ARP-Request - Reply from the perspective of a router

Question

Presume I have a route table with such entries:

typedef struct route_table_entry {
    uint32_t prefix;
    uint32_t next_hop;
    uint32_t mask;
    int interface;
} route_table_entry;

I have a network with 4 hosts and a router, all host are only bound to the router.

I try to ping from h0 to h1. The first request is an ARP request. So as the router I have to first: cache h0 IP and MAC into an ARP table like this:

typedef struct arp_table_entry {
    uint32_t ipaddr;                /* IP address */
    uint8_t mac[ETH_ALEN];          /* MAC address */
    int interface;                  /* Interface number */
    int ttl;                        /* Time to live. */
} arp_table_entry;

But how do I find the interface if the ARP request or ether header doesn't include the interface? Should I use the route table? If so, how? Nevertheless, I get the ARP Request and I send it to all my available interfaces, right?

for (size_t i = 0; i < ROUTER_NUM_INTERFACES; ++i)  {
                rc = send_packet(interfaces[i], &m);
                DIE (rc < 0, "send_message");
            }

Isn't that what broadcast means? then I wait for a response, again I get a response an ARP-Reply this time, but I don't know to whom to send it, because I don't how from which interface the request came from, how should I go around it?

Answer 1

I have a network with 4 hosts and a router, all host are only bound to the router. I try to ping from h0 to h1. The first request is an ARP request.

Hosts within the same subnet don't use a router but talk directly to each other, by using ARP for the destination IP and using the resulting MAC for encapsulation. No router is involved.

When hosts are located in different subnets, the local routing table tells a host to use a gateway. The host ARPs the gateway (if not in cache) and sends the IP packet to that MAC.

In turn, the router ARPs the destination IP (or another gateway, as determined by its routing table) and sends the packet to that MAC.

In detail, sending from H0 to H1 across the router looks like this:

1. H0 checks its routing table for H1, finding it requires R-0 as gateway.
2. H0 ARPs R-0's IP for its MAC address.
3. H0 sends the packet to R-0's MAC address.
4. The router checks its routing table for H1, finding it attached locally to R-1.
5. The router ARPs H1's IP address for its MAC on R-1.
6. The router sends the packet out of R-1 to H1's MAC.

But how do I find the interface if the ARP request or ether header doesn't include the interface?

The interface is the one the ARP request was sent from (by broadcast) and the ARP response was received on. The ARP request is sent from the interface that is attached to the destination IP's subnet. That interface is determined by the local routing table.

In case you're asking about the router's use of ARP from the perspective of a host: whether or not and how a router (or anyone else) uses ARP is of no consequence to a host in another subnet. It doesn't and can't know about it.

Answer 2

The answer to your question is, you need a packet descriptor which you populate upon receiving a packet, with information including which interface received it.

But how do I find the interface if the ARP request or ether header doesn't include the interface? Should I use the route table?

Don't use the route table to figure out where you received a packet from. If you are using a raw socket (e.g. in Linux) receiving packets from an interface, you know which interface delivered that packet to your program, because you know which socket it arrived on. Save that information for your further processing steps.