Re: [lwip-users] IP forwarding to/from PPP working for one netif, but not another

[Sylvain Rochet]

Hi Simon,

On Thu, Jan 30, 2020 at 09:31:11PM +0100, address@hidden wrote:
> Am 30.01.2020 um 20:42 schrieb Sylvain Rochet:
> > On Wed, Jan 29, 2020 at 08:15:11PM -0800, Andrew Pullin wrote:
> > > 
> > > Host2:
> > >     - Micontroller device, ESP32
> > >     - has both WiFi and Ethernet PHY hardware (IP101GRI Ethernet PHY), 
> > > used
> > >mutually exclusively
> > >     - Running lwIP
> > >     - WiFi and Ethernet are independently verified to work to reach WAN 
> > > from
> > >this host
> > >     - PPPoS server
> > >     - ppp0 : 10.0.0.1/32
> > >         - IP for server and client are hard-coded in
> > >         - I believe it is /32, as the netmask is reported as 
> > > 255.255.255.255
> > >     - When running w/ WiFi:
> > >         - wlan0 : ip 10.0.200.136/16  (IP and /16 from DHCP server)
> > >     - When running w/ Ethernet:
> > >         - eth0 : ip 10.0.200.113/16  (IP and /16 from DHCP server)
> 
> One odd thing I find here is that lwIP does not support 2 interfaces in
> the same subnet. So you would have to take care that only one netif at a
> time has its link set up (or its address set or its administrative
> status up, or whatever). In any case, having both wlan0 and eth0 up and
> running with an address in the same subnet will result in packets being
> accepted on both netifs but transmitted on whichever was registered last
> (or was it first?).

I assumed that both were not active at the same time, good catch, this 
might be an issue as well.


> > This is not "connection sharing", this is IP routing, not NAT !,
> > "connection sharing" is usually used for hosts that do SNAT on their
> > Interface facing Internet.
> >
> > IP routing needs a complete routing table on all hosts telling how to
> > reach (i.e. which next-hop to use) every host of the whole network.
> >
> > Example with 2 subnets and 3 Hosts:
> >
> > A-eth0  --------------------- eth0-B-eth1 --------------------- eth0-C
> >    192.168.0.1/24    192.168.0.2/24   172.16.1.1/24     172.16.1.2/24
> >
> > Host A routing table:
> > 192.168.0.0/24 dev eth0
> > 172.16.1.0/24 via 192.168.0.2
> >
> > Host B routing table:
> > 192.168.0.0/24 dev eth0
> > 172.16.1.0/24 dev eth1
> >
> > Host C routing table:
> > 192.168.0.0/24 via 172.16.1.1
> > 172.16.1.0/24 dev eth0
> >
> > dev = directly connected
> > via = next-hop, gateway if you prefer
> >
> > What you could see is that each host have a similar routing table, just
> > the next-hop changes. Default route is just a way to reduce the routing
> > table size by removing all entries that have an identical next-hop.
> 
> Which brings me to a question: how do we define such routing rules in
> lwIP? Hmm...

I'm surprised by the question :-) In lwIP, "dev" rules don't need to be 
added, the ip4_route function "create" them dynamically using the 
interface address/netmask.

But thanks you for asking that because there is a huuuuuge issue about 
that with the aforementioned case. Since "dev" rules are not ordered 
because we just foreach on interfaces list until a match, we don't 
respect that smaller subnets should be checked and matched first. So if 
an IP subnet is within the IP subnet of another interface, although 
perfectly legal configuration but *bad* design, it does not work in 
lwIP. In lwIP it actually depends on the order interfaces were added :-)

For "via" routes, we have LWIP_HOOK_IP4_ROUTE.

Sylvain

signature.asc
Description: Digital signature