[Freeipa-devel] Generic support for unknown DNS RR types (RFC 3597)

Wed Mar 11 07:13:56 UTC 2015

On 03/10/2015 07:24 PM, Petr Spacek wrote:
> On 10.3.2015 18:36, Simo Sorce wrote:
>> On Tue, 2015-03-10 at 18:26 +0100, Petr Spacek wrote:
>>> On 10.3.2015 17:35, Simo Sorce wrote:
>>>> On Tue, 2015-03-10 at 16:19 +0100, Petr Spacek wrote:
>>>>> On 10.3.2015 15:53, Simo Sorce wrote:
>>>>>> On Tue, 2015-03-10 at 15:32 +0100, Petr Spacek wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> I would like to discuss Generic support for unknown DNS RR types (RFC 3597
>>>>>>> [0]). Here is the proposal:
>>>>>>>
>>>>>>> LDAP schema
>>>>>>> ===========
>>>>>>> - 1 new attribute:
>>>>>>> ( <OID> NAME 'GenericRecord' DESC 'unknown DNS record, RFC 3597' EQUALITY
>>>>>>> caseIgnoreIA5Match SYNTAX 1.3.6.1.4.1.1466.115.121.1.26 )
>>>>>>>
>>>>>>> The attribute should be added to existing idnsRecord object class as MAY.
>>>>>>>
>>>>>>> This new attribute should contain data encoded according to RFC 3597 section
>>>>>>> 5 [5]:
>>>>>>>
>>>>>>> The RDATA section of an RR of unknown type is represented as a
>>>>>>>    sequence of white space separated words as follows:
>>>>>>>
>>>>>>>       The special token \# (a backslash immediately followed by a hash
>>>>>>>       sign), which identifies the RDATA as having the generic encoding
>>>>>>>       defined herein rather than a traditional type-specific encoding.
>>>>>>>
>>>>>>>       An unsigned decimal integer specifying the RDATA length in octets.
>>>>>>>
>>>>>>>       Zero or more words of hexadecimal data encoding the actual RDATA
>>>>>>>       field, each containing an even number of hexadecimal digits.
>>>>>>>
>>>>>>>    If the RDATA is of zero length, the text representation contains only
>>>>>>>    the \# token and the single zero representing the length.
>>>>>>>
>>>>>>> Examples from RFC:
>>>>>>>       a.example.   CLASS32     TYPE731         \# 6 abcd (
>>>>>>>                                                ef 01 23 45 )
>>>>>>>       b.example.   HS          TYPE62347       \# 0
>>>>>>>       e.example.   IN          A               \# 4 0A000001
>>>>>>>       e.example.   CLASS1      TYPE1           10.0.0.2
>>>>>>>
>>>>>>>
>>>>>>> Open questions about LDAP format
>>>>>>> ================================
>>>>>>> Should we include "\#" constant? We know that the attribute contains record in
>>>>>>> RFC 3597 syntax so it is not strictly necessary.
>>>>>>>
>>>>>>> I think it would be better to follow RFC 3597 format. It allows blind
>>>>>>> copy&pasting from other tools, including direct calls to python-dns.
>>>>>>>
>>>>>>> It also eases writing conversion tools between DNS and LDAP format because
>>>>>>> they do not need to change record values.
>>>>>>>
>>>>>>>
>>>>>>> Another question is if we should explicitly include length of data represented
>>>>>>> in hexadecimal notation as a decimal number. I'm very strongly inclined to let
>>>>>>> it there because it is very good sanity check and again, it allows us to
>>>>>>> re-use existing tools including parsers.
>>>>>>>
>>>>>>> I will ask Uninett.no for standardization after we sort this out (they own the
>>>>>>> OID arc we use for DNS records).
>>>>>>>
>>>>>>>
>>>>>>> Attribute usage
>>>>>>> ===============
>>>>>>> Every DNS RR type has assigned a number [1] which is used on wire. RR types
>>>>>>> which are unknown to the server cannot be named by their mnemonic/type name
>>>>>>> because server would not be able to do name->number conversion and to generate
>>>>>>> DNS wire format.
>>>>>>>
>>>>>>> As a result, we have to encode the RR type number somehow. Let's use attribute
>>>>>>> sub-types.
>>>>>>>
>>>>>>> E.g. a record with type 65280 and hex value 0A000001 will be represented as:
>>>>>>> GenericRecord;TYPE65280: \# 4 0A000001
>>>>>>>
>>>>>>>
>>>>>>> CLI
>>>>>>> ===
>>>>>>> $ ipa dnsrecord-add zone.example owner \
>>>>>>>   --generic-type=65280 --generic-data='\# 4 0A000001'
>>>>>>>
>>>>>>> $ ipa dnsrecord-show zone.example owner
>>>>>>> Record name: owner
>>>>>>> TYPE65280 Record: \# 4 0A000001
>>>>>>>
>>>>>>>
>>>>>>> ACK? :-)
>>>>>>
>>>>>> Almost.
>>>>>> We should refrain from using subtypes when not necessary, and in this
>>>>>> case it is not necessary.
