freebsd stable - Feb 2014 - [patch] bootp_subr.c support for jumbo frames on BOOTP intf

I am attaching a patch for those that wish to test or provide feedback. 
Currently there is no support for adjusting the MTU on an interface a diskless
client boots from.  Additionally if you attempt to adjust the MTU on the
interface with ifconfig after the system comes up, the result is only cosmetic
and the booting interface still cannot pass frames larger than 1500 bytes.

There is a RFC standard DHCP TAG to provide a hit to a client what interface MTU
should be set to.  The attached patch will use this DHCP option in bootp and set
the interface MTU accordingly.  This allows diskless clients to be booted on a
jumbo frame enabled network.

Before using this, make sure your network and NIC is jumbo frame
enabled/capable.

Additionally the attached patch removes the client from setting itself as the
gateway (proxy-arp) if there is no router provided from DHCP.  The reasoning
behind this is that it is more common for hosts to be multi-homed and that the
booting interface may not necissarily be the interface a gateway is to be
installed on.  My erroneously setting gateway to hosts self, a default route
cannot be easily changed at boot time.  If the host needs to set itself as the
gateway, return that in an option in DHCP.


For tracking:

PR:  http://www.freebsd.org/cgi/query-pr.cgi?pr=misc/187094
     (should probably be moved from misc to kern)


Patch is against: 10.0-RELEASE r261846

--
Robert Blayzor
Network Architect & Engineer
CCIE 42148 (SP)
rblayzor at inoc.net
INOC, LLC
80 State Street, 7th Flr
Albany, NY  12207
USA


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