Assorted notes about udns (library).

UDP-only mode
~~~~~~~~~~~~~

First of all, since udns is (currently) UDP-only, there are some
shortcomings.

It assumes that a reply will fit into a UDP buffer.  With adoption of EDNS0,
and general robustness of IP stacks, in most cases it's not an issue.  But
in some cases there may be problems:

 - if an RRset is "very large" so it does not fit even in buffer of size
   requested by the library (current default is 4096; some servers limits
   it further), we will not see the reply, or will only see "damaged"
   reply (depending on the server).

 - many DNS servers ignores EDNS0 option requests.  In this case, no matter
   which buffer size udns library will request, such servers reply is limited
   to 512 bytes (standard pre-EDNS0 DNS packet size).  (Udns falls back to
   non-EDNO0 query if EDNS0-enabled one received FORMERR or NOTIMPL error).

The problem is that with this, udns currently will not consider replies with
TC (truncation) bit set, and will treat such replies the same way as it
treats SERVFAIL replies, thus trying next server, or temp-failing the query
if no more servers to try.  In other words, if the reply is really large, or
if the servers you're using don't support EDNS0, your application will be
unable to resolve a given name.

Yet it's not common situation - in practice, it's very rare.

Implementing TCP mode isn't difficult, but it complicates API significantly.
Currently udns uses only single UDP socket (or - maybe in the future - two,
see below), but in case of TCP, it will need to open and close sockets for
TCP connections left and right, and that have to be integrated into an
application's event loop in an easy and efficient way.  Plus all the
timeouts - different for connect(), write, and several stages of read.

IPv6 vs IPv4 usage
~~~~~~~~~~~~~~~~~~

This is only relevant for nameservers reachable over IPv6, NOT for IPv6
queries.  I.e., if you've IPv6 addresses in 'nameservers' line in your
/etc/resolv.conf file.  Even more: if you have BOTH IPv6 AND IPv4 addresses
there.  Or pass them to udns initialization routines.

Since udns uses a single UDP socket to communicate with all nameservers,
it should support both v4 and v6 communications.  Most current platforms
supports this mode - using PF_INET6 socket and V4MAPPED addresses, i.e,
"tunnelling" IPv4 inside IPv6.  But not all systems supports this.  And
more, it has been said that such mode is deprecated.

So, list only IPv4 or only IPv6 addresses, but don't mix them, in your
/etc/resolv.conf.

An alternative is to use two sockets instead of 1 - one for IPv6 and one
for IPv4.  For now I'm not sure if it's worth the complexity - again, of
the API, not the library itself (but this will not simplify library either).

Single socket for all queries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using single UDP socket for sending queries to all nameservers has obvious
advantages.  First it's, again, trivial, simple to use API.  And simple
library too.  Also, after sending queries to all nameservers (in case first
didn't reply in time), we will be able to receive late reply from first
nameserver and accept it.

But this mode has disadvantages too.  Most important is that it's much easier
to send fake reply to us, as the UDP port where we expects the reply to come
to is constant during the whole lifetime of an application.  More secure
implementations uses random port for every single query.  While port number
(16 bits integer) can not hold much randomness, it's still of some help.
Ok, udns is a stub resolver, so it expects sorta friendly environment, but
on LAN it's usually much easier to fire an attack, due to the speed of local
network, where a bad guy can generate alot of packets in a short time.

Choosing of DNS QueryID
~~~~~~~~~~~~~~~~~~~~~~~

Currently, udns uses sequential number for query IDs.  Which simplifies
attacks even more (c.f. the previous item about single UDP port), making
them nearly trivial.  The library should use random number for query ID.
But there's no portable way to get random numbers, even on various flavors
of Unix.  It's possible to use low bits from tv_nsec field returned by
gettimeofday() (current time, nanoseconds), but I wrote the library in
a way to avoid making system calls where possible, because many syscalls
means many context switches and slow processes as a result.  Maybe use some
application-supplied callback to get random values will be a better way,
defaulting to gettimeofday() method.

Note that a single query - even if (re)sent to different nameservers, several
times (due to no reply received in time), uses the same qID assigned when it
was first dispatched.  So we have: single UDP socket (fixed port number),
sequential (= trivially predictable) qIDs, and long lifetime of those qIDs.
This all makes (local) attacks against the library really trivial.

See also comments in udns_resolver.c, udns_newid().

And note that at least some other stub resolvers out there (like c-ares
for example) also uses sequential qID.

Assumptions about RRs returned
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Currently udns processes records in the reply it received sequentially.
This means that order of the records is significant.  For example, if
we asked for foo.bar A, but the server returned that foo.bar is a CNAME
(alias) for bar.baz, and bar.baz, in turn, has address 1.2.3.4, when
the CNAME should come first in reply, followed by A.  While DNS specs
does not say anything about order of records - it's an rrSET - unordered, -
I think an implementation which returns the records in "wrong" order is
somewhat insane...

CNAME recursion
~~~~~~~~~~~~~~~

