Ubuntu Manpage: lacme - ACME client written with process isolation and minimal privileges in mind

NAME

       lacme - ACME client written with process isolation and minimal privileges in mind

SYNOPSIS

       lacme [--config=FILENAME] [--socket=PATH] [OPTION ...] COMMAND [ARGUMENT ...]

DESCRIPTION

lacme is a small ACME client written with process isolation and minimal privileges in
mind. It is divided into four components, each with its own executable:

1. A lacme-accountd(1) process to manage the account key and issue SHA-256 signatures
needed for each ACME command. (This process binds to a UNIX-domain socket to reply to
signature requests from the ACME client.) One can use the UNIX-domain socket forwarding
facility of OpenSSH 6.7 and later to run lacme-accountd(1) and lacme on different
hosts. Alternatively, the lacme-accountd(1) process can be spawned by the “master”
lacme process below; in that case, the two processes communicate through a socket pair.

2. A “master” lacme process, which runs as root and is the only component with access to
the private key material of the server keys. It is used to fork the ACME client (and
optionally the ACME webserver) after dropping root privileges. For certificate
issuances (newOrder command), it also generates Certificate Signing Requests, then
verifies the validity of the issued certificate, and optionally reloads or restarts
services when the notify setting is set.

3. An actual ACME client (specified with the command setting of the [client] section of
the configuration file), which builds ACME commands and dialogues with the remote ACME
server. Since ACME commands need to be signed with the account key, the “master” lacme
process passes the lacme-accountd(1) UNIX-domain socket to the ACME client: data
signatures are requested by writing the data to be signed to the socket.

4. For certificate issuances (newOrder command), an optional webserver (specified with the
command setting of the [webserver] section of the configuration file), which is spawned
by the “master” lacme. (The only challenge type currently supported by lacme is
http-01, which requires a webserver to answer challenges.) That webserver only
processes GET and HEAD requests under the /.well-known/acme-challenge/ URI. Moreover
temporary iptables(8) rules can be automatically installed to open the HTTP port.

COMMANDS

       lacme account [--tos-agreed] [--register] [CONTACT ...]
              Register  (if --registered is set) a lacme-accountd(1)-managed account key.  A list
              of CONTACT information (such as maito: URIs) can be specified in order for the ACME
              server  to  contact  the  client  for  issues related to this registration (such as
              notifications  about   server-initiated   revocations).    --tos-agreed   indicates
              agreement  with  the  ACME  server’s  Terms  of  Service (and might be required for
              registration).

              If the account key is already registered, update the contact info  with  the  given
              list of CONTACT information.

              Upon success, lacme prints the new or updated Account Object from the ACME server.

       lacme newOrder [--config-certs=FILE] [--min-days=INT|--force] [SECTION ...]
              Read  the  certificate  configuration  FILE (see the certificate configuration file
              section below for the configuration options), and request new Certificate  Issuance
              for  each  of  its  sections  (or the given list of SECTIONs).  Command alias: new-
              order.

              The flag --force is an alias for --min-days=-1, which forces renewal regardless  of
              the expiration date of existing certificates.

       lacme revokeCert FILE [FILE ...]
              Request that the given certificate(s) FILE(s) be revoked.  For this command, lacme-
              accountd(1) can be pointed to either  the  account  key  or  the  certificate  key.
              Command alias: revoke-cert.

GENERIC SETTINGS

       --config=filename
              Use  filename  as  configuration file instead of %E/lacme/lacme.conf.  The value is
              subject to %-specifier expansion.

              See the configuration file section below for the configuration options.

       --socket=path
              Use path as the lacme-accountd(1) UNIX-domain socket to connect  to  for  signature
              requests  from  the  ACME  client.   The value is subject to %-specifier expansion.
              lacme aborts if path exists or if its parent directory is writable by other  users.
              Default: %t/S.lacme.

              This  command-line  option  overrides the socket setting of the [client] section of
              the configuration file; it also causes the [accountd] section to be ignored.

       -h, --help
              Display a brief help and exit.

       -q, --quiet
              Be quiet.

       --debug
              Turn on debug mode.

CONFIGURATION FILE

Valid settings are:

DEFAULT SECTION
config-certs
For certificate issuances (newOrder command), specify the space-separated list of
certificate configuration files or directories to use (see the certificate
configuration file section below for the configuration options). Each item in that
list is independently subject to %-specifier expansion.

Paths not starting with / (after %-expansion) are relative to the parent directory
of the configuration filename. The list of files and directories is processed in
the specified order, with the later items taking precedence. Files in a directory
are processed in lexicographic order, only considering the ones with suffix .conf.

Default: lacme-certs.conf lacme-certs.conf.d/.

