router with working DHCP server

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

Does anyone have a broadband router with a working DHCP server?

By working, I mean a router that after power cycle, would remeber or
figure out the IP addresses it gave out previously, or are currently in
use on the network.

I had tried many routers and APs with DHCP servers before and all of
them would forget the IP addresses they gave out, and give out duplicate
IP address already in use.

One way to test your router is this (assuming the DHCP server's starting
address is 100):

Connect two DHCP client machines to the router. Say machine A gets
address *.*.*.100 and machine B gets *.*.*.101. Power cycle the router.
Reboot machine B and see if it gets an IP address. If your router has
a simple DHCP implemenation it will give out *.*.*.100 address which is
still in use by machine A. So B will never get an IP address and
machine A's console probably will report that there's a duplicate IP
address on the network.

I would expect the DHCP server to at least remember the IP addresses it
gave out for the duration of the lease.

 

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

On Thu, 15 Jul 2004 07:16:10 -0700, Jeff Liebermann
<jeffl@comix.santa-cruz.ca.us> wrote:

>>I had tried many routers and APs with DHCP servers before and all of
>>them would forget the IP addresses they gave out, and give out duplicate
>>IP address already in use.

A bit more on the subject.

From RFC1541:
(...) As a consistency check, the allocation
mechanism may probe the reused address, e.g., with an ICMP echo
request, before allocating the address, and the client will
probe the newly received address, e.g., with ARP.

What this means is that if the DHCP server feels the need to reassign
an IP address (due to overflow, IP exhaustion, or a reboot), it may
ping probe for the previous lease holder of the IP address to see if
it is available. Since you so cleverly disconnected the previous
lease holder of the specific IP address involved, ping will fail, DHCP
will declare that the address is available, and reassign the IP
address to the next victim. In simpler terms, if nobody is home after
a reboot, there's no requirement to restore a bunch of stale
addresses.


--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558

 

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

WD wrote:

>
> Does anyone have a broadband router with a working DHCP server?
>
> By working, I mean a router that after power cycle, would remeber or
> figure out the IP addresses it gave out previously, or are currently in
> use on the network.
>
> I had tried many routers and APs with DHCP servers before and all of
> them would forget the IP addresses they gave out, and give out duplicate
> IP address already in use.
>
> One way to test your router is this (assuming the DHCP server's starting
> address is 100):
>
> Connect two DHCP client machines to the router. Say machine A gets
> address *.*.*.100 and machine B gets *.*.*.101. Power cycle the router.
> Reboot machine B and see if it gets an IP address. If your router has
> a simple DHCP implemenation it will give out *.*.*.100 address which is
> still in use by machine A. So B will never get an IP address and
> machine A's console probably will report that there's a duplicate IP
> address on the network.
>
> I would expect the DHCP server to at least remember the IP addresses it
> gave out for the duration of the lease.
>

Uh, the D in DHCP means Dynamic, which means you cannot count on getting
the same IPA-to-PC assignments. If it matters to one PC, give it a
static IPA in the same subnet as the rest of the clients, and exclude
that IPA from being handed out via DHCP; works for me.
--
Cheers, Bob

 

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

I wasn't concerned about getting a different address; I was concerned
about getting a working address (i.e. one that's not already in use).

Bob Willard wrote:
> WD wrote:
>
>>
>> Does anyone have a broadband router with a working DHCP server?
>>
>> By working, I mean a router that after power cycle, would remeber or
>> figure out the IP addresses it gave out previously, or are currently
>> in use on the network.
>>
>> I had tried many routers and APs with DHCP servers before and all of
>> them would forget the IP addresses they gave out, and give out
>> duplicate IP address already in use.
>>
>> One way to test your router is this (assuming the DHCP server's
>> starting address is 100):
>>
>> Connect two DHCP client machines to the router. Say machine A gets
>> address *.*.*.100 and machine B gets *.*.*.101. Power cycle the
>> router. Reboot machine B and see if it gets an IP address. If your
>> router has a simple DHCP implemenation it will give out *.*.*.100
>> address which is still in use by machine A. So B will never get an IP
>> address and machine A's console probably will report that there's a
>> duplicate IP address on the network.
>>
>> I would expect the DHCP server to at least remember the IP addresses
>> it gave out for the duration of the lease.
>>
>
> Uh, the D in DHCP means Dynamic, which means you cannot count on getting
> the same IPA-to-PC assignments. If it matters to one PC, give it a
> static IPA in the same subnet as the rest of the clients, and exclude
> that IPA from being handed out via DHCP; works for me.

