On Wed, 13 Sep 2017 14:56:38 +0200 Tomaz Buh <hubber.iskratel@xxxxxxxxx> wrote: > I'm trying to modify packets within the BPF program and send them to > another IP address through the same NIC with the XDP_TX call > (ultimately I'm trying to do a RTP proxy sample). > > What I'm missing is the MAC address for the IP, which can be normally > obtained from the ARP table. > I can see two solutions for dynamic sending: > -search the kernel tables for ARP entries, > -produce ARP reqests and have own ARP table. > > Is any of the solutions possible with the current implementation? > Is there any better solution? I don't think the kernel have a bpf helper call that can do lookups in the arptable, and I don't think we need to add it, as via current facilities we can get the same thing. The XDP program will need to have a bpf map for maintaining the IP to MAC-addr mapping, like the the arptable. But the XDP program can delegate a userspace program to maintain and update this map. First of all you let ETH_P_ARP return XDP_PASS (like [1]), which cause the OS/kernel to handle ARP packets. Second you can register to get rt_netlink notifications when the arp/neigh table change (group RTNLGRP_NEIGH). Look at how the iproute2 tool[2] does it, via the tool rtmon or "ip monitor" command (man ip-monitor) like: $ ip mon neigh 192.168.100.1 dev mlx5p1 lladdr e4:1d:2d:af:60:80 REACHABLE For the C-code look at ip/ipmonitor.c [3] and e.g. print_neigh() in ip/ipneigh.c [4]. [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/samples/bpf/xdp_ddos01_blacklist_kern.c#L245-L249 [2] https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git [3] https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/tree/ip/ipmonitor.c [4] https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/tree/ip/ipneigh.c#n194 -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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