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Howto/Client Certificate Authentication with LDAP


We can use user certificates to authenticate our ldap session.

generate user certificate for user account

Follow instructions in this blog.

Short version:

  • create csr (certificate signing request).

I usually create a new directory and name it after the name of the user/host we want to create a certificate for. For user10, create a user10 folder.

Inside this folder, create a text file user10.inf like this:

[ req ]
prompt = no
encrypt_key = no

distinguished_name = dn
req_extensions = exts

[ dn ]
commonName = "user10"

[ exts ]
subjectAltName=email:user10@yourdomain.tld
  • generate a key:
openssl genrsa -out user10.key 2048
  • generate the csr:
openssl req -new -key user10.key -out user10.csr -config user10.inf
  • verify csr:
 
openssl req -in user10.csr -text -noout 
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: CN=user10
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:c2:d2:0c:44:c8:e3:8b:d7:e5:bc:b6:5d:fc:cf:
                    xxxxx
                Exponent: 65537 (0x10001)
        Attributes:
        Requested Extensions:
            X509v3 Subject Alternative Name: 
                email:user10@yourdomain.tld
    Signature Algorithm: sha1WithRSAEncryption
         05:7b:a7:51:1e:28:25:8d:78:fb:d9:08:43:6d:54:51:db:10:
         xxxxxxxxxxxxxxxxxxxxx
  • request the certificate (as the user self or as an admin user):
$ ipa cert-request user10.csr --principal user10 
....

If everything goes according to plan, you know have a certificate coupled to the user account

$ ipa user-show user10
  User login: user10
  First name: ipa
  Last name: user
  Home directory: /home/user10
  Login shell: /bin/sh
  Email address: user10@yourdomain.tld
  UID: 1076200013
  GID: 1076200013
  Certificate: MIIEMjCCAxqgAwIBAgIBDjANBgkqhkiG9w0BAQsFADA5MRcwFQYDVQQKDA5VTklYxxxxxxxxxxxxxxxxxxxxxxxxxxxx==
  Account disabled: False
  Password: True
  Member of groups: ipausers
  Kerberos keys available: True
  • retrieve the certificate:

first we need to get the certificate's serial number.

ipa cert-find
...
  Serial number (hex): 0xE
  Serial number: 14
  Status: VALID
  Subject: CN=user10,O=YOURDOMAIN.TLD
<pre>
So, number 14.

<pre> 
ipa cert-show 14 --out user10.pem 
  • eventually, verify certificate:
openssl x509 -in user10.pem -noout -text

which will give you all the certificate output on screen.

map certificate to user account

Canonical info:

http://directory.fedoraproject.org/docs/389ds/howto/howto-certmapping.html

https://access.redhat.com/documentation/en-US/Red_Hat_Directory_Server/9.0/html/Administration_Guide/Managing_SSL-Using_Certificate_Based_Authentication.html

  • verify /etc/dirsrv/slapd-INSTANCE-NAME/certmap.conf looks like this:
certmap default         default
#default:DNComps
#default:FilterComps    e, uid
#default:verifycert     on
#default:CmapLdapAttr   certSubjectDN
#default:library        <path_to_shared_lib_or_dll>
#default:InitFn         <Init function's name>
default:DNComps
default:FilterComps     uid
certmap ipaca           CN=Certificate Authority,O=SUB.DOMAIN.TLD
ipaca:CmapLdapAttr      seeAlso
ipaca:verifycert        on

As you see, there is a 'default' mapping and an 'ipaca' mapping.

WARNING!!!

Do not modify anything of the ipaca mapping unless you know what you are doing. You risk messing up your pki tomcat service and plenty of things will stop working.

WARNING!!!

As you see, the ipaca mapping is your ipa server PKI. It has a CmapLdapAttr mapping attribute to the ldap object attribute seeAlso.

When I searched a test ipa environment, the only account with a seeAlso attribute was the "DN: uid=pkidbuser,ou=people,o=ipaca" user, with this value: "CN=CA Subsystem,O=SUB.DOMAIN.TLD" (substitute O=SUB.DOMAIN.TLD with your own REALM name, obviously). This is an internal ipa user, do not modify it! We cannot modify this mapping or the PKI subsystem will stop working.

