[ipxe-devel] Subnet mask 255.255.255.255 doesn't work

[Clive Eisen]

> On 26 Jun 2015, at 20:11, Michael Brown <mcb30 at ipxe.org> wrote:
> 
> On 26/06/15 18:55, Josh Jameson wrote:
>> I am having an issue where I need to provide /32 DHCP request where the
>> gateway is outside the subnet.
>> 
>> For example consider the following DHCP offer;
>>  IP: 10.0.0.50
>>  Netmask: 255.255.255.255
>>  Gateway: 10.1.0.1
>> 
>> iPXE doesn't seem to communicate right. It times out when trying to get
>> a HTTP request at http://10.1.0.1/.
> 
> That doesn't surprise me.  By specifying a subnet mask of 255.255.255.255, you are telling iPXE (or any other client) that there is one and only one IP address on that network, which is the 10.0.0.50 address itself.  You have not provided any way for the client to reach 10.1.0.1.
> 
>> If I boot the same machine and
>> network config into Linux or Windows I can access the URL.
> 
> That _does_ surprise me.  I tested it just now: as expected, Linux fails to communicate with 10.1.0.1.  (Specifically, it refuses to even add 10.1.0.1 as the default gateway, since that address is not within the designated local network range of 10.0.0.50-10.0.0.50).
> 
> I'm not sure what network topology you're actually trying to create.  Is the iPXE client in the same broadcast domain as the gateway (i.e. if you send a broadcast such as an ARP request from the iPXE node, do you expect it to reach the gateway)?  If so, then you are probably using a subnet mask of 255.0.0.0.
> 
> See https://en.wikipedia.org/wiki/Subnetwork#IPv4_subnetting for some basic introduction to subnet masks.

Sorry to butt in but this is bridging

And we only have half the story here - the bit that’s missing is the route commands

Have a look at how OVHs explain it for virtual machines  bridging to the host NIC and thus its gateway - much better than I could

http://docs.ovh.ca/en/guides-network-bridging.html

You CAN have the gateway outside your subnet with a /32 netmask

They key is in the routing scripts AIUI

Now don’t forget this is on the same NIC there is only one

--
Clive Eisen
GPG: 75056DD0

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