[NTLUG:Discuss] Re: DHCP Vulnerability? -- IPv4 "link local"

[Bryan J. Smith]

On Fri, 2004-08-13 at 16:28, Jack Snodgrass wrote:
> 169.254/16 are what windows PCs ( and maybe some linux boxes ) assign 
> themselves when they can't talk to a DCHP server. The 169.254/16 range
> is documented someplace..... it may be a MS thing... 
> See http://www.win2000mag.com/Articles/Index.cfm?ArticleID=7464

On Fri, 2004-08-13 at 17:11, Cameron, Thomas wrote: 
> The 169.254.x.x range is part of APIPA (Automatic Private Internet
> Protocol Addressing):
> http://www.google.com/search?hl=en&ie=UTF-8&q=apipa&btnG=Google+Search.
> It means that your desktop never got a response from your DHCP server
> so it magicked up an IP address from that range (after checking that
> noone else was using it).

The short name for it is the IPv4 "Link Local" block.  Long story short,
there are many issues that arose the need for this block's reservation. 

The big one is that some OS network subsystems and/or DHCP clients
refuse to work if an IP adderss is not assigned with the interface comes
up.  So you have a chicken v. egg issue, I can't bring up the interface
until I have an IP address, but I have to have the interface up to send
out the RARP packets to get one.

In developing layer 2 drivers for the Linux kernel (and other OS
kernels) in 2001, finally seeing such a block reserved removed a _lot_
of headaches or potential gotchas in arbitrarily assigning a default
address.

-- Bryan

P.S.  Under IPv6, there is a dedicated "Link Local" block that is always
used.  Nodes have additional IPv6 addresses for non-local routing.


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Bryan J. Smith                                   b.j.smith at ieee.org