Maszyna VM:
############################################################
$ cat /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

#Local interface
auto ens19
iface ens19 inet static
        address 192.168.0.1
        netmask 255.255.255.0



# Public interface
auto ens18
iface ens18 inet static
        address IP.FAILOVER.z.OVH
        netmask 255.255.255.255
        network IP.FAILOVER.z.OVH
        post-up route add IP.HOSTA.OVH.254 dev ens18
        post-up route add default gw IP.HOSTA.OVH.254
        dns-nameservers 8.8.8.8 8.8.4.4
##############################################################
$ ip r

default via IP.HOSTA.OVH.254 dev ens18
10.8.0.0/24 via 10.8.0.2 dev tun0
10.8.0.2 dev tun0  proto kernel  scope link  src 10.8.0.1
IP.HOSTA.OVH.254 dev ens18  scope link
192.168.0.0/24 dev ens19  proto kernel  scope link  src 192.168.0.1

################################################################
$ iptables-save

*nat
:PREROUTING ACCEPT [60031:32103183]
:INPUT ACCEPT [1797:67898]
:OUTPUT ACCEPT [53:3769]
:POSTROUTING ACCEPT [168:11007]
-A POSTROUTING -s 10.8.0.0/24 -o ens18 -j MASQUERADE
-A POSTROUTING -s 192.168.0.0/24 -o ens18 -j MASQUERADE
-A POSTROUTING -s 192.168.0.0/24 -o ens19 -j MASQUERADE
COMMIT
*filter
:INPUT ACCEPT [8000:1226673]
:FORWARD ACCEPT [272:15164]
:OUTPUT ACCEPT [7121:1105759]
:f2b-sshd - [0:0]
-A INPUT -p tcp -m multiport --dports 22 -j f2b-sshd
-A INPUT -p tcp -m multiport --dports 22 -j f2b-sshd
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -s 10.8.0.0/24 -d 192.168.0.1/32 -i tun0 -o 192.168.0.0
-A FORWARD -i ens19 -o ens18 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i ens19 -o ens18 -j ACCEPT
-A f2b-sshd -j RETURN
-A f2b-sshd -j RETURN
COMMIT
##############################################################
$  tcpdump -n -l -i any -c 10 host 192.168.0.10

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
13:30:45.254957 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:30:45.254988 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:30:48.271421 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:30:48.271442 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:30:50.255199 ARP, Request who-has 192.168.0.10 tell 192.168.0.1, length 28
13:30:50.276644 ARP, Reply 192.168.0.10 is-at 8e:43:f1:69:b6:30, length 46
13:30:54.282025 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:30:54.282043 IP 10.8.0.6.52960 > 192.168.0.10.22: Flags [S], seq 2245353329, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
#################################################################
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Maszyna z problemem (B)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
##################################################################
$ cat /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

#Local interface
auto ens18
iface ens18 inet static
        address 192.168.0.10
        netmask 255.255.255.0
        dns-nameservers 8.8.8.8 8.8.4.4

# Public interface
auto ens19
iface ens19 inet static
        address IP.FAILOVER.z.OVH2
        netmask 255.255.255.255
        network IP.FAILOVER.z.OVH2
        post-up route add IP.HOSTA.OVH.254 dev ens19
        post-up route add default gw IP.HOSTA.OVH.254
        dns-nameservers 8.8.8.8 8.8.4.4
#################################################################
$ ip r

IP.HOSTA.OVH.254 dev ens19  scope link
192.168.0.0/24 dev ens18  proto kernel  scope link  src 192.168.0.10
#################################################################
$ iptables-save
(pusty output)
#################################################################
$ tcpdump -n -l -i any -c 10 host 192.168.0.1

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
13:46:29.558380 ARP, Request who-has 192.168.0.10 tell 192.168.0.1, length 4                                                            6
13:46:29.558399 ARP, Reply 192.168.0.10 is-at 8e:43:f1:69:b6:30, length 28
2 packets captured
2 packets received by filter
0 packets dropped by kernel
#################################################################
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Ale jak zrobię:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
$ tcpdump -n -l -i any -c 10 host 10.8.0.6
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
13:50:57.528736 IP 10.8.0.6.53268 > 192.168.0.10.22: Flags [S], seq 2744405898, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:51:00.533416 IP 10.8.0.6.53268 > 192.168.0.10.22: Flags [S], seq 2744405898, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0
13:51:06.543333 IP 10.8.0.6.53268 > 192.168.0.10.22: Flags [S], seq 2744405898, win 64240, options [mss 1353,nop,wscale 8,nop,nop,sackOK], length 0

3 packets captured
3 packets received by filter
0 packets dropped by kernel