Bridging

Hello,

This question has already been asked in a more specific form others in
the OpenWRT forum.

I've a linux box with say, eth0, eth1. This is using PPPoE to connect
to the internet, hence I have a ppp0 interface tied to eth0. Now, my
ISP gives me a 8 address subnet, say, a.b.c.0-7. The linux box is the
router, and is .6, and I'd like to be able IPs .1-.5 for my own hosts,
connected via a switch to eth1.

My first thought was to create a bridge containing ppp0 and eth1,
although I understand that this isn't possible. (Could one bridge,
say ppp0 and ppp1?) What is the correct way to achieve this?

Regards,

Chris

Hello,

Chris a écrit :
>
> I've a linux box with say, eth0, eth1. This is using PPPoE to connect
> to the internet, hence I have a ppp0 interface tied to eth0. Now, my
> ISP gives me a 8 address subnet, say, a.b.c.0-7. The linux box is the
> router, and is .6, and I'd like to be able IPs .1-.5 for my own hosts,
> connected via a switch to eth1.
>
> My first thought was to create a bridge containing ppp0 and eth1,
> although I understand that this isn't possible.

I'm afraid there is no straightforward way to bridge together link layer
protocols as different as ethernet and PPP.

> (Could one bridge, say ppp0 and ppp1?)

Theroetically yes, but AFAIK the bridging code in Linux only works with
ethernet.

> What is the correct way to achieve this?

Routing. Simple good old plain static routing.
Enable IP forwarding on the box and give eth1 the address a.b.c.6/29.
Give each host an address within a.b.c.1-5/29 and default gateway a.b.c.6.

Thanks Pascal,

On 5 May, 15:32, Pascal Hambourg <boite-a-s...@plouf.fr.eu.org> wrote:

> > I've a linux box with say, eth0, eth1. This is using PPPoE to connect
> > to the internet, hence I have a ppp0 interface tied to eth0. Now, my
> > ISP gives me a 8 address subnet, say, a.b.c.0-7. The linux box is the
> > router, and is .6, and I'd like to be able IPs .1-.5 for my own hosts,
> > connected via a switch to eth1.
>
> > What is the correct way to achieve this?
>
> Routing. Simple good old plain static routing.
> Enable IP forwarding on the box and give eth1 the address a.b.c.6/29.
> Give each host an address within a.b.c.1-5/29 and default gateway a.b.c.6.

ppp0 is currently a.b.c.6, is making eth1 the same likely to cause any
problems? Is it then simply a matter of running,

route add a.b.c.0 netmask 255.255.255.242 dev eth1?

Regards,

Chris

Chris a écrit :
>>
>>Enable IP forwarding on the box and give eth1 the address a.b.c.6/29.
>>Give each host an address within a.b.c.1-5/29 and default gateway a.b.c.6.
>
> ppp0 is currently a.b.c.6,

Allocated by the IPCP negotiation ?

> is making eth1 the same likely to cause any problems?

None that I am aware of. What kind of problems do you think of ?

> Is it then simply a matter of running,
>
> route add a.b.c.0 netmask 255.255.255.242 dev eth1?
^^^
Don't you mean 255.255.255.248 ?
I guess this could do the trick too. Except that it would not consider
a.b.c.7 as a directed broadcast address. But you can add it by hand with
'ip route add broadcast a.b.c.7 dev eth1' if required.

On 5 May, 23:17, Pascal Hambourg <boite-a-s...@plouf.fr.eu.org> wrote:
> Chris a écrit :
>
>
>
> >>Enable IP forwarding on the box and give eth1 the address a.b.c.6/29.
> >>Give each host an address within a.b.c.1-5/29 and default gateway a.b.c..6.
>
> > ppp0 is currently a.b.c.6,
>
> Allocated by the IPCP negotiation ?
>

I had thought this was being set manually, but whatever IP/netmask I
supply does seem to be overridden.

ppp0 Link encap:Point-Point Protocol
inet addr:88.97.163.222 P-t-P:62.3.83.5 Mask:
255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1492 Metric:1
RX packets:3 errors:0 dropped:0 overruns:0 frame:0
TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:114 (114.0 B) TX bytes:170 (170.0 B)


> > is making eth1 the same likely to cause any problems?
>
> None that I am aware of. What kind of problems do you think of ?
>

Routing issues I think, when I first thought about it, I wasn't aware
you could specify 'dev eth1' when adding static routes.

> > Is it then simply a matter of running,
>
> > route add a.b.c.0 netmask 255.255.255.242 dev eth1?
>
> ^^^
> Don't you mean 255.255.255.248 ?

Oops, yes.

> I guess this could do the trick too. Except that it would not consider
> a.b.c.7 as a directed broadcast address. But you can add it by hand with
> 'ip route add broadcast a.b.c.7 dev eth1' if required.

Again, thanks. Can you point me to a good article on the use of
static routing, I had thought I understood it, and that 'a.b.c.0
netmask 255...248 ' would match any address in the desired range