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

sorry, that's not how they work. when the router goes down all the clients
need resetting. at least that's how my routers have always worked.

jtm

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

I certainly hope this is not true in general. If true we might as well
throw the DHCP out the window. If true, imagine what you have to do in
a corporate network, or what an ISP has to do if their DHCP server crashes.

On second thought, that might just be what some ISPs do - kick off
everyone off when their DHCP server crashes ;-)

I have read some DHCP servers address this problem by saving the
assigned IP addresses in a file and read it back on reboot (e.g. Linux
implementation), or save the addresses in non-volatile memory (router
implementation). Yet another way to address this issue is to "probe" an
IP address before assignig it to determine if it's in use. This
algorithm, while it works most of the time, is still flawed: it won't
work if a computer which has been assigned a valid address is
temporarily off line; the DHCP server might take this address and give
it to the next computer requesting one.



Jim Miller wrote:

> sorry, that's not how they work. when the router goes down all the clients
> need resetting. at least that's how my routers have always worked.
>
> jtm
>
>

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

I certainly hope this is not true in general. If true we might as well
throw the DHCP out the window. If true, imagine what you have to do in
a corporate network, or what an ISP has to do if their DHCP server crashes.

On second thought, that might just be what some ISPs do - kick off
everyone off when their DHCP server crashes ;-)

I have read some DHCP servers address this problem by saving the
assigned IP addresses in a file and read it back on reboot (e.g. Linux
implementation), or save the addresses in non-volatile memory (router
implementation). Yet another way to address this issue is to "probe" an
IP address before assignig it to determine if it's in use. This
algorithm, while it works most of the time, is still flawed: it won't
work if a computer which has been assigned a valid address is
temporarily off line; the DHCP server might take this address and give
it to the next computer requesting one.



Jim Miller wrote:

> sorry, that's not how they work. when the router goes down all the clients
> need resetting. at least that's how my routers have always worked.
>
> jtm
>
>

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

"WD" <wd@nowhereclose.com> wrote in message
news:40F6972E.8080301@nowhereclose.com...
>
> I certainly hope this is not true in general. If true we might as well
> throw the DHCP out the window. If true, imagine what you have to do in
> a corporate network, or what an ISP has to do if their DHCP server
crashes.
>
> On second thought, that might just be what some ISPs do - kick off
> everyone off when their DHCP server crashes ;-)

Actually since I work at an ISP I know the answer to this one. If I did
have to reboot one of the terminal servers, maybe once a year tops to reset
the modems, everyone that was connected to that one would be kicked off
because it would be off. They are pretty reliable machines though. Once
everything is configured, a job that takes about 4 hours, since I don't do
it very often, they are pretty much automatic as far as their functions.


>
> I have read some DHCP servers address this problem by saving the
> assigned IP addresses in a file and read it back on reboot (e.g. Linux
> implementation), or save the addresses in non-volatile memory (router
> implementation). Yet another way to address this issue is to "probe" an
> IP address before assignig it to determine if it's in use. This
> algorithm, while it works most of the time, is still flawed: it won't
> work if a computer which has been assigned a valid address is
> temporarily off line; the DHCP server might take this address and give
> it to the next computer requesting one.
I've never used Linux as a DHCP server since it's cheaper all around to use
a router to do the job. Administration, upkeep, power and just about
everything else is pretty much automatic for a cheap router that can act as
a DHCP.

