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Today's Topics:
1. Re: failover and loadbalance for relayed requests (Gregory Sloop)
2. Re: failover and loadbalance for relayed requests (Simon Hobson)
3. RE: failover and loadbalance for relayed requests
(Cuttler, Brian (HEALTH))
4. Re: dhcpd doesn't acknowledge dhcp requests (Sean McMurray)
5. Is there any way to include dynamic info in the DHCP REQUEST?
(Fredrik Persson)
----------------------------------------------------------------------
Message: 1
Date: Wed, 3 Jun 2015 08:58:06 -0700
From: Gregory Sloop <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: Re: failover and loadbalance for relayed requests
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-15"
Just a note - it's not really "fail-over" but more load-balancing with the
ability to handle the loss of one of the peers. [At least when *I* think if
fail-over, I think of a server in stand-by, which isn't doing anything. Then
when the primary active server fails, the "fail-over" comes up and fully takes
over for the primary. And that's not the "fail-over" that ISC DHCPd does.]
I'm not knocking it at all - just that it's got some interesting nuance that
can bite you.
Some of these potential pitfalls in peer/fail-over/load-balancing have been, at
least partially, mitigated in versions 4.2.0+ [I think] and newer.
A search of the archives and release notes will help you find some of these.
[See: auto-partner-down, and the "rewind" feature also added in 4.2.0+
versions.]
Read the docs carefully - it's quite a bit more complex than you might think at
first glance. Not terribly hard or difficult, but it may not function exactly
as you'd imagined it to do.
[Corner cases always bite you when you lease(t) {pun intended} expect.]
HTH
-Greg
L> Thanks Simon !!!
L> On 03/06/15 12:18, Simon Hobson wrote:
>> Leandro <[email protected]> wrote:
>>> I would like to ask for some documentation to read
>> start with "man dhcpd.conf"
>>> and also ask for the dhcp isc failover capabilities and recommendations.
>> You can have 2 (and no more than 2) servers in a failover pair per pool. You
>> essentially clone the config file from the primary to the secondary - and
>> add the relevant failover peer declarations.
>>> I imagine to setup two servers for instance with lease database sync and
>>> two ips configured on the relays ... is that possible ?
>> Yes, that's how it works
>>> how does iscp manage the leases db sync?
>> It's internal. When one server issues a lease, it notifies the other which
>> updates it's local lease file.
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Message: 2
Date: Wed, 3 Jun 2015 16:58:30 +0100
From: Simon Hobson <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: Re: failover and loadbalance for relayed requests
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Sean McMurray <[email protected]> wrote:
> ISC DHCP can only do 2 coupled servers. If you are looking for more
> redundancy/load balancing than that, you will have to be creative.
You can have more, but only 2 for any pool. So, for example, with three servers
you could have 3 pools each served by 2 servers. Eg S1 and S2 server P1, S2 and
S3 server P2, and S1 and S3 server P3.
> For instance, if you are just looking for load balancing, you could put your
> leases file on a clustered filesystem and have multiple servers serve
> different pools in the same subnet, using the same leasefile.
I believe that will lead to massive problems. The leasefile is write only
(append) except during server startup, and while it may appear to work for a
short time (accumulating leases from all the servers), there is the cleanup ...
By defalt it's once an hour - the server will write out a completely new lease
file from internal tables. Each server will therefore write a lease file that
does not include any leases from the other servers. If a server has to restart
for any reason, it's russian roulette whether the lease file is one with it's
own data or one without ! Starting up a DHCP server with no knowledge of what
leases it has previously given out is "not a good thing".
> The two servers communicate back and forth about what leases they have
> served. When one fails, the other takes over and eventually recovers the
> other's leases.
But note that this isn't automatic - because there are failure modes which can
interrupt inter-server communications without stopping either of them serving
clients. It's a manual process to put a server into partner down state - though
I gather some people do run automated tasks to do this.
------------------------------
Message: 3
Date: Wed, 3 Jun 2015 15:58:51 +0000
From: "Cuttler, Brian (HEALTH)" <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: RE: failover and loadbalance for relayed requests
Message-ID:
<cy1pr09mb02014769334296fae607a865ba...@cy1pr09mb0201.namprd09.prod.outlook.com>
Content-Type: text/plain; charset="us-ascii"
I had wondered about that, seemed to me that was the mechanism to determine
which server replied what percentage of the time, since allocation of IP
addresses is not going to do the job, you have to decide to answer based on the
question, not on the pool size.
