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-- With these modifications it is now possible to store and retrieve keys from an AD without manually tweaking the schema. Permissions need to be set manuallay, though.
446 lines
16 KiB
Org Mode
446 lines
16 KiB
Org Mode
# README.ldap -*- org -*-
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#+TITLE: How to use LDAP with GnuPG
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#+AUTHOR: GnuPG.com
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#+DATE: 2020-10-07
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#
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# The following comment lines are for use by Org-mode.
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#+EXPORT_FILE_NAME: gnupg-and-ldap
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#+LANGUAGE: en
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#+OPTIONS: H:3 num:t toc:t \n:nil @:t ::t |:t ^:{} -:t f:t *:t TeX:t LaTeX:t skip:nil d:(HIDE) tags:not-in-toc
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#+HTML_HEAD: <link rel="stylesheet" type="text/css" href="https://gnupg.org/share/site.css" />
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#+LATEX_CLASS: article
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#+LATEX_CLASS_OPTIONS: [a4paper,11pt]
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#+LATEX_HEADER: \usepackage{a4wide}
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#+LATEX_HEADER_EXTRA: \parindent0mm
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#+STARTUP: showall
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* How to use LDAP with GnuPG
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In GnuPG the handling of LDAP is done by its Dirmngr component. This
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is due to the architecture of the system where Dirmngr is the sole
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process responsible for network related tasks. Network access is
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required for:
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- CRL fetching and caching for S/MIME
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- OCSP checking
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- S/MIME (X.509) certificate search via LDAP
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- OpenPGP keyserver access (HTTP, LDAP, etc.)
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- Checking for software updates (if enabled)
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In the following we describe how S/MIME and OpenPGP certificate search
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is implemented. If you want to skip this background information feel
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free to continue with the next section where LDAP installation and
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configuration is described. In any case we need to explain a few
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terms used with LDAP:
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- DIT :: /Directory Information Tree/ also known as /naming context/.
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This is is often referred to as the /LDAP directory/. It is
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where the data for a single organization described by a DNS
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name is stored (e.g. "example.org").
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- DN :: /Distinguished Name/ is the key for an entry in the DIT. It
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is a similar concept as used in the DNS system.
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- RDN :: /Relative Distinguished Name/ is a component or part of a
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DN. For example the DN "cn=admin,dc=example,dc=com" consist
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of the 3 RDNs "cn=admin", "dc=example", and "dc=com". Each
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RDN has a name (e.g. "cn" for /common name/ or "dc" for
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/domain component/) and a values (e.g. "admin").
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- LDIF :: /LDAP Data Interchange Format/ is a description for the
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human readable data exchange format used with LDAP.
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** OpenPGP
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To serve OpenPGP certificates via LDAP a dedicated schema needs to be
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installed. The schema supported by GnuPG was originally defined by
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PGP Inc. in the end of the 1990ies. This is today still the schema
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installed on LDAP servers for access by PGP or GnuPG. However, this
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schema has a couple of deficits which need to be fixed. For that
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reason we have defined additional attributes. These new attributes
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eventually allow to lookup certificates by their fingerprints and not
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just by the shorter and thus non-unique Key-ID. The new schema also
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supports storing of information on the subkeys and the UTF-8 encoded
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mail addresses. Current versions of GnuPG do not yet make use of
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these new attributes but for new LDAP installations it is highly
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recommended to use the new schema so that a future version of the
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software can make use if these attributes.
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Note that the OpenPGP certificates are stored in the DIT under a
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separate organizational unit using the long Key-ID to distinguish
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them. An example for such an DN is:
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: pgpCertID=63113AE866587D0A,ou=GnuPG Keys,dc=example,dc=com
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This design means that entries stored under "GnuPG Keys" are not
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connected to the users commonly found on an LDAP server. This allows
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to store arbitrary OpenPGP certificates in the directory and is
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commonly used to make the certificates of external communication
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partners easily available.
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** S/MIME
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Standard X.509 LDAP semantics apply for S/MIME certificate search.
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The current version of Dirmngr (2.2.23) supports 3 pattern formats
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which are translated from GnuPG's User-ID syntax, as given to the gpg
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and gpgsm commands, to the LDAP syntax:
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- Mail :: Indicated by a leading left angle and translated to the
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query:
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: "<ADDRSPEC>" -> "mail=ADDRSPEC"
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- Subject DN :: Indicated by a leading slash. The DN is formatted
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according to RFC-2253 rules and thus directly usable
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for an LDAP query.