So the solution is quite simple. We need to populate the seeAlso attribute of the user10 account with this value:

cn=user10,o=SUB.DOMAIN.TLD

You can add this value to the seeAlso attribute using your favourite ldap client, like the very nice apache ds studio

configure ldap client

we can easily test this using ldapsearch. We need to set two environment variables in ~/.ldaprc:

TLS_CERT /path/to/user10.pem
TLS_KEY /path/to/user10.key

And now search:

$ ldapsearch -h kdc.domain.tld -ZZ -Y EXTERNAL objectclass=person -s sub -b dc=sub,dc=domain,dc=tld cn
...
# search result
search: 3
result: 0 Success

# numResponses: 1002
# numEntries: 1001

And in the log files (/var/log/dirsrv/slapd-INSTANCE-NAME/access) of the ldap server we see this:

[04/Mar/2016:23:34:57 +0100] conn=100 fd=111 slot=111 connection from 192.168.0.124 to 192.168.5.10
[04/Mar/2016:23:34:57 +0100] conn=100 op=0 EXT oid="1.3.6.1.4.1.1466.20037" name="startTLS"
[04/Mar/2016:23:34:57 +0100] conn=100 op=0 RESULT err=0 tag=120 nentries=0 etime=0
[04/Mar/2016:23:34:57 +0100] conn=100 TLS1.2 256-bit AES; client CN=user10,O=SUB.DOMAIN.TLD issuer CN=Certificate Authority,O=SUB.DOMAIN.TLD
[04/Mar/2016:23:34:57 +0100] conn=100 TLS1.2 client bound as uid=user10,cn=users,cn=accounts,dc=sub,dc=domain,dc=tld
[04/Mar/2016:23:34:57 +0100] conn=100 op=1 BIND dn="" method=sasl version=3 mech=EXTERNAL
[04/Mar/2016:23:34:57 +0100] conn=100 op=1 RESULT err=0 tag=97 nentries=0 etime=0 dn="uid=user10,cn=users,cn=accounts,dc=sub,dc=domain,dc=tld"
[04/Mar/2016:23:34:57 +0100] conn=100 op=2 SRCH base="dc=sub,dc=domain,dc=tld" scope=2 filter="(objectClass=person)" attrs="cn"
[04/Mar/2016:23:34:57 +0100] conn=100 op=2 RESULT err=0 tag=101 nentries=1001 etime=0

perl5 example

you need the perl-LDAP and perl-IO-Socket-SSL packages for this (fedora/rhel/centos).

#!/usr/bin/env perl

use strict;
use warnings;
use utf8;
use Net::LDAP;

my $base = "dc=sub,dc=domain,dc=tld";

my $ldap = Net::LDAP->new( 'kdc.sub.domain.tld', debug => 0 ) || die "$@";

my $msg = $ldap->start_tls(
    verify     => 'require',
    sslversion => 'tlsv1',
    clientcert => "/path/to/user10.pem",
    clientkey  => "/path/to/user10.key",
);

$msg->code && warn "could not starttls: " . $msg->error;

# no bind needed, we are already authenticated!

my $search = $ldap->search(
    base   => $base,
    scope  => "sub",
    filter => "(objectclass=person)",
    attrs => [ 'uid', ],
);

$search->code && warn "failed to get persons: " . $search->error;

print "found " . $search->count . " persons\n";

for my $entry ( $search->entries ) {
    print $entry->get_value('uid'), "\n";
    print $entry->dn, "\n";
}

disabling access to the user certificate

if this certificate (or its key) has been compromised you need to disable its access to the directory.

  • revoke it:
ipa cert-revoke <serialnr>
  • remove the seeAlso attribute from the user account.

This is necessary because the DS apparently does not check the revocation status of the certificate. Having revoked it, I can still use it to access the ldap server. Removing the ldap value of seeAlso solves this problem.

Author

Howto provided by Natxo Asenjo on freeipa-users.