[client] SECTION
This section is used for configuring the ACME client (which takes care of ACME commands
and dialogues with the remote ACME server).

socket See --socket=.

user The username to drop privileges to (setting both effective and real uid). Skip
privilege drop if the value is empty (not recommended). Default: _lacme-client.

group The groupname to drop privileges to (setting both effective and real gid, and also
setting the list of supplementary gids to that single group). Skip privilege drop
if the value is empty (not recommended). Default: nogroup.

command
The ACME client command. It is split on whitespace, with the first item being the
command to execute, the second its first argument etc. (Note that lacme might
append more arguments when executing the command internally.) Default:
/usr/libexec/lacme/client.

server Root URI of the ACME server. Default: https://acme-
v02.api.letsencrypt.org/directory.

timeout
Timeout in seconds after which the client stops polling the ACME server and
considers the request failed. Default: 30.

SSL_verify
Whether to verify the server certificate chain. Default: Yes.

SSL_version
Specify the version of the SSL protocol used to transmit data.

SSL_cipher_list
Specify the cipher list for the connection, see ciphers(1ssl) for more information.

[webserver] SECTION
This section is used to configure how ACME challenge responses are served during
certificate issuance.

listen Comma- or space-separated list of addresses to listen on. Valid addresses are of
the form IPV4:PORT, [IPV6]:PORT (where the :PORT suffix is optional and defaults to
the HTTP port 80), or an absolute path of a UNIX-domain socket (created with mode
0666). Default: /run/lacme-www.socket.

Note: The default value is only suitable when an external HTTP daemon is publicly
reachable and passes all ACME challenge requests to the webserver component through
the UNIX-domain socket /run/lacme-www.socket (for instance using the provided
/etc/lacme/apache2.conf or /etc/lacme/nginx.conf configuration snippets for each
virtual host requiring authorization). If there is no HTTP daemon bound to port 80
one needs to set listen to [::] (or 0.0.0.0 [::] when dual IPv4/IPv6 stack is
disabled or unavailable), and possibly also set iptables to Yes.

challenge-directory
Directory under which an external HTTP daemon is configured to serve GET requests
for challenge files under /.well-known/acme-challenge/ (for each virtual host
requiring authorization) as static files. The directory must exist beforehand,
must be empty, and the lacme client user (by default _lacme-client) needs to be
able to create files under it.

This setting is required when listen is empty. Moreover its value is subject to
%-specifier expansion before privilege drop.

user The username to drop privileges to (setting both effective and real uid). Skip
privilege drop if the value is empty (not recommended). Default: _lacme-www.

command
The ACME webserver command. It is split on whitespace, with the first item being
the command to execute, the second its first argument etc. (Note that lacme might
append more arguments when executing the command internally.) A separate process is
spawned for each address to listen on. (In particular no webserver process is
forked when the listen setting is empty.) Default: /usr/libexec/lacme/webserver.

iptables
Whether to automatically install temporary iptables(8) rules to open the
ADDRESS[:PORT] specified with listen. The rules are automatically removed once
lacme exits. This setting is ignored when challenge-directory is set. Default:
No.

[accountd] SECTION
This section is used for configuring the lacme-accountd(1) child process. If the section
(including its header) is absent or commented out, or if the CLI option --socket is
specified, then lacme connects to an existing lacme-accountd(1) process via the specified
UNIX-domain socket.

user The username to drop privileges to (setting both effective and real uid). Skip
privilege drop if the value is empty (the default).

command
The lacme-accountd(1) command. It is split on whitespace, with the first item
being the command to execute, the second its first argument etc. (Note that lacme
appends more arguments when executing the command internally.) Each item in that
list is independently subject to %-specifier expansion after privilege drop.
Default: /usr/bin/lacme-accountd.

Use for instance `ssh -T lacme@account.example.net lacme-accountd` in order to
spawn a remote lacme-accountd(1) server.

config Path to the lacme-accountd(1) configuration file. The value is subject to
%-specifier expansion after privilege drop.

quiet Be quiet. Possible values: Yes/No.

CERTIFICATE CONFIGURATION FILE

For certificate issuances (newOrder command), a separate file is used to configure paths
to the certificate and key, as well as the subject, subjectAltName, etc. to generate
Certificate Signing Requests. Each section denotes a separate certificate issuance.
Valid settings are:

certificate
Where to store the issued certificate (in PEM format). At least one of certificate
or certificate-chain is required.

certificate-chain
Where to store the issued certificate along with its chain of trust (in PEM
format). At least one of certificate or certificate-chain is required.

certificate-key
Path to the service’s private key. This setting is required. The genpkey(1ssl)
command can be used to generate a new service RSA key:

$ install -vm0600 /dev/null /path/to/service.rsa.key
$ openssl genpkey -algorithm RSA -out /path/to/service.rsa.key

Alternatively, for an ECDSA key using the NIST P-256 curve:

$ install -vm0600 /dev/null /path/to/service.ecdsa.key
$ openssl genpkey -algorithm EC -out /path/to/service.ecdsa.key \
-pkeyopt ec_paramgen_curve:P-256 \
-pkeyopt ec_param_enc:named_curve

lacme supports any key algorithm than the underlying libssl (OpenSSL) version is
able to manipulate, but the ACME server might reject CSRs associated with private
keys of deprecated and/or “exotic” algorithms.

