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

Tue Mar 10 17:36:05 UTC 2015

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 :)

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York