[CentOS] Need help getting two NICs to work on CentOS 7

[lejeczek]

On 07/11/16 07:00, Sean Son wrote:
> Hello all
>
> Here is the scenario:  We have a mail server VM which currently has two
> virtual NICs attached to it. One NIC is has an IP on a subnet with a
> default gateway defined and the other NIC has an IP on a different subnet
> with a different gateway on a different VLAN defined. Now when I activate
> both NICs, and run an ifconfig -a, I see that both IP addresses are
> showing. Now here is the problem. When I ping the VM, the first NIC's IP is
> not pingable at all, but the second NIC's IP is pingable. How do I
> configure the networking so that both IPs are pingable and the VM is
> reachable via both IPs? Please let me know what I may be doing wrong!
>
>
> Thank you!
>
> Sean
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policy based routing is useful concept but in my mind should 
be the very last resort.
It is the kernel's networking functions which are 
responsible for routing tables construction and if you can 
shed more details (like mentioned earlier) on what can(not) 
ping what(VM hosting box <-> VM guest / outside internal net 
<-> VM / etc.) centbuddies could share more concrete 
thoughts then.
Basically - with policy/source based routing you need to be 
very careful, test everything - best is to leave it to the 
kernel and only use main table, kernel should be the 
smartest, for more complex setups maybe think "routing daemons".