Another interesting point is the handling of CNAMEs returned as replies
to non-CNAME queries.  If we asked for foo.bar A, but it's a CNAME, udns
expects BOTH the CNAME itself and the target DN to be present in the reply.
In other words, udns DOES NOT RECURSE CNAMES.  If we asked for foo.bar A,
but only record in reply was that foo.bar is a CNAME for bar.baz, udns will
return no records to an application (NXDOMAIN).  Strictly speaking, udns
should repeat the query asking for bar.baz A, and recurse.  But since it's
stub resolver, recursive resolver should recurse for us instead.

It's not very difficult to implement, however.  Probably with some (global?)
flag to en/dis-able the feature.  Provided there's some demand for it.

To clarify: udns handles CNAME recursion in a single reply packet just fine.

Note also that standard gethostbyname() routine does not recurse in this
situation, too.

Error reporting
~~~~~~~~~~~~~~~

Too many places in the code (various failure paths) sets generic "TEMPFAIL"
error condition.  For example, if no nameserver replied to our query, an
application will get generic TEMPFAIL, instead of something like TIMEDOUT.
This probably should be fixed, but most applications don't care about the
exact reasons of failure - 4 common cases are already too much:
  - query returned some valid data
  - NXDOMAIN
  - valid domain but no data of requested type - =NXDOMAIN in most cases
  - temporary error - this one sometimes (incorrectly!) treated as NXDOMAIN
    by (naive) applications.
DNS isn't yes/no, it's at least 3 variants, temp err being the 3rd important
case!  And adding more variations for the temp error case is complicating things
even more - again, from an application writer standpoint.  For diagnostics,
such more specific error cases are of good help.

Planned API changes
~~~~~~~~~~~~~~~~~~~

At least one thing I want to change for 0.1 version is a way how queries are
submitted and how replies are handled.

I want to made dns_query object to be owned by an application.  So that instead
of udns library allocating it for the lifetime of query, it will be pre-
allocated by an application.  This simplifies and enhances query submitting
interface, and complicates it a bit too, in simplest cases.

Currently, we have:

dns_submit_dn(dn, cls, typ, flags, parse, cbck, data)
dns_submit_p(name, cls, typ, flags, parse, cbck, data)
dns_submit_a4(ctx, name, flags, cbck, data)

and so on -- with many parameters missed for type-specific cases, but generic
cases being too complex for most common usage.

Instead, with dns_query being owned by an app, we will be able to separately
set up various parts of the query - domain name (various forms), type&class,
parser, flags, callback...  and even change them at runtime.  And we will also
be able to reuse query structures, instead of allocating/freeing them every
time.  So the whole thing will look something like:

 q = dns_alloc_query();
 dns_submit(dns_q_flags(dns_q_a4(q, name, cbck), DNS_F_NOSRCH), data);

The idea is to have a set of functions accepting struct dns_query* and
returning it (so the calls can be "nested" like the above), to set up
relevant parts of the query - specific type of callback, conversion from
(type-specific) query parameters into a domain name (this is for type-
specific query initializers), and setting various flags and options and
type&class things.

One example where this is almost essential - if we want to support
per-query set of nameservers (which isn't at all useless: imagine a
high-volume mail server, were we want to direct DNSBL queries to a separate
set of nameservers, and rDNS queries to their own set and so on).  Adding
another argument (set of nameservers to use) to EVERY query submitting
routine is.. insane.  Especially since in 99% cases it will be set to
default NULL.  But with such "nesting" of query initializers, it becomes
trivial.

Another way to do the same is to manipulate query object right after a
query has been submitted, but before any events processing (during this
time, query object is allocated and initialized, but no actual network
packets were sent - it will happen on the next event processing).  But
this way it become impossible to perform syncronous resolver calls, since
those calls hide query objects they use internally.

Speaking of replies handling - the planned change is to stop using dynamic
memory (malloc) inside the library.  That is, instead of allocating a buffer
for a reply dynamically in a parsing routine (or memdup'ing the raw reply
packet if no parsing routine is specified), I want udns to return the packet
buffer it uses internally, and change parsing routines to expect a buffer
for result.  When parsing, a routine will return true amount of memory it
will need to place the result, regardless of whenever it has enough room
or not, so that an application can (re)allocate properly sized buffer and
call a parsing routine again.

Another modification I plan to include is to have an ability to work in
terms of domain names (DNs) as used with on-wire DNS packets, not only
with asciiz representations of them.  For this to work, the above two
changes (query submission and result passing) have to be completed first
(esp. the query submission part), so that it will be possible to specify
some additional query flags (for example) to request domain names instead
of the text strings, and to allow easy query submissions with either DNs
or text strings.
