Re: DHCP off topic

[Tomas Pelka <tompelka zavinac gmail bod com>]

Petr Prcek Janda napsal(a):

> On Tue, Apr 01, 2008 at 11:12:21AM +0200, Tomas Pelka wrote:
>   
> > 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.
> >
> >     
> Hlavne se jedna o otazku na OS dhcp klienta, ne o OS na DHCP serveru,
> jak jsem to puvodne pochopil.
> Jako nejjednodussi zpusob, jak to zjistit me napada tcpdump (wireshark),
> ale o nejakem seznamu kde se jaky pristup vyuziva nevim.
>
>         
> 	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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>   
Asi ano. Zkousel sem zatim Windows XP Profesional SP2 a Linux s Internet 
Systems Consortium DHCP Client V3.0.4 a V3.0.10. Posilaji na unicast.

Diky.

--

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