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- Substring search :: If no other syntax matches or the pattern is
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prefixed with an asterisk the User-ID is translated to:
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: "USERID" -> "(|(sn=*USERID*)(|(cn=*USERID*)(mail=*USERID*)))"
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or in other word a substring search on the serial-number, the
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common-name, and the mail attribute is done.
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The result is expected to be in one of the attributes
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"userCertificate", "cACertificate", or "x509caCert". In cases where
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we are looking for the issuer certificate only "cACertificate" is
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used. "ObjectClass=*" is always used a filter.
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Note: The attribute "mail" with the OID 0.9.2342.19200300.100.1.3 was
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originally defined with this OID under the name "rfc822Mailbox" using
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a different although similar syntax. Take care: This is not an UTF-8
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encoded mail address and in theory GnuPG should use IDN mapping here.
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However, it is questionable whether any real world installation
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would be able to handle such a mapping.
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* How to install OpenLDAP
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To install a standard LDAP server to provide S/MIME certificate lookup
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follow the instructions of your OS vendor. For example on Debian
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based systems this is:
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: apt-get install slapd ldap-utils libsasl2-modules
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Follow the prompts during installation, set an initial admin password,
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and, most important, the domain you want to serve. Note that we use
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"example.com" in following. If you ever need to change the
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configuration on a Debian based system you can do so by running
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: dpkg-reconfigure slapd
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Serving LDAP requests for S/MIME (X.509) certificates will then work
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out of the box. Use your standard tools to maintain these
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entries. Some hints on how to manually add certificates can be found
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below in the section "Useful LDAP Commands".
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Please read on if you want to serve also OpenPGP certificates.
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** Installation of the OpenPGP Schema
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Assuming a standard OpenLDAP installation, it is easy to add a new
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schema to store OpenPGP certificate. We describe this now step by
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step.
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First you need to download the two LDIF files
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- https://gnupg.org/misc/gnupg-ldap-schema.ldif
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- https://gnupg.org/misc/gnupg-ldap-init.ldif.
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As administrator (root) on your LDAP server use the command
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: ldapadd -v -Y EXTERNAL -H ldapi:/// -f ./gnupg-ldap-schema.ldif
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to install the schema. The options given to the ldapadd tool are:
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- -v :: Given some diagnostic output (be verbose). To be even more
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verbose you may use =-vv= or =-vvv=. The diagnostics are
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written to stdout.
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- -Y :: Specify the authentication mechanism. Here we use =EXTERN=
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which is in this case local socket based authentication
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(ldapi).
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- -H :: The URL to access the LDAP server. Only scheme, host, and
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port are allowed. In our case we use =ldapi:///= to request
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a connection on the standard OpenLDAP socket (usually this is
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=/var/run/slapd/ldapi=).
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- -f :: Specify a file with data to add to the directory. The file
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used here is the specification of the keyserver schema. If
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this option is not used ldapadd expects this data on stdin.
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The new schema should now be installed. Check this by using this
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command:
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: ldapsearch -Q -Y EXTERNAL -L -H ldapi:/// \
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: -b 'cn=schema,cn=config' cn | grep cn:
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(on Unix the backslash indicates that the line is continued with the
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next line)
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The options not used by ldapsearch which have not yet been explained
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above are:
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- -Q :: Be quiet about authentication and never prompt.
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- -b :: Specify the search base. In this case we want the internal
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OpenLDAP schema which stores the server's own configuration.
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The final argument =cn= restricts the output to the DN and the CN
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attribute; the grep then shows only the latter. With a freshly
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installed OpenLDAP system you should get an output like:
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#+begin_example
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cn: schema
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cn: {0}core
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cn: {1}cosine
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cn: {2}nis
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cn: {3}inetorgperson
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cn: {4}gnupg-keyserver
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#+end_example
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This tells you that the keyserver schema has been installed under (in
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this case) the index "{4}".
