The documentation in xfer.h and xfer.c does not say that the metadata
parameter is optional in calls such as xfer_deliver_iob_meta() and the
deliver_iob() method.  However, some code in net/ is prepared to
accept a NULL pointer, and xfer_deliver_as_iob() passes a NULL pointer
directly to the deliver_iob() method.

Fix this mess of conflicting assumptions by making everything assume
that the metadata parameter is mandatory, and fixing
xfer_deliver_as_iob() to pass in a dummy metadata structure (as is
already done in xfer_deliver_iob()).

Various combinations of options 43.6, 43.7 and 43.8 dictate which
servers we send Boot Server Discovery requests to, and which servers
we should accept responses from.  Obey these options, and remove the
explicit specification of a single Boot Server from start_pxebs() and
dependent functions.

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

The DHCP client code now implements only the mechanism of the DHCP and
PXE Boot Server protocols.  Boot Server Discovery can be initiated
manually using the "pxebs" command.  The menuing code is separated out
into a user-level function on a par with boot_root_path(), and is
entered in preference to a normal filename boot if the DHCP vendor
class is "PXEClient" and the PXE boot menu option exists.

PXE dictates a mechanism for boot menuing, involving prompting the
user with a variable message, waiting for a predefined keypress,
displaying a boot menu, and waiting for a selection.

This breaks the currently desirable abstraction that DHCP is a process
that can happen in the background without any user interaction.

Remove the lazy assumption that ProxyDHCP == "DHCP with option 60 set
to PXEClient", and explicitly separate the notion of ProxyDHCP from
the notion of packets containing PXE options.

It is possible to configure a DHCP server to hand out PXE options
without a ProxyDHCP server present.  This requires setting option 60
to "PXEClient", which will cause gPXE to attempt ProxyDHCP.

We assume in several places that dhcp->proxydhcpack is set to the
DHCPACK packet containing option 60 set to "PXEClient".  When we
transition into ProxyDHCPREQUEST, set dhcp->proxydhcpack=dhcp->dhcpack
so that this assumption holds true.

We ought to rename several references to "proxydhcp" to something more
accurate, such as "pxedhcp".  Treating a single DHCP response as
potentially both DHCPOFFER and ProxyDHCPOFFER does make the code
smaller, but the variable names get confusing.

Pick out the first boot menu item from the boot menu (option 43.9) and
pass it to the boot server as the boot menu item (option 43.71).

Also improve DHCP debug messages to include more details of the
packets being transmitted.

Some PXE configurations require us to perform a third DHCP transaction
(in addition to the real DHCP transaction and the ProxyDHCP
transaction) in order to retrieve information from a "Boot Server".

This is an experimental implementation, since the actual behaviour is
not well specified in the PXE spec.

Move all the DHCP state transition logic into a single function
dhcp_next_state().  This will make it easier to add support for PXE
Boot Servers, since it abstracts away the difference between "mark
DHCP as complete" and "transition to boot server discovery".

The Linux PXE server (http://www.kano.org.uk/projects/pxe) does not
set the server identifier in its ProxyDHCP responses.  If the server
ID is missing, do not treat this as an error.

This resolves the "vague and unsettling memory" mentioned in commit
fdb8481d ("[dhcp] Verify server identifier on ProxyDHCPACKs").

Note that we already accept ProxyDHCPOFFERs without a server
identifier; they get treated as potential BOOTP packets.

Settings can be constructed using a dotted-decimal notation, to allow
for access to unnamed settings.  The default interpretation is as a
DHCP option number (with encapsulated options represented as
"<encapsulating option>.<encapsulated option>".

In several contexts (e.g. SMBIOS, Phantom CLP), it is useful to
interpret the dotted-decimal notation as referring to non-DHCP
options.  In this case, it becomes necessary for these contexts to
ignore standard DHCP options, otherwise we end up trying to, for
example, retrieve the boot filename from SMBIOS.

Allow settings blocks to specify a "tag magic".  When dotted-decimal
notation is used to construct a setting, the tag magic value of the
originating settings block will be ORed in to the tag number.
Store/fetch methods can then check for the magic number before
interpreting arbitrarily-numbered settings.