I did not understand that after reading most of the FAQ yesterday, I did not
follow up with the referenced FAQ having to do with the pool split, which is
not the correct term for this.
Thanks - about to implement this at my site and this discussion was very timely
for me.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Steven Carr
Sent: Wednesday, June 03, 2015 11:53 AM
To: Users of ISC DHCP
Subject: Re: failover and loadbalance for relayed requests
On 3 June 2015 at 16:45, Sean McMurray <[email protected]> wrote:
> You define which pools they share and what the split is. For example,
> you could have the primary serve 2/3 of the pool, and have the
> secondary serve 1/3.
That must be the most misunderstood part of DHCP failover.
The split *does not* determine the share of the pool. The pool is
*always* balanced 50/50. The split value determines which peer will respond to
the client based on the hashed value of the client identifier (MAC address).
Steve
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------------------------------
Message: 4
Date: Wed, 03 Jun 2015 10:15:58 -0700
From: Sean McMurray <[email protected]>
To: Users of ISC DHCP <[email protected]>
Subject: Re: dhcpd doesn't acknowledge dhcp requests
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 06/03/2015 08:52 AM, dave c wrote:
> Sean,
>
> I don't see a subnet defined for the 10.64.0.104 local subnet.
>
> Without it, broadcasts aren't going to get answered as there is no
> dhcpd pool or configuration telling it to answer local requests.
There is a pool for 10.112.0.0/13 and broadcasts are coming from that
interface. Those are the broadcasts that are being ignored.
The 10.64.0.0 network is just being used to sync the peers.
[snip]
> Dave
>
> On 6/3/15 10:10, Sean McMurray wrote:
>> On 06/02/2015 03:40 PM, Graham Clinch wrote:
>>>> I have a dhcp server that won't give out leases and doesn't
>>>> acknowledge
>>>> DHCPDISCOVERs.
>>>>
>>>> I see the requests come in with tcpdump, but the daemon doesn't log
>>>> them
>>>> and doesn't respond.
>>>>
>>>> [snip]
>>>>
[snip]
>> On startup it says that it is Listening and Sending on em2. It also
>> lists interfaces it will
>> ignore because there is no matching subnet declared in dhcpd.conf.
>>
>> It also says:
>>
>> Sending on Socket/fallback/fallback-net
>>
>>> ss -anp | grep dhcpd
>> p_raw UNCONN 0 0 *:em2 *
>> users:(("dhcpd",21614,5))
>> u_str ESTAB 0 0 *
>> 558943 * 559749
>> users:(("dhcpd",21614,2),("dhcpd",21614,1))
>> u_dgr UNCONN 0 0 *
>> 558954 * 8044
>> users:(("dhcpd",21614,3))
>> raw UNCONN 0 0 *:1 *:*
>> users:(("dhcpd",21614,4))
>> tcp UNCONN 0 0 *:14659 *:*
>> users:(("dhcpd",21614,20))
>> tcp UNCONN 0 0 *:67 *:*
>> users:(("dhcpd",21614,9))
>> tcp UNCONN 0 0 :::23465 :::*
>> users:(("dhcpd",21614,21))
>> tcp LISTEN 0 1 10.64.0.104:647 *:*
>> users:(("dhcpd",21614,11))
>> tcp ESTAB 0 0 10.64.0.104:647 10.64.0.112:56061
>> users:(("dhcpd",21614,10))
------------------------------
Message: 5
Date: Thu, 4 Jun 2015 13:33:33 +0200
From: Fredrik Persson <[email protected]>
To: [email protected]
Subject: Is there any way to include dynamic info in the DHCP REQUEST?
Message-ID:
<camy-fmlmd0xzk-o1blsagehmoyjfwukqskblhjs8sn9gs95...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi, new to the list!
Is there a way to include dynamic information in a "send" to the dhcp
server?
Something like this (I know that this does not work, but it gives an idea
of what I am looking for):
interface "eth0" {
send <some parameter> $SOME_ENV_VAR;
request subnet-mask,
broadcast-address,
/* ... */
}
Thanks in advance,
Fredrik Persson
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