>>>>>>
>>>>>> Use:
>>>>>> GenericRecord: 65280 \# 4 0A000001
>>>>>
>>>>> I was considering that too but I can see two main drawbacks:
>>>>>
>>>>> 1) It does not work very well with DS ACI (targetattrfilter, anyone?). Adding
>>>>> generic write access to GenericRecord == ability to add TLSA records too,
>>>>> which you may not want. IMHO it is perfectly reasonable to limit write access
>>>>> to certain types (e.g. to one from private range).
>>>>>
>>>>> 2) We would need a separate substring index for emulating filters like
>>>>> (type==65280). AFAIK GenericRecord;TYPE65280 should work with presence index
>>>>> which will be handy one day when we decide to handle upgrades like
>>>>> GenericRecord;TYPE256->UriRecord.
>>>>>
>>>>> Another (less important) annoyance is that conversion tools would have to
>>>>> mangle record data instead of just converting attribute name->record type.
>>>>>
>>>>>
>>>>> I can be convinced that subtypes are not necessary but I do not see clear
>>>>> advantage of avoiding them. What is the problem with subtypes?
>>>>
>>>> Poor support by most clients, so it is generally discouraged.
>>> Hmm, it does not sound like a thing we should care in this case. DNS tree is
>>> not meant for direct consumption by LDAP clients (compare with cn=compat).
>>>
>>> IMHO the only two clients we should care are FreeIPA framework and
>>> bind-dyndb-ldap so I do not see this as a problem, really. If someone wants to
>>> access DNS tree by hand - sure, use a standard compliant client!
>>>
>>> Working ACI and LDAP filters sounds like good price for supporting only
>>> standards compliant clients.
>>>
>>> AFAIK OpenLDAP works well and I suspect that ApacheDS will work too because
>>> Eclipse has nice support for sub-types built-in. If I can draw some
>>> conclusions from that, sub-types are not a thing aliens forgot here when
>>> leaving Earth one million years ago :-)
>>>
>>>> The problem with subtypes and ACIs though is that I think ACIs do not
>>>> care about the subtype unless you explicit mention them.
>>> IMHO that is exactly what I would like to see for GenericRecord. It allows us
>>> to write ACI which allows admins to add any GenericRecord and at the same time
>>> allows us to craft ACI which allows access only to GenericRecord;TYPE65280 for
>>> specific group/user.
>>>
>>>> So perhaps bind_dyndb_ldap should refuse to use a generic type that
>>>> shadows DNSSEC relevant records ?
>>> Sorry, this cannot possibly work because it depends on up-to-date blacklist.
>>>
>>> How would the plugin released in 2015 know that highly sensitive OPENPGPKEY
>>> type will be standardized in 2016 and assigned number XYZ?
>>
>> Ok, show me an example ACI that works and you get my ack :)
> 
> Am I being punished for something? :-)
> 
> Anyway, this monstrosity:
> 
> (targetattr = "objectclass || txtRecord;test")(target =
> "ldap:///idnsname=*,cn=dns,dc=ipa,dc=example")(version 3.0;acl
> "permission:luser: Read DNS Entries";allow (compare,read,search) userdn =
> "ldap:///uid=luser,cn=users,cn=accounts,dc=ipa,dc=example";)
> 
> Gives 'luser' read access only to txtRecord;test and *not* to the whole
> txtRecord in general.
> 
> $ kinit luser
> $ ldapsearch -Y GSSAPI -s base -b
> 'idnsname=txt,idnsname=ipa.example.,cn=dns,dc=ipa,dc=example'
> SASL username: luser at IPA.EXAMPLE
> 
> # txt, ipa.example., dns, ipa.example
> dn: idnsname=txt,idnsname=ipa.example.,cn=dns,dc=ipa,dc=example
> objectClass: top
> objectClass: idnsrecord
> tXTRecord;test: Guess what is new here!
> 
> Filter '(tXTRecord;test=*)' works as expected and returns only objects with
> subtype ;test.
> 
> The only weird thing I noticed is that search filter '(tXTRecord=*)' does not
> return the object if you have access only to an subtype with existing value
> but not to the 'vanilla' attribute.
> 
> Maybe it is a bug? I will think about it for a while and possibly open a
> ticket. Anyway, this is not something we need for implementation.

Ludwig? IIRC, DS does not return object for LDAP search, if the user does not
have access to *all* searched attributes. So I am on fence whether this is a
bug or not. As user does not really have access to the base attribute...

Martin