Re: Centos 7 dhcpd failure to allow a 2nd network over same interal nic

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]



On 01/15/2017 09:11 AM, Gregory P. Ennis wrote:
> All I can say is that when I looked at the dhcpd.conf examples and read
> the man pages as well as the explanations of how dhcpd works, we should
> be able to use dhcpd for more than one subnet :



You can, provided they're on different physical interfaces.

I'm mostly certain you can have two DHCP scopes on one physical 
interface, provided that the DHCP server itself only has addresses on 
one of them.  That is, if eth0 has 192.168.1.9 and only that address, 
you should be able to offer addresses for 192.168.1.0/24 and also 
192.168.2.0/24 on that interface.  Any host you want to assign an 
address in 192.168.2.0/24 will have to be manually added to that subnet 
with a "host" entry in dhcpd.conf.  Otherwise, imagine that you have an 
Ethernet LAN that includes a WAP.  When the DHCP server gets a request 
from a new host, how does it know whether that client is on Ethernet or 
WiFi?  There's no indication in the request the server receives that 
indicates which media the client is using.

However, attaching two IP subnets to the same broadcast domain is 
usually a bad idea.  Networks are typically segregated for one of two 
reasons: either to establish access controls or to reduce traffic to 
improve service.  You'll accomplish neither.  Hosts on each subnet won't 
be able to communicate with each other directly, but they will all see 
all of the address discovery traffic broadcast on the network.  A host 
that wanted to communicate with a host in another subnet could simply 
add a new address manually and bypass any access controls that the 
router had in place. Worse, because any communication you *do* want to 
allow has to pass to the router and then be sent back out the same 
network interface, you've actually doubled the amount of traffic on your 
LAN.

Having multiple subnets on a single broadcast domain can be an 
interesting, inexpensive way to experiment with access control or 
simulate multihosting, but you don't want to do it for any longer than 
is necessary for experimental purposes.

-------------------------------------------------------------------

Gordan,

Thank you for such a good explanation.  It seems apparent to me that a
better way to do what I wanted would be to have two wireless routers,
one wifi being controlled by the dhcpd server that assigns ip addresses
through it to known and trusted connections with one subnet, and the
other wifi router assigning addresses on a different subnet to less
trusted users with less access.

Greg

_______________________________________________
CentOS mailing list
CentOS@xxxxxxxxxx
https://lists.centos.org/mailman/listinfo/centos



[Index of Archives]     [CentOS]     [CentOS Announce]     [CentOS Development]     [CentOS ARM Devel]     [CentOS Docs]     [CentOS Virtualization]     [Carrier Grade Linux]     [Linux Media]     [Asterisk]     [DCCP]     [Netdev]     [Xorg]     [Linux USB]
  Powered by Linux