -outform DER

And you’re ready to run the following script from cron

#!/bin/bash
date=$(date +%Y-%m-%d)
dir=/etc/ssl/certs
cert="${dir}/letsencrypt-${date}.crt"
fullchain="${dir}/letsencrypt-${date}.pem"
chain="${dir}/letsencrypt-chain-${date}.pem"
csr=/etc/ssl/letsencrypt.der
out=/tmp/certbot.out

##
# certbot handling
#
# first it cannot replace certs, so ensure new locations (date suffix)
# each time mean the certificate is unique each time.  Next, it's
# really chatty, so the only way to tell if there was a failure is to
# check whether the certificates got updated and then get cron to
# email the log
##

certbot certonly --webroot --csr ${csr} --preferred-challenges http-01 -w /var/www --fullchain-path ${fullchain} --chain-path ${chain} --cert-path ${cert} > ${out} 2>&1

if [ ! -f ${fullchain} -o ! -f ${chain} -o ! -f ${cert} ]; then
    cat ${out}
    exit 1;
fi

# link into place

# cert only (apache needs)
ln -sf ${cert} ${dir}/letsencrypt.crt
# cert with chain (stunnel needs)
ln -sf ${fullchain} ${dir}/letsencrypt.pem
# chain only (apache needs)
ln -sf ${chain} ${dir}/letsencrypt-chain.pem

# reload the services
sudo systemctl reload apache2
sudo systemctl restart stunnel4
sudo systemctl reload postfix

Note that this script needs the ability to write files and create links in /etc/ssl/certs (can be done by group permission) and the systemctl reloads need the following in /etc/sudoers

%LimitedAdmins ALL=NOPASSWD: /bin/systemctl reload apache2
%LimitedAdmins ALL=NOPASSWD: /bin/systemctl reload postfix
%LimitedAdmins ALL=NOPASSWD: /bin/systemctl restart stunnel4

And finally you can run this as a cron script under whichever user you’ve chosen to have sufficient privilege to write the certificates.  I run this every month, so I know I if anything goes wrong I have at least two months to fix it.

Oh, and just in case you need proof that I got this all working, here you are!

One thought on “Using letsencrypt certificates with DANE

  1. inshizu

    IIRC the reason LE uses a non-LE cert on their website has to do with what Akamai had available to them as the CDN that fronts their service. Akamai is a sponsor, though, so you’d think there’d be a way to sort that out.

    Reply

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