Re: DHCP off topic

[Tomas Pelka <tompelka zavinac gmail bod com>]

Petr Prcek Janda napsal(a):
> On Sun, Mar 30, 2008 at 11:43:13PM +0200, Tomas Pelka wrote:
>   
> > Dobry vecer,
> > resim takovy drobny problem, trochu off topic priznavam. S kolegou jsem 
> > se preli o nasledujicim problemu.
> > Princip cinnosti DHCP je jiste kazdemu znam. Nejprve klientska stanice 
> > posila DHCP_DISCOVER a server odpovida zpravou DHCP_OFFER. Soucasti 
> > DHCP_OFFER je i tzv. transaction_ID (podle toho server pozna ktera 
> > relace kam patri, alespon tak si to vysvetluji). Otazkou je na jakou MAC 
> > adresu je DHCP_OFFER odesilan? V RFC 2131 je napsano ze existuji dva 
> > pristupy:
> >   1)posila na FF:FF:FF... (striktni varianta)
> >   2)na MAC klienta, ktery o IP zadal (liberalnejsi varianta).
> > Opravte me pokud jsem neco uvedl nepresne.
> >
> > Vi nekdo jestli existuji zaznamy a kde je najit o tom ktery OS jakou 
> > varinatu pouziva?
> >
> >     
>
> Tak jste me zviklal kouknout do toho rfc, jestli jsem blbej ja, nebo
> spatne ctete
>
> xid (4) Transaction ID, a random number chosen by the
>         client, used by the client and server to
>         associate messages and responses
>         between a client and a server.
>
> A k vasemu dotazu - imho to vubec nezalezi na libovuli OS 
> (nevim, co ten do toho na co kecat), nebo DHCP serveru, ale je dost 
> striktne definovano, kdy se to na jakou adresu posila.
>
>    If the 'giaddr' field in a DHCP message from a client is non-zero,
>    the server sends any return messages to the 'DHCP server' port on the
>    BOOTP relay agent whose address appears in 'giaddr'. If the 'giaddr'
>    field is zero and the 'ciaddr' field is nonzero, then the server
>    unicasts DHCPOFFER and DHCPACK messages to the address in 'ciaddr'.
>    If 'giaddr' is zero and 'ciaddr' is zero, and the broadcast bit is
>    set, then the server broadcasts DHCPOFFER and DHCPACK messages to
>    0xffffffff. If the broadcast bit is not set and 'giaddr' is zero and
>    'ciaddr' is zero, then the server unicasts DHCPOFFER and DHCPACK
>    messages to the client's hardware address and 'yiaddr' address.  In
>    all cases, when 'giaddr' is zero, the server broadcasts any DHCPNAK
>    messages to 0xffffffff.
>
>
> 	S pozdravem 
>
>                                 Petr Janda
> --
> email: /bin/sh -c 'A=netbox; B=janda; printf "%s zavinac %s bod cz\n" ${B} ${A}'
>
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>   
Jeste z RFC:

/Normally, DHCP servers and BOOTP relay agents attempt to deliver 
DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using 
uicast delivery. The IP destination address (in the IP header) is set to 
the DHCP 'yiaddr' address and the link-layer destination address is set 
to the DHCP 'chaddr' address. Unfortunately, some client implementations 
are unable to receive such unicast IP datagrams until the implementation 
has been configured with a valid IP address (leading to a deadlock in 
which the client's IP address cannot be delivered until the client has 
been configured with an IP address).//

A client that cannot receive unicast IP datagrams until its protocol 
software has been configured with an IP address SHOULD set the BROADCAST 
bit in the 'flags' field to 1 in any DHCPDISCOVER or DHCPREQUEST 
messages that client sends. The BROADCAST bit will provide a hint to the 
DHCP server and BOOTP relay agent to broadcast any messages to the 
client on the client's subnet. A client that can receive unicast IP 
datagrams before its protocol software has been configured SHOULD clear 
the BROADCAST bit to 0. The BOOTP clarifications document discusses the 
ramifications of the use of the BROADCAST bit [21].//

A server or relay agent sending or relaying a DHCP message directly to a 
DHCP client (i.e., not to a relay agent specified in the 'giaddr' field) 
SHOULD examine the BROADCAST bit in the 'flags' field. If this bit is 
set to 1, the DHCP message SHOULD be sent as an IP broadcast using an IP 
broadcast address (preferably 0xffffffff) as the IP destination address 
and the link-layer broadcast address as the link-layer destination 
address. If the BROADCAST bit is cleared to 0, the message SHOULD be 
sent as an IP unicast to the IP address specified in the 'yiaddr' field 
and the link-layer address specified in the 'chaddr' field. If 
unicasting is not possible, the message MAY be sent as an IP broadcast 
using an IP broadcast address (preferably 0xffffffff) as the IP 
destination address and the link- layer broadcast address as the 
link-layer destination address.

/Takze to asi bude zaviset na linkove vrstve a ta je dle meho nazoru 
implementovana v OS, ne? Ale treba se pletu.

Koukam ze jsem spatne polozil otazku, asi me melo jednat o IP a ne MAC.

--

S pozdravem Tomáš Pelka
e-mail: tompelka zavinac gmail bod com