Existe um recurso Linux que faz com que sua máquina responda para cada endereço IP atribuído, em cada interface, quando compartilham a mesma sub-rede IP, independentemente das atribuições específicas da interface IP. Isso pode ou não ser desejável para você.
Este recurso está ativado por padrão, e você pode configurá-lo por meio de < href="http://linux.die.net/man/8/sysctl"> sysctl .
Para o tráfego de saída, sua máquina funciona assim: para cada interface configurada como UP, uma entrada de rota será inserida na tabela de roteamento, independentemente de haver ou não um link, e independentemente de quaisquer outros nós estarem acessíveis através dessa interface ou não. Como você tem duas interfaces para a mesma sub-rede IP, você terá duas rotas idênticas na tabela de roteamento. O sistema operacional usará apenas um deles e você não terá controle sobre qual deles será usado! Além disso, qual deles será usado é independente do endereço de entrada do pacote em resposta ao qual o pacote de saída será enviado. Isso significa que o failover em geral não funciona como esperado.
arp_filter - BOOLEAN
1 - Allows you to have multiple network interfaces on the same subnet, and have the ARPs for each interface be answered based on whether or not the kernel would route a packet from the ARP'd IP out that interface (therefore you must use source based routing for this to work). In other words it allows control of which cards (usually 1) will respond to an arp request.
0 - (default) The kernel can respond to arp requests with addresses from other interfaces. This may seem wrong but it usually makes sense, because it increases the chance of successful communication. IP addresses are owned by the complete host on Linux, not by particular interfaces. Only for more complex setups like load-balancing, does this behaviour cause problems.
arp_filter for the interface will be enabled if at least one of conf/{all,interface}/arp_filter is set to TRUE, it will be disabled otherwise
arp_announce - INTEGER
Define different restriction levels for announcing the local source IP address from IP packets in ARP requests sent on interface:
0 - (default) Use any local address, configured on any interface
1 - Try to avoid local addresses that are not in the target's subnet for this interface. This mode is useful when target hosts reachable via this interface require the source IP address in ARP requests to be part of their logical network configured on the receiving interface. When we generate the request we will check all our subnets that include the target IP and will preserve the source address if it is from such subnet. If there is no such subnet we select source address according to the rules for level 2.
2 - Always use the best local address for this target. In this mode we ignore the source address in the IP packet and try to select local address that we prefer for talks with the target host. Such local address is selected by looking for primary IP addresses on all our subnets on the outgoing interface that include the target IP address. If no suitable local address is found we select the first local address we have on the outgoing interface or on all other interfaces, with the hope we will receive reply for our request and even sometimes no matter the source IP address we announce.
The max value from conf/{all,interface}/arp_announce is used. Increasing the restriction level gives more chance for receiving answer from the resolved target while decreasing the level announces more valid sender's information.