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ACMEv2 client written in plain C with minimal dependencies

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uacme manual


lightweight client for the RFC8555 ACMEv2 protocol, written in plain C with minimal dependencies (libcurl and one of GnuTLS, OpenSSL or mbedTLS. The ACMEv2 protocol allows a Certificate Authority (Let's Encrypt is a popular one) and an applicant to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation.


  • Written in C - It runs on any unix machine, including Linux, BSD, ...
  • Minimal dependencies - Other than the standard C library, uacme depends only on libcurl and one of GnuTLS, OpenSSL or mbedTLS. It does all the cryptography and network communications without spawning external processes. Particularly when using mbedTLS, it is small enough to run on embedded systems with severe RAM and program memory restrictions (such as OpenWRT routers, for example). This is in contrast to solutions based on python or shell scripts, which may well be a few hundred lines but require many other large applications such as python or openssl to work.
  • Native ECC support - Elliptic Curve keys and certificates can be generated with a commmand line option (-t EC)
  • Easily extensible - It optionally calls an external hook program with the tokens required for domain authorization by the server. The hook program can be an executable, shell script, perl script, python script, or any file that the operating system can execute.
  • ACME challenge agnostic - It provides the user or hook program with all tokens and information required to complete any challenge type but leaves the task of setting up and cleaning up the challenge environment to the user or hook. Example shell scripts to handle http-01, dns-01 and tls-alpn-01 challenges are provided.
  • Zero downtime tls-alpn-01 support - The distribution also includes ualpn, a lightweight proxying tls-alpn-01 challenge responder compliant with RFC8737 and RFC8738.
  • Can run as a cron job - to renew certificates automatically when needed, even for remote machines
  • Robust - It checks every operation, retrying or failing gracefully as appropriate
  • Detailed error reporting - By default totally quiet when everything works ok, it reports precise and detailed error information on stderr when something goes wrong. Optionally it can also print debug information by specifying the --verbose flag once or more.


Note: pristine releases are in the upstream/latest branch, tagged as upstream/x.x.x

mkdir uacme
wget -O - | tar zx -C uacme --strip-components=1
cd uacme
./configure --disable-maintainer-mode
make install
If you just want to check out the latest pristine release from github:
git clone -b upstream/latest
uacme packages are available for several distributions:

Getting started

Once you have obtained uacme (see Installation above) the next step is creating an ACME account:

uacme -v -c /path/to/uacme.d new
The configuration directory and account private key should have been created:
You can then issue a certificate for your domain by doing
uacme -v -c /path/to/uacme.d issue
If everything goes well uacme asks you to set up a challenge, for example
uacme: challenge=http-01 token=kZjqYgAss_sl4XXDfFq-jeQV1_lqsE76v2BoCGegFk4
Note the challenge type in the example is http-01 which means you should set up your web server to serve a URL based on the token:
The URL must return a text file containing a single line with the key authorization:
After setting up the web server you can then type 'y' followed by a newline. This notifies the ACME server that it can proceed with the challenge verification. If the procedure is successful uacme saves the certificate and the key at:
Note several challenge types are possible. If you type anything other than 'y', uacme skips the challenge and proposes a different one. The easiest is http-01 but any other type can be dealt with. Keep in mind that challenge types may be served in random order by the server. Do not make any assumptions and read what uacme outputs carefully.

Automating updates

Use the -h flag to manage the challenge with a hook script:

uacme -v -c /path/to/uacme.d -h /usr/share/uacme/ issue
or (depending on your installation)
uacme -v -c /path/to/uacme.d -h /usr/local/share/uacme/ issue
This will use the example hook script included in the distribution to manage http-01 challenges. You might need to edit the script to match your webserver's environment.

Once everything works correctly you can also set up cron, for example

6 15 * * * /usr/bin/uacme -c /path/to/uacme.d -h /usr/share/uacme/ issue 
The cron job will automatically update the certificate when needed. Note the absence of -v flag, this makes uacme only produce output upon errors.

Note also that you will need to restart or reload any service that uses the certificate, to make sure it uses the renewed one. This is system and installation dependent. I normally put the necessary instructions in another script (for example /usr/share/uacme/ that is executed by cron when uacme returns 0 (indicating the certificate has been reissued).

6 15 * * * /usr/bin/uacme -c /path/to/uacme.d -h /usr/share/uacme/ issue && /usr/share/uacme/

Check for a complete, ready-to-go solution.

dns-01 challenge support

The hook script included in the distribution allows managing dns-01 challenges with nsupdate. This only works if your name server supports RFC2136 (bind does, nsd doesn't). is another example that works with nsd. shows how to integrate with Cloudflare API.

tls-alpn-01 challenge support

ualpn is a lightweight proxying tls-alpn-01 challenge responder, designed to handle incoming HTTPS connections on port 443. Most of the time it just transparently proxies connections to the real web server (which can be on either another machine, or a different TCP port on the same machine). When a tls-alpn-01 challenge handshake comes in ualpn handles it on the fly instead of proxying it to the webserver. This means that unlike other available tls-alpn-01 responders, ualpn does not require your webserver to stop during the challenge (zero downtime).

The high performance event-driven implementation is based on libev which considerably reduces the cost of context switches and memory usage. In addition on systems such as Linux supporting the splice() system call, ualpn is able to move network data entirely in kernel memory without a round trip to user space, which further enhances performance.

ualpn also listens to a UNIX domain socket so that it can be fed the necessary tls-alpn-01 key authorizations for the domains being validated by the ACME server. ualpn was designed to be easy to integrate with not only uacme (check the example hook script) but also other ACME clients. A certbot plugin is also available.

To get started with ualpn: * move your real HTTPS server to port 4443 which doesn't need to be open to the outside (only ualpn will connect to it) and set it up to accept the PROXY protocol: * for nginx:

    server {
        listen ssl proxy_protocol;
        real_ip_header proxy_protocol;
        proxy_set_header X-Real-IP $proxy_protocol_addr;
        proxy_set_header X-Forwarded-For $proxy_protocol_addr;
* for apache:
    Listen 4443
        RemoteIPProxyProtocol On
* launch ualpn as a daemon and check the logs (by default in syslog)
  sudo ualpn -v -d -u nobody:nogroup -c [email protected] -S 666
* create an ACME account
  uacme -v -s -c /path/to/uacme.d -y new
* try obtaining a certificate with tls-alpn-01 challenge
  uacme -v -s -c /path/to/uacme.d -h /usr/share/uacme/ issue
or, depending on your installation
  uacme -v -s -c /path/to/uacme.d -h /usr/local/share/uacme/ issue


There are regular unix man pages in the distribution, also available in HTML: uacme ualpn

Bugs and suggestions

If you believe you have found a bug, please log it at

If you have any suggestions for improvements, pull requests are welcome.

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