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

[Simo Sorce]

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.
The problem with subtypes and ACIs though is that I think ACIs do not
care about the subtype unless you explicit mention them.
So perhaps bind_dyndb_ldap should refuse to use a generic type that
shadows DNSSEC relevant records ?

Simo.

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