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The next step is to connect the new schema with your DIT. This means
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that entries to actually store the certificates and meta data are
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created. This way GnuPG will be able to find the data. For this you
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need to edit the downloaded file =gnupg-ldap-init.ldif= and replace
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all the RDNs with name "dc" with your own. For example, in our own
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LDAP we would change
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: dn: cn=PGPServerInfo,dc=example,dc=com
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to
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: dn: cn=PGPServerInfo,dc=gnupg,dc=com
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and do that also for the other 3 appearances of the "dc" RDNs. In case
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you use a 3-level domain, add another "dc" in the same way you did when
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setting up OpenLDAP. With that modified file run
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: ldapadd -v -x -H ldapi:/// -D 'cn=admin,dc=example,dc=com' \
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: -W -f ./gnupg-ldap-init.ldif
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Remember to change the "dc" RDNs also here to what you actually use.
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We use simple authentication by means of these options:
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- -x :: Use simple authentication
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- -D :: The Bind-DN used to bind to the LDAP directory
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- -W :: Ask for the admin's passphrase. You may also use a lowercase
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=-w= followed by the passphrase but that would reveal the
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passphrase in the shell's history etc.
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All users with access right to the LDAP server may now retrieve
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OpenPGP certificates. But wait, we also need a user allowed to insert
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or update OpenPGP certificates. Choose a useful name for that user
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and create a file =newuser.ldif=. In our example domain we name that
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user "LordPrivySeal" and thus the file is:
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#+begin_src
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dn: uid=LordPrivySeal,ou=GnuPG Users,dc=example,dc=com
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objectClass: inetOrgPerson
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objectClass: uidObject
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sn: Lord Keeper of the Privy Seal
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cn: Lord Privy Seal
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userPassword: {SSHA}u6oxl9ulaS57RPyjApyPcE7mNECNK1Tg
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#+end_src
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The =userPassword= has been created by running
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: /usr/sbin/slappasswd
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entering the password, and paste the output into the file (the
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password used in the above example is "abc").
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Now run
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: ldapadd -v -x -H ldapi:/// -D 'cn=admin,dc=gnupg,dc=com' \
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: -W -f ./newuser.ldif
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On the password prompt enter the admin's password (not the one of the
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new user). Note that the user is created below the "GnuPG Users"
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organizational unit and not in the standard name space. Thus this is
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a dedicated user for OpenPGP certificates.
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See below how you can list the entire DIT. With
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a fresh install you should see these DNs:
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#+begin_example
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dn: dc=example,dc=com
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dn: cn=admin,dc=example,dc=com
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dn: cn=PGPServerInfo,dc=example,dc=com
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dn: ou=GnuPG Keys,dc=example,dc=com
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dn: ou=GnuPG Users,dc=example,dc=com
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dn: uid=LordPrivySeal,ou=GnuPG Users,dc=example,dc=com
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#+end_example
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Finally we need to give all users read access to the server's database
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and allow an authenticated user to modify the database. To do this
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you need to figure out the used database; run the command
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: ldapsearch -Q -Y EXTERNAL -H ldapi:/// -b 'cn=config' dn | grep olcDatabase=
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which should give you a list like this:
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#+begin_example
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dn: olcDatabase={-1}frontend,cn=config
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dn: olcDatabase={0}config,cn=config
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dn: olcDatabase={1}mdb,cn=config
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#+end_example
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The first two databases are for internal purposes, the last one is our
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database. Now create a file =grantaccess.ldif= with this content:
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#+begin_example
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dn: olcDatabase={1}mdb,cn=config
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changetype: modify
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replace: olcAccess
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olcAccess: {0} to dn.subtree="dc=example,dc=com"
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by dn.regex="^uid=LordPrivySeal,ou=GnuPG Users,dc=example,dc=com" write
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by * read
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#+end_example
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As usual replace all "dc=example,dc=com" accordingly. Take care not
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to insert a blank line anywhere. The first line needs to give the DN
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of the database as determined above. Excute the rules from that file
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using the command:
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: ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f grantaccess.ldif
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Now all users have read access and the user LordPrivySeal has write
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access. In case you want to give several users permissions to update the
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keys replace the regex line in =grantaccess.ldif= with
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: by dn.regex="^uid=([^,]+),ou=GnuPG Users,dc=example,dc=com" write
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and create those users below the RDN "ou=GnuPG Users".
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That's all you need to do at the server.
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** Configuration for GnuPG
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The easiest way to enable LDAP for S/MIME is to put
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#+begin_src
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keyserver ldap.example.com::::dc=example,dc=com:
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#+end_src
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into =gpgsm.conf=. If you prefer to use a dedicated configuration
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file you can do this with dirmngr by adding a line
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: ldap.example.com::::dc=example,dc=com:
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to =dirmngr_ldapservers.conf=.
