21.Where is DHCP defined?
In Internet RFCs.
22. Can DHCP support remote access?
PPP has its own non-DHCP way in which communications servers can hand
clients an IP address called IPCP (IP Control Protocol) but doesn't have the same
flexibility as DHCP or BOOTP in handing out other parameters. Such a
communications server may support the use of DHCP to acquire the IP addresses
it gives out. This is sometimes called doing DHCP by proxy for the client. I know
that Windows NT's remote access support does this.
A feature of DHCP under development (DHCPinform) is a method by which a
DHCP server can supply parameters to a client that already has an IP number.
With this, a PPP client could get its IP number using IPCP, then get the rest of its
parameters using this feature of DHCP.
SLIP has no standard way in which a server can hand a client an IP address, but
many communications servers support non-standard ways of doing this that can
be utilized by scripts, etc. Thus, like communications servers supporting PPP,
such communications servers could also support the use of DHCP to acquire the
IP addressees to give out.
The DHCP protocol is capable of allocating an IP address to a device without an
IEEE-style MAC address, such as a computer attached through SLIP or PPP, but
to do so, it makes use of a feature which may or may not be supported by the
DHCP server: the ability of the server to use something other than the MAC
address to identify the client. Communications servers that acquire IP numbers
for their clients via DHCP run into the same roadblock in that they have just one
MAC address, but need to acquire more than one IP address. One way such a
communications server can get around this problem is through the use of a set
of unique pseudo-MAC addresses for the purposes of its communications with
the DHCP server. Another way (used by Shiva) is to use a different "client ID
type" for your hardware address. Client ID type 1 means you're using MAC
addresses. However, client ID type 0 means an ASCII string.
23.How can I relay DHCP if my router does not support it?
A server on a net(subnet) can relay DHCP or BOOTP for that net. Microsoft has
software to make Windows NT do this.
24.What is DHCP Spoofing?
Ascend Pipeline ISDN routers (which attach Ethernets to ISDN lines) incorporate
a feature that Ascend calls "DHCP spoofing" which is essentially a tiny server
implementation that hands an IP address to a connecting Windows 95 computer,
with the intention of giving it an IP number during its connection process.
25. How long should a lease be?
A very relevant factor is that the client starts trying to renew the lease when it is
halfway through: thus, for example, with a 4 day lease, the client which has lost
access to its DHCP server has 2 days from when it first tries to renew the lease
until the lease expires and the client must stop using the network. During a 2-
day outage, new users cannot get new leases, but no lease will expire for any
computer turned on at the time that the outage commences.
Another factor is that the longer the lease the longer time it takes for client
configuration changes controlled by DHCP to propogate.
No comments:
Post a Comment