>
>
>
> Jim Miller wrote:
>
> > sorry, that's not how they work. when the router goes down all the
clients
> > need resetting. at least that's how my routers have always worked.
> >
> > jtm
> >

I agree with the last. Usually if the router has gone down so has the rest
of the network, usually due to an extended power failure so everything has
to be rebooted anyway.

AG

 

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

On Thu, 15 Jul 2004 08:45:22 -0400, WD <wd@nowhereclose.com> wrote:

>
>Does anyone have a broadband router with a working DHCP server?
>
>By working, I mean a router that after power cycle, would remeber or
>figure out the IP addresses it gave out previously, or are currently in
>use on the network.

The DLink 604 (and probably a bunch of other routers) allow you to
assign an IP address to a MAC address, so every time a given device
requests an IP address, it gets the same one. DLink calls it "Static
DHCP".

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

di624a prismgt gives same machine same ip everytime
can also set specific mac specific dhcp ip
"Jim Miller" <jim@NOSPAMjtmiller.com> wrote in message
news:taydnVcg9KuVGGvdRVn-vw@comcast.com...
> sorry, that's not how they work. when the router goes down all the clients
> need resetting. at least that's how my routers have always worked.
>
> jtm
>
>

 

[Guest]

Archived from groups: alt.internet.wireless,comp.dcom.xdsl (More info?)

"Bob Willard" <BobwBSGS@TrashThis.comcast.net> wrote in message
news:_bvJc.82786$a24.41222@attbi_s03...
> WD wrote:
>
> >
> > Does anyone have a broadband router with a working DHCP server?
> >
> > By working, I mean a router that after power cycle, would remeber or
> > figure out the IP addresses it gave out previously, or are currently in
> > use on the network.
> >
> > I had tried many routers and APs with DHCP servers before and all of
> > them would forget the IP addresses they gave out, and give out duplicate
> > IP address already in use.
> >
> > One way to test your router is this (assuming the DHCP server's starting
> > address is 100):
> >
> > Connect two DHCP client machines to the router. Say machine A gets
> > address *.*.*.100 and machine B gets *.*.*.101. Power cycle the router.
> > Reboot machine B and see if it gets an IP address. If your router has
> > a simple DHCP implemenation it will give out *.*.*.100 address which is
> > still in use by machine A. So B will never get an IP address and
> > machine A's console probably will report that there's a duplicate IP
> > address on the network.
> >
> > I would expect the DHCP server to at least remember the IP addresses it
> > gave out for the duration of the lease.
> >
>
> Uh, the D in DHCP means Dynamic, which means you cannot count on getting
> the same IPA-to-PC assignments. If it matters to one PC, give it a
> static IPA in the same subnet as the rest of the clients, and exclude
> that IPA from being handed out via DHCP; works for me.
> --
> Cheers, Bob

Actually, WD's expectation is correct. Although DHCP is dynamic, it gives
out a LEASE (with an expiration time) for an IP address. You are entitled
to use that address without further communication with the DHCP server until
the lease expires. The DHCP server ought to keep track of leases until they
expire. But for <$100 you may not get that level of fault-tolerance.

On the other hand, WD's test for the DHCP server remembering leases across a
router reset will not work. The client computers also remember their
leases. If the router assigns .100 to A and assigns .101 to B, and then the
router is reset, when B is booted it will explicitly request address .101
again, unless it has leased other addresses on the same adapter in the
interim.

Ron Bandes, CCNP, CTT+, etc.

 

[Guest]

Archived from groups: alt.internet.wireless (More info?)

On Thu, 15 Jul 2004 10:40:11 -0400, WD spoketh

>
>I certainly hope this is not true in general. If true we might as well
>throw the DHCP out the window. If true, imagine what you have to do in
>a corporate network, or what an ISP has to do if their DHCP server crashes.
>

It's not true in general. DHCP servers may check with ICMP if an address
is in use. That's why blocking ICMP echo requests on your firewall may
be a bad idea.


Lars M. Hansen
www.hansenonline.net
Remove "bad" from my e-mail address to contact me.
"If you try to fail, and succeed, which have you done?"