The ProxyDHCPREQUEST is a unicast packet, so the first request will
almost always be lost due to not having the IP address in the ARP
cache.  If the minimum retry time is set to one second (as per commit
ff2b6a5), then ProxyDHCP will time out and give up before managing to
successfully transmit a request.

The DHCP timers need to be reworked anyway, so this mild hack is
acceptable for now.

New min_timeout and max_timeout fields in struct retry_timer allow
users of this timer to set their own desired minimum and maximum
timeouts, without being constrained to a single global minimum and
maximum. Users of the timer can still elect to use the default global
values by leaving the min_timeout and max_timeout fields as 0.

Altiris erroneously cares about the ordering of DHCP options, and will
get confused if we don't construct them in the order it expects.

This is observed (so far) only when attempting to deploy 64-bit Win2k3.

Verifying server ID and DHCP transaction ID is insufficient to
differentiate between DHCPACK and ProxyDHCPACK when the DHCP server and
Proxy DHCP server are the same machine.

Perform the same test for a matching DHCP_SERVER_IDENTIFIER on
ProxyDHCPACKs as we do for DHCPACKs.  Otherwise, a retransmitted
DHCPACK can end up being treated as the ProxyDHCPACK.

I have a vague and unsettling memory that this test was deliberately
omitted, but I can't remember why, and can't find anything in the VC
logs.

A missing test for dhcp->dhcpoffer in dhcp_timer_expired() was causing
the client to transition to DHCPREQUEST after timing out on waiting
for ProxyDHCP even if no DHCPOFFERs had been received.

In particular, expose the system UUID as a setting ("smbios/uuid").

The PXE spec is (as usual) unclear on precisely when ProxyDHCPREQUESTs
should be issued.  We adapt the following, slightly paranoid approach:

  If an offer contains an IP address, then it is a normal DHCPOFFER.

  If an offer contains an option #60 "PXEClient", then it is a
  ProxyDHCPOFFER.  Note that the same packet can be both a normal
  DHCPOFFER and a ProxyDHCPOFFER.

  After receiving the normal DHCPACK, if we have received a
  ProxyDHCPOFFER, we unicast a ProxyDHCPREQUEST back to the ProxyDHCP
  server on port 4011.  If we time out waiting for a ProxyDHCPACK, we
  treat this as a non-fatal error.

Copy DHCP responses to a standard malloc()ed buffer, rather than
retaining the I/O buffer that they arrived in.

Allow for settings to be described by something other than a DHCP option
tag if desirable.  Currently used only for the MAC address setting.

Separate out fake DHCP packet creation code from dhcp.c to fakedhcp.c.

Remove notion of settings from dhcppkt.c.

Rationalise dhcp.c to use settings API only for final registration of the
DHCP options, rather than using {store,fetch}_setting throughout.

DHCP code was using an incorrect check for whether to construct a
DHCPDISCOVER or DHCPREQUEST packet.

Add dedicated functions create_dhcpdiscover(), create_dhcpack() and
create_proxydhcpack() for use by external code such as the PXE preboot
code.

Register ProxyDHCP options under the global scope "proxydhcp".

Unregister previously-acquired DHCP and ProxyDHCP settings when DHCP
succeeds.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

RFC 4390 provides for the DHCP client identifier to contain the link-layer
hardware type and MAC address when the MAC address exceeds 16 bytes.
However, the hardware type field is only 8 bits; we were assuming 16 bits.

separate xfer method.

Add missing .alloc_iob entries to several xfer_interface_operations
structures.

their equivalent DHCP-option-specified values.

also avoid waiting for ProxyDHCP offers).

Also reduce the ProxyDHCP timeout, because it's already irritating me.

to construct DHCP packets suitable for Infiniband.

to construct DHCP packets suitable for Infiniband.

to generate the initial transmission; this cuts off around 0.3s per
instantiated connection.

and generate DHCP options to indicate features to DHCP server (and to
PXE NBPs).