Provide an example to name constrain a CA

It's imperfect, and abetter procedure should be written up
for the more security conscious.

misc/makealtca.py

@@ -0,0 +1,103 @@
+# This creates a locked down variant of a confluent CA
+
+# The confluent CA naturally doesn't have any name constraints, which
+# is great for flexibility.
+
+# However, if actually being imported into a web browser, it is likely
+# to want to limit the CA to apply to cluster resources rather
+# than having a blank check to vouch for any and all subjects.
+
+# This particular approach causes a certificate authority that
+# can be imported and vouch for the subset of certificates
+# that were issued by the normal CA that match name constraints
+
+# Unfortunately, *ALL* names are checked, whether they are relevant
+# to the conversation or not, so we reproduce the logic and the resultant
+# CA is fragile to IP addresses.
+
+# The better approach would be to issue a separate, full name only certificate
+# and have a simpler alt CA.  The same CA can be used to sign both certificates,
+# and still import the limited one
+
+
+import subprocess
+import socket
+import confluent.certutil as certutil
+import sys
+def create_alt_ca(certout, permitdomains):
+    # This is to create a constrained variant of the existing authority
+    # this will allow a client browser to only trust it for select domains
+    # while the nodes can more broadly trust it (e.g. to vouch for IP addresses)
+    sslcfg = certutil.get_openssl_conf_location()
+    newcfg = '/etc/confluent/tls/ca-alt/openssl-alt.cfg'
+    subj = subprocess.check_output(
+        ['openssl', 'x509', '-subject', '-noout', '-in', '/etc/confluent/tls/ca/cacert.pem']
+        ).decode().replace('subject=', '')
+    serial = subprocess.check_output(
+        ['openssl', 'x509', '-serial', '-noout', '-in', '/etc/confluent/tls/ca/cacert.pem']
+        ).decode().replace('serial=', '')
+    with open('/etc/confluent/tls/ca-alt/serial', 'w') as srl:
+        srl.write(serial)
+    settings = {
+        'dir': '/etc/confluent/tls/ca-alt',
+        'certificate': '$dir/cacert.pem',
+        'private_key': '$dir/private/cakey.pem',
+        'countryName': 'optional',
+        'stateOrProvinceName': 'optional',
+        'organizationName': 'optional',
+    }
+    keyin = '/etc/confluent/tls/ca/private/cakey.pem'
+    csrin = '/etc/confluent/tls/ca/ca.csr'
+
+    shortname = 'r3u20'
+
+
+    ipaddrs = list(certutil.get_ip_addresses())
+    san = [] # 'IP:{0}'.format(x) for x in ipaddrs]
+    # It is incorrect to put IP addresses as DNS type.  However
+    # there exists non-compliant clients that fail with them as IP
+    # san.extend(['DNS:{0}'.format(x) for x in ipaddrs])
+    dnsnames = set(ipaddrs)
+    dnsnames.add(shortname)
+    for currip in ipaddrs:
+        dnsnames.add(socket.getnameinfo((currip, 0), 0)[0])
+    for currname in dnsnames:
+        san.append('DNS:{0}'.format(currname))
+
+
+
+    if permitdomains[0] == '':
+        permitdomains = []
+    nameconstraints = ['permitted;DNS:{}'.format(x) for x in permitdomains]
+    nameconstraints.extend(['permitted;{}'.format(x) for x in san])
+    nameconstraints = ','.join(nameconstraints)
+    if nameconstraints:
+        nameconstraints = f'nameConstraints = critical,{nameconstraints}\n'
+    certutil.mkdirp('/etc/confluent/tls/ca-alt/newcerts')
+    with open('/etc/confluent/tls/ca-alt/index.txt', 'w') as idx:
+        pass
+
+    with open(sslcfg, 'r') as cfgin:
+        with open(newcfg, 'w') as cfgfile:
+            for line in cfgin.readlines():
+                cfg = line.split('#')[0]
+                if '=' in cfg:
+                    key, val = cfg.split('=', 1)
+                    for stg in settings:
+                        if certutil.substitute_cfg(stg, key, val, settings[stg], cfgfile, line):
+                            break
+                    else:
+                        cfgfile.write(line.strip() + '\n')
+                    continue
+                cfgfile.write(line.strip() + '\n')
+            cfgfile.write(f'\n[CACert]\nbasicConstraints = CA:true\n{nameconstraints}[ca_confluent]\n')
+    subprocess.check_call(
+        ['openssl', 'ca', '-config', newcfg, '-batch', '-selfsign',
+        '-extensions', 'CACert', '-extfile', newcfg,
+        '-notext', '-startdate',
+         '19700101010101Z', '-enddate', '21000101010101Z', '-keyfile',
+         keyin, '-out', certout, '-in', csrin]
+    )
+if __name__ == '__main__':
+    create_alt_ca(sys.argv[1], sys.argv[2].split(','))
+