dhcp relay - stateless - debug

foomail123 foomail123 at yahoo.com
Thu Jul 20 19:05:16 UTC 2006


Thank you for information.

Question on DHCP debugging,
I see syslog interface is being used.
Is this right ? Is this only way ?
I was looking for an option for debug logs to
be sent to stdout and/or file.

Thanks,

--- Allwyn Carvalho <allwyn at lucent.com> wrote:

> One inline.
> 
> Simon Hobson wrote:
> 
> >foomail123 wrote:
> >
> >  
> >
> >>Since dhcp relay is stateless, if there are
> multiple
> >>interfaces, how does relay know which interface to
> >>foward DHCP offer received from server ?
> >>
> >>How does relay work in vlan case (multiple
> interfaces
> >>in a vlan) ?
> >>
> >>Also, being stateless if same host is connected to
> >>multiple interfaces and/or vlans, how does relay
> >>process DHCP offer packet received from server ?
> >>    
> >>
> >
> >The process is fairly simple.
> >
> >On receiving a broadcast packet from a client, it
> puts a Gateway 
> >Interface Address (GIAddr) into the header of the
> packet before 
> >unicasting it to the server(s).
> >
> >Replies from the server are unicast to the GIAddr,
> so the relay agent 
> >can tell from that which interface to broadcast it
> back out on.
> >  
> >
> 
> The relay agent may also insert "relay agent
> options" -- option-82, to 
> identify the client interface.  See RFC 3046.  This
> is better than using 
> giaddr, because the same giaddr may be used to
> fulfill DHCP requests 
> from multiple client interfaces.
> 
> Allwyn.
> 
> >GIAddr is the key - it must be an address that both
> the server and 
> >agent can identify with a subnet or shared network
> (in the case of 
> >the server) and an interface (for the relay agent).
> It is typically 
> >the IP address of the interface through which the
> broadcast request 
> >was received - it can be a different address, but
> if it isn't in the 
> >same subnet then it must still be unique to the
> interface (ie you 
> >can't use the same loopback address for three
> different networks) and 
> >you would need a shared network statement on the
> server to associate 
> >the network and the GIAddr.
> >
> >
> >VLANs are no different. A client will normally only
> be in one VLAN 
> >(typically determined by the setting for the
> physical port of the 
> >switch it connects to) - in network terms it's no
> different to having 
> >a separate switch for the VLAN. The relay agent
> must use a GIAddr 
> >appropriate to the VLAN it detects the client as
> being connected to.
> >
> >
> >If a host is connected to multiple networks or
> VLANs, then the host 
> >is responsible for making dhcp requests on each
> interface (or virtual 
> >interface) it wishes to configure - but each
> request is uniquely 
> >linked to a network as above.
> >
> >
> >If you have a shared network (ie multiple IP
> subnets on one physical 
> >network), then the relay agent cannot differentiate
> clients  - that 
> >is the servers job IF the administrator has
> configured it accordingly 
> >(eg by using classes to assign clients to subnets).
> >
> >
> >Simon
> >  
> >
> 
> 


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