For a dual cert setup (for instance RSA+ECDSA), duplicate the certificate section
and use a distinct certificate-key resp. certificate (or certificate-chain) value
for each key algorithm.

min-days
For an existing certificate, the minimum number of days before its expiration date
the section is considered for re-issuance. A negative value forces reissuance,
while the number 0 limits reissuance to expired certificates. Default: the value
of the CLI option --min-days, or 21 if there is no such option.

subject
Subject field of the Certificate Signing Request, in the form
/type0=value0/type1=value1/type2=.... This setting is required.

subjectAltName
Comma-separated list of Subject Alternative Names, in the form
type0:value1,type1:value1,type2:... The only type currently supported is DNS, to
specify an alternative domain name.

CAfile Path to the bundle of trusted issuer certificates. This is used for validating
each certificate after issuance or renewal. Specifying an empty value skips
certificate validation. Default: /usr/share/lacme/ca-certificates.crt.

hash Message digest to sign the Certificate Signing Request with, overriding the
req(1ssl) default.

keyUsage
Comma-separated list of Key Usages, for instance digitalSignature, keyEncipherment,
to include in the Certificate Signing Request. See x509v3_config(5ssl) for a list
of possible values. Note that the ACME server might override the value provided
here.

tlsfeature
Comma-separated list of TLS extension identifiers, such as status_request for OCSP
Must-Staple. See x509v3_config(5ssl) for a list of possible values. Note that the
ACME server might override the value provided here.

chown An optional username[:groupname] to chown the issued certificate and certificate-
chain to.

chmod An optional octal mode to chmod the issued certificate and certificate-chain to.

notify Command to pass the the system’s command shell (`/bin/sh -c`) after successful
installation of the certificate and/or certificate-chain.

%-SPECIFIERS

       Some CLI options and configuration settings are subject to %-expansion for  the  following
       specifiers.  Check the documentation of each setting to see which ones are affected.

       %C       /var/cache  for  the  root  user,  and $XDG_CACHE_HOME for other
                users  (or  $HOME/.cache  if  the   XDG_CACHE_HOME   environment
                variable is unset or empty).
       %E       /etc for the root user, and $XDG_CONFIG_HOME for other users (or
                $HOME/.config if the  XDG_CONFIG_HOME  environment  variable  is
                unset or empty).
       %g       Current group name.
       %G       Current group ID.
       %h       Home directory of the current user.
       %t       /run  for  the  root user, and $XDG_RUNTIME_DIR for other users.
                Non-root  users  may  only  use  %t  when  the   XDG_RUNTIME_DIR
                environment variable is set to a non-empty value.
       %T       $TMPDIR,  or /tmp if the TMPDIR environment variable is unset or
                empty.
       %u       Current user name.
       %U       Current user ID.
       %%       A literal %.

EXAMPLES

              $ sudo lacme account --register --tos-agreed mailto:noreply@example.com
              $ sudo lacme newOrder
              $ sudo lacme revokeCert /path/to/service.crt

       Automatic renewal can be scheduled via crontab(5) or systemd.timer(5).  In order to  avoid
       deploying  a  single  account key onto multiple nodes and/or dealing with multiple account
       keys, one can install a single lacme-accountd(1) instance on a dedicated host, generate  a
       single  account  key  there  (and  keep  it well), and set the following in the [accountd]
       section:

              command = ssh -T lacme@account.example.net lacme-accountd

       If the user running lacme can connect to  lacme@account.example.net  using  (passwordless)
       key  authentication, this setting will spawn a remote lacme-accountd(1) and use it to sign
       ACME  requests.   Further  hardening  can  be  achieved  by  means  of  authorized_keys(5)
       restrictions:

              restrict,from="...",command="/usr/bin/lacme-accountd --quiet --stdio" ssh-rsa ...

BUGS AND FEEDBACK

       Bugs  or  feature requests for lacme should be filed with the Debian project’s bug tracker
       at <https://www.debian.org/Bugs/>.

AUTHORS

       Guilhem Moulin (mailto:guilhem@fripost.org).

                                          December 2015                                  lacme(8)