Hi folks, A few tunnel devices, like geneve or vxlan, are using udp_tunnel_xmit_skb, or related functions for transmitting packets, and are doing the usual FIB lookup to get the dst entry. I see a lot of code like this:
if (rt->dst.dev == dev) { netdev_dbg(dev, "circular route to %pI4\n", &dst->sin.sin_addr.s_addr); dev->stats.collisions++; goto rt_tx_error; } This one is from vxlan, but there are other similar blocks elsewhere. The basic idea is "am I about to send this packet to my own device?" This is a bit crude. For starters, two interfaces could be pointed at each other, bouncing the packet back and forth indefinitely, causing the feared routing loop. Hopefully as more headers got tacked on, allocations would eventually fail, and the queen would be saved. But what about in devices for which self-routing might actually be useful? For example, let's say that if an incoming skb is headed for dst X, it gets encapsulated and sent to dst A, and for dst Y it gets encapsulated and sent to dst B, and for dst Z it gets encapsulated and sent to dst C. I can imagine situations in which setting A==Y and B==Z might be useful to do multiple levels of encapsulation on one device, so that skbs headed for dst X get sent to dst C, but with intermediate transformations of dst A and dst B. This isn't merely theoretical. I'm working on a driver right now that could benefit from this. So, in implementing this, the question of avoiding routing loops comes into play. The most straight forward way to do this is to use a TTL value that's decreased. But we have a problem. A packet sent to dst X that is encapsulated and sent to dst A will have a ttl calculated for its journey to dst A. How do we preserve TTLs across multiple traversals of the networking stack? We can't simply stay with the TTL of the packet when it comes in, because it's tunnel destination might require a different TTL. The best thing would be to have a "tunnel TTL" value as part of skb->cb, except the cb gets overwritten when traversing the networking stack. The best thing I can think of is some other member of sk_buff, but I don't see any that look good for this. So perhaps it would be worthwhile to add this to struct sk_buff? David - are you interested in this if I submit a patch? Or, alternatively, does a fast solution for this already exist that I overlooked? Thanks, Jason -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html