[SOLVED] IP address scope global secondary

Vieri · l33t Joined: 18 Dec 2005 Posts: 877

Hi,

I configure a network interface in Gentoo with a setting like this one:

NeddySeagoon · Posted: Fri Mar 26, 2021 2:12 pm Post subject:

Veri,

None oi those IP addresses are global. They are all private
The difference is one of convention, more than anything. private IPs should never appear on the the internet.
Its a convention that boundary routers drop packets with destinations in these address ranges, hence they are private.

We need both the ifconfig and route output from the system sending the ping and the hos with IP 10.215.145.241 to know why it replied.

What happens is that the kernel loots at the routing table, from bottom to top and routs the ping to 192.168.216.100 to the first match.
The default gateway is used if there are no matches as that matches everything.

This process is repeated at every hop along the way until the time to live is reached, the packet is delivered or the host that has it is unable to route it any further.
In this case, that host is 10.215.145.241, so it returns Destination Host Unreachable.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Vieri · l33t Joined: 18 Dec 2005 Posts: 877

Hi Neddy,

I'll get the info you asked for asap. In the meantime, what does "secondary" mean?

pietinger · Posted: Fri Mar 26, 2021 9:39 pm Post subject:

Vieri · l33t Joined: 18 Dec 2005 Posts: 877

Hi,

Thanks for explaining what "secondary" means here. It's all as expected. The netmasks are as I need them to be for the moment.

Now I'd like to know why the ICMP "replier" is one of my Gentoo Linux router's IP addresses and not another one.

I ran this on the host sending the ping request:

pietinger · Posted: Sat Mar 27, 2021 2:49 am Post subject:

Vieri,

what you are doing is called "multihome". Multihoming is no problem if you have no overlapping subnets. But your subnet 10.215.145.0/23 is part of your bigger subnet 10.215.144.0/22.

Linux is using a "weak host model" by default (WikiPedia tells you more); this means your host can send packets on an interface that has not assigned the source IP address of the packet being sent (weak host send behavior). I dont know exactly how the kernel internally deals with more than one possible source address for a reply; but I assume he will take the address with the "closest" netmask to the netmask of the destination address, or maybe - in other words - with the smallest possible. I dont know how to change this behavier by configuration (because you cannot work with routing priorities); it could be possible by using a second ethernet adapter in this host.

pietinger · Posted: Sat Mar 27, 2021 2:56 am Post subject:

Vieri · l33t Joined: 18 Dec 2005 Posts: 877

My network is in a transient state as I'm trying to prune some netmasks off, but it's delicate when in a very large corporate network so I need to take it one step at a time.
My goal is to change the Gentoo router's IP addr. 10.215.144.91/22 to 10.215.144.91/23.
There are plenty of hosts however that are still not within the right mask, hence the use of /22 to "easily and lazily" cover them.
It might not be best practice so I guess I'll change the strategy now either by changing the LAN hosts' network settings to 10.215.144.0/23 (if my local addr. pool allows me to) or adding another IP address+netmask on the Gentoo router and adjusting the clients.

Thank you very much for the helpful information.
It's greatly appreciated.

Vieri

pietinger · Posted: Sat Mar 27, 2021 11:42 am Post subject:

pietinger · Posted: Sat Mar 27, 2021 12:07 pm Post subject:

P.S.: I have just looked in an old guide http://linux-ip.net/html/index.html

and found this page: http://linux-ip.net/html/tools-ip-address.html
explaining the "scope". Maybe you are interested in it.

Greetings,
Peter