How to find I/F to destination

Hi,

I would like to determine through which interface packets sent to a particular UDP destination will go through, and so determine the MTU size for that interface. Can anyone suggest a good way of doing this from within the kernel?

Doing this will permit AF_RXRPC to obtain a better initial guess as to the maximum size of the packets that can be sent that way.

I could use the code Patrick gave me to allow AFS to iterate through all the interfaces and then pick the smallest MTU, but that seems wrong somehow - though it probably will result in the correct answer 99% of the time.

David

Hi David.

I used following code in netchannels:

static int netchannel_ip_route_output_flow(struct rtable **rp, struct flowi *flp, int flags)

{
	int err;

	err = __ip_route_output_key(rp, flp);
	if (err)
		return err;

	if (flp->proto) {
		if (!flp->fl4_src)
			flp->fl4_src = (*rp)->rt_src;
		if (!flp->fl4_dst)
			flp->fl4_dst = (*rp)->rt_dst;
	}

	return 0;
}

struct dst_entry *route_get_raw(u32 saddr, u32 daddr, u16 sport, u16 dport, u8 proto)

{
	struct rtable *rt;
	struct flowi fl = { .oif = 0,
			    .nl_u = { .ip4_u =
				      { .saddr = saddr,
					.daddr = daddr,
					.tos = 0 } },
			    .proto = proto,
			    .uli_u = { .ports =
				       { .sport = sport,
					 .dport = dport } } };

	if (netchannel_ip_route_output_flow(&rt, &fl, 0))
		goto no_route;
	return dst_clone(&rt->u.dst);

no_route:

	return NULL;
}

This is basically a copied input route code.
dst entry, obtained from route_get_raw() holds a pointer to network device.

	Evgeniy Polyakov

Is there any way to get at this without having to link against the ipv4 module?

David

Ah, nevermind. ipv4 can't be a module.

David

Should route_get_raw() release the rtable if gets back?

David

Yes, dst entry should be released when not used anymore.

	Evgeniy Polyakov

I meant the rtable returned by __ip_route_output_key(), not the dst.

David

That is the same, dst is dereferenced as rtable.
Cloned dst is returned, so it must be put back at the ned of the usage.

	Evgeniy Polyakov

If that's the case, then why do you bother to clone it in route_get_raw()? Surely that'll give you *two* clones...

BTW, it seems to work. The attached function gives me:

	[0mount ] <== rxrpc_assess_MTU_size() [if_mtu 1500]

Thanks!

David

---
static void rxrpc_assess_MTU_size(struct rxrpc_peer *peer)

{
	struct rtable *rt;
	struct flowi fl;
	int ret;

	peer->if_mtu = 1500;

	memset(&fl, 0, sizeof(fl));

	switch (peer->srx.transport.family) {
	case AF_INET:
		fl.oif = 0;
		fl.proto = IPPROTO_UDP,
		fl.nl_u.ip4_u.saddr = 0;
		fl.nl_u.ip4_u.daddr = peer->srx.transport.sin.sin_addr.s_addr;
		fl.nl_u.ip4_u.tos = 0;
		/* assume AFS.CM talking to AFS.FS */
		fl.uli_u.ports.sport = htonl(7001);
		fl.uli_u.ports.dport = htonl(7000);
		break;
	default:
		BUG();
	}

	ret = ip_route_output_key(&rt, &fl);
	if (ret < 0) {
		kleave(" [route err %d]", ret);
		return;
	}

	peer->if_mtu = dst_mtu(&rt->u.dst);
	kleave(" [if_mtu %u]", peer->if_mtu);
}

htons()

You need dst_release(&rt->u.dst) here.

Blech. Thanks. Does it actually matter what ports are specified?

Thanks. I think Evgeniy's code may be wrong then. He ends with a dst_clone(), which I think is superfluous.

David

It matters when IPsec port selectors are used to find the correct policy.

Yes, that looks wrong.

Main idea is to get a reference, and then clone it for each user. Then each user drops its reference, and when system is not used anymore, main reference is dropped too.

	Evgeniy Polyakov

This one is quite slow compared to atomic reference increase, so yes, there are two clones - one for main 'route', which then is being cloned for each used in fast path. When it is not needed anymore (netchannel is removed), the first one is dropped.

	Evgeniy Polyakov

You can also cache returned entry and then just clone it and check ->obsolete() callback.

Something like this:
struct dst_entry *route_get(struct dst_entry *dst)

{
	if (dst && dst->obsolete && dst->ops->check(dst, 0) == NULL) {
		dst_release(dst);
		return NULL;
	}
	return dst_clone(dst);
}

Copied from route code too.

	Evgeniy Polyakov

What's proto? Should that be IPPROTO_UDP?

David

Yep.

	Evgeniy Polyakov

Do a route lookup (ip_route_output_key), then either use dst_mtu to get the PMTU value or dst->dev in case you really want the device's MTU.