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Assuming you want to use the machine running the LDAP server also to
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maintain OpenPGP certificates, put the following line into the
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=dirmngr.conf= configuration of a dedicated user for this task:
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#+begin_src
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keyserver ldapi:///????bindname=uid=LordPrivySeal
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%2Cou=GnuPG%20Users%2Cdc=example%2Cdc=com,password=abc
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#+end_src
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(Enter this all on one line; "%2C" directly at the end of "Seal")
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That is a pretty long line with weird escaping rules. Just enter it
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verbatim but replace the "dc" RDNs accordingly. Remember that =ldapi=
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uses local socket connection instead of TCP to connect to the server.
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The password given in that file is the password of the OpenPGP
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maintainer (LordPrivySeal). Use appropriate permissions for that
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file to make it not too easy to access that password. See the GnuPG
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manual for other ways to configure an LDAP keyserver.
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With that configuration in place you may add arbitrary OpenPGP keys to
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your LDAP. For example user "joe@example.org" sends you a key and
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asks to insert that key. If you feel comfortable with that you should
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first check the key, import it into your local keyring, and then send
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it off to your LDAP server:
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: gpg --show-key < file-with-joes-key.asc
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Looks good? Note the fingerprint of the key and run
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: gpg --import < file-with-joes-key.asc
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: gpg --send-keys FINGERPRINT
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That's all. If you want to work from a different machine or use the
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Kleopatra GUI you need to make sure that ldaps has been correctly
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configured (for example on the machine =ldap.example.org=) and you
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need to use this keyserver line:
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#+begin_src
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keyserver ldaps://ldap.example.com/????bindname=uid=LordPrivySeal
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%2Cou=GnuPG%20Users%2Cdc=example%2Cdc=com,password=abc
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#+end_src
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(Enter this all on one line; "%2C" directly at the end of "Seal")
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The easier case is the configuration line for anonymous users which is
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a mere
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#+begin_src
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keyserver ldaps://ldap.example.com
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#+end_src
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This assumes that you have a valid TLS server certificate for that
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domain and ldaps is enabled on the server.
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* Useful LDAP Commands
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** List the entire DIT
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To list the entire DIT for the domain "example.com" use this command:
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: ldapsearch -Q -Y EXTERNAL -LLL -H ldapi:/// -b dc=example,dc=com dn
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This lists just the DNs. If you need the entire content of the DIT
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leave out the "dn" argument. The option "-LLL" selects useful
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formatting options for the output.
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** Insert X.509 Certficate
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If you don't have a handy tool to insert a certificate via LDAP you
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can do it manually. First put the certificate in binary (DER) format
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into a file. For example using gpgsm:
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: gpgsm --export berta.boss@example.com >berta.crt
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Then create a file =addcert.ldif=:
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#+begin_example
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dn: CN=Berta Boss,dc=example,dc=com
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objectclass: inetOrgPerson
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cn: Berta Boss
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sn: Boss
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gn: Berta
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uid: berta
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mail: berta.boss@example.com
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usercertificate;binary:< file:///home/admin/berta.crt
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#+end_example
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(Note that an absolute file name is required.)
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Finally run
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: ldapadd -x -H ldapi:/// -D 'cn=admin,dc=example,dc=com' -W -f adduser.ldif
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** Change RootDN Password:
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Create temporary file named =passwd.ldif=:
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#+begin_src
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dn: olcDatabase={1}mdb,cn=config
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changetype: modify
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replace: olcRootPW
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olcRootPW: XXXX
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#+end_src
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For XXXX insert the output of slappasswd and run
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: ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f passwd.ldif
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followed by
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: ldappasswd -x -D cn=admin,dc=example,dc=com -W -S
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and enter the new and old password again.
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** Show ACLs
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: ldapsearch -Q -Y EXTERNAL -H ldapi:/// -b 'cn=config' olcAccess
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** Show a list of databases
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: ldapsearch -Q -Y EXTERNAL -H ldapi:/// -b 'cn=config' | grep ^olcDatabase:
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** Change the log level
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To debug access problems, it is useful to change the log level:
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: printf "dn: cn=config\nchangetype: %s\nreplace: %s\n%s: %s\n" \
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: modify olcLogLevel olcLogLevel ACL | ldapadd -Q -Y EXTERNAL -H ldapi:///
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to revert replace "ACL" by "none".
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