How does the networking works in CentOS 7? Specifically, ifconfig?

Question

Goal: To configure the routing of the traffic from VM to the PC and to the internet. to configure the forwarding of the traffic from wlan0 to the eth0 on CentOS7, and vice versa.

My topology:

VM - (cable) --- eth0=RPI=wlan0 -(wireless)- Router -- ISP
                                               | 
                                           (wireless)
                                               PC

• I have used this article to configure the traffic forwarding from eth0 to wlan0: https://gist.github.com/tzermias/5408466
• wlan0 has been configured using nmtui.

Commands being issued:

sudo ifconfig eth0 192.168.56.1 netmask 255.255.255.0
sudo sysctl -w net.ipv4.ip_forward=1
sudo iptables -A FORWARD --in-interface eth0 -j ACCEPT
sudo iptables --table nat -A POSTROUTING --out-interface wlan0 -j MASQUERADE

Now, I checked the network-scripts folder:

[pi@rtr001 network-scripts]$ ls
ifcfg-FunBox-84A8  ifdown-ppp       ifup-ippp    ifup-TeamPort
ifcfg-link         ifdown-routes    ifup-ipv6    ifup-tunnel
ifcfg-lo           ifdown-sit       ifup-isdn    ifup-wireless
ifdown             ifdown-Team      ifup-plip    init.ipv6-global
ifdown-bnep        ifdown-TeamPort  ifup-plusb   keys-FunBox-84A8
ifdown-eth         ifdown-tunnel    ifup-post    network-functions
ifdown-ippp        ifup             ifup-ppp     network-functions-ipv6
ifdown-ipv6        ifup-aliases     ifup-routes
ifdown-isdn        ifup-bnep        ifup-sit
ifdown-post        ifup-eth         ifup-Team

Noticed, that, the ifcfg-eth0 is not present in there, even, the output of ifconfig, below, is showing the address is assigned, and I am able to ping device, plugged into the Ethernet port, with IP 10.10.10.2:

[pi@rtr001 network-scripts]$ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.10.10.1  netmask 255.255.255.0  broadcast 10.10.10.255
        ether b8:27:eb:d7:d7:9a  txqueuelen 1000  (Ethernet)
        RX packets 3  bytes 214 (214.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 50  bytes 8296 (8.1 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 32  bytes 2592 (2.5 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 32  bytes 2592 (2.5 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.26  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::47ae:281a:680f:11eb  prefixlen 64  scopeid 0x20<link>
        ether b8:27:eb:82:82:cf  txqueuelen 1000  (Ethernet)
        RX packets 649  bytes 42528 (41.5 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 492  bytes 69881 (68.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Also, the VM, with IP: 10.10.10.2, is able to ping 10.10.10.1, but failing to ping 192.168.1.22, any IP in subnet 192.168.X.X, which is between RPI -- Router.

Questions:

1. What does sudo ifconfig eth0 10.10.10.1 netmask 255.255.255.0 do, in terms of modifying of the files in Linux? In other words, it looks, as per output of ifconfig, that the address of eth0 has been assigned, but, there is no specific ifcfg-eth0 present.
2. How to make the configuration of eth0 present after the reboot?
3. What I should modify in RPI, in order to be able to ping from VM, 192.168.X.X subnet?
4. How can I see the NAT ip table, which command I should run? It looks, iptables -L is not showing the NAT rules...

Answer 1

The command sudo ifconfig... do not make any change in files. It change till next reboot or next ifconfig the values of IP, netmask, etc.

To make persistent changes create file ifcfg-eth0 and enter there the info (something like).

DEVICE=eth0
BOOTPROTO=static
ONBOOT=yes
NETWORK=192.168.56.0
NETMASK=255.255.255.0
IPADDR=192.168.56.1
USERCTL=no