Summary: I would like to be able to read the Gateway address for an IP Packet that has been routed to my Network Device.

Note: I am using Linux Kernel 3.18, gcc 4.8.4, glibc 2.9, and compiled for x86.

My routing table looks something like this:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
148.1.1.0 * 255.255.255.0 U 0 0 0 eth0
148.2.1.0 150.1.4.2 255.255.255.0 UG 0 0 0 net0
148.3.1.0 150.1.4.3 255.255.255.0 UG 0 0 0 net0
148.4.1.0 150.1.4.4 255.255.255.0 UG 0 0 0 net0


When I send a packet to 148.2.1.0, the kernel will route it to my NIC driver, and calls the "xmit()" function with "struct sk_buff *skb" and "struct net_device *dev" as parameters.

My question is, how can I determine which gateway was selected, as 3 of them are all pointing at "net0".

The nexthop appears to be available, as in file "linux-3-18/net/ipv4/arp.c", there is an line which says
"paddr = rt_nexthop(skb_rtable(skb), ip_hdr(skb)->daddr);"

However, in my driver, "skb_rtable(skb)" is NULL. The nexthop/gateway must be available/accessible, but I am having difficulties.
In an example from a much earlier kernel (2.4), the gateway is passed in with the "sk_buff", but not in this later Kernel.

Any ideas please??

Normally a NIC driver doesn't need that information. However, the packet will be passed after the ARP resolution so the MAC destination field in the Ethernet header normally contains the MAC address of the destination (in your case the gateway).

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