Yrurtj

How shall I understand the routing tables of a guest OS and a host OS via KVM?

When a packet reaches the guest OS

1. 
   

   If the packet is destined for the loop back address (127.0.0.1) of the guest OS,

   
   
   
   
   
   • will it be transmitted to ent3 (192.168.122.202) and then to gateway (192.168.122.1), according to the first rule in the routing
     table of the guest OS? Isn't that wrong, since the intended destination (127.0.0.1) is in the guest OS?
   
   
   
   


2. 
   

   if the packet is destined for a private IP address (192.168.122.2) in the virtual machine's private network

   
   
   
   
   
   • is the packet delivered to ent3 (192.168.122.202) as the final destination, according to the second entry in the routing table of
     the guest OS? Isn't that wrong, because the intened destination
     192.168.122.2 isn't in the guest OS, and the packet should be transmitted to gateway 192.168.122.1, and then to 192.168.122.2?
   
   
   
   


3. 
   

   If the packet is destined for a private address (192.168.1.100) in the LAN of the host,

   
   
   
   
   

   • will it be transmitted to ent3 (192.168.122.202) and then to gateway (192.168.122.1), according to the first rule in the routing
     table of the guest OS?

   
   

   • when the packet reaches the host OS, will it be transmitted to wlx801f02b5c389 (192.168.1.97), according to the third rule in the
     routing table of the guest OS? If yes, isn't that wrong, because
     wlx801f02b5c389 (192.168.1.97) isn't the intended destination
     192.168.1.100?

   
   
   
   


4. 
   

   if the packet is destined for a public IP address (e.g. the one for
   www.google.com),

   
   
   
   
   

   • is the packet transmitted to ent3 (192.168.122.202) and then to gateway (192.168.122.1), according to the first rule in the routing
     table of the guest OS?

   
   

   • when the packet arrives at the host OS, is it transmitted to wlx801f02b5c389 (192.168.1.97), and then to gateway (192.168.1.1),
     according to the first entry in the routing table of the host OS?

   
   

   • will gateway (192.168.1.1) transmit the packet to www.google.com?

   
   
   
   

Thanks.

On the guest OS:

$ /sbin/route -n 

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

0.0.0.0         192.168.122.1   0.0.0.0         UG    0      0        0 ens3

192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 ens3

and

$ /sbin/ifconfig

ens3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

        inet 192.168.122.202  netmask 255.255.255.0  broadcast 192.168.122.255

        inet6 fe80::5054:ff:fe99:5eee  prefixlen 64  scopeid 0x20<link>

        ether 52:54:00:99:5e:ee  txqueuelen 1000  (Ethernet)

        RX packets 14906  bytes 18020195 (17.1 MiB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 7390  bytes 786783 (768.3 KiB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 35568



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 1  (Local Loopback)

        RX packets 496  bytes 39840 (38.9 KiB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 496  bytes 39840 (38.9 KiB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

On the host OS:

$ route -n

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

0.0.0.0         192.168.1.1     0.0.0.0         UG    600    0        0 wlx801f02b5c389

192.168.1.0     0.0.0.0         255.255.255.0   U     600    0        0 wlx801f02b5c389

192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

and

$ ifconfig 



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 3119811  bytes 177332278 (177.3 MB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 3119811  bytes 177332278 (177.3 MB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0



virbr0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

        inet 192.168.122.1  netmask 255.255.255.0  broadcast 192.168.122.255

        ether 52:54:00:b1:aa:1f  txqueuelen 1000  (Ethernet)

        RX packets 7865  bytes 728783 (728.7 KB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 14988  bytes 18248805 (18.2 MB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0



vnet0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

        inet6 fe80::fc54:ff:fe99:5eee  prefixlen 64  scopeid 0x20<link>

        ether fe:54:00:99:5e:ee  txqueuelen 1000  (Ethernet)

        RX packets 7414  bytes 789007 (789.0 KB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 54379  bytes 20281975 (20.2 MB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0



wlx801f02b5c389: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

        inet 192.168.1.97  netmask 255.255.255.0  broadcast 192.168.1.255

        inet6   prefixlen 64  scopeid 0x20<link>

        ether 80:1f:02:b5:c3:89  txqueuelen 1000  (Ethernet)

        RX packets 115011  bytes 102852430 (102.8 MB)

        RX errors 0  dropped 0  overruns 0  frame 0

        TX packets 80310  bytes 14401568 (14.4 MB)

        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

The host and the guest systems are -- in the case of a virtual machine running via KVM, virtualbox, VMware, etc. -- completely separate computer systems.

They will route traffic the same as two physical computers configured in each particular manner.

There is no special configuration, that I know of or have seen evidence of, on the host OR the guest with the network configuration.

All of the network functionality on the host that the libvirt/KVM packages and applications provide (such as providing DHCPd, NAT, etc.) can be provided via other packages and utilized over a physical network connection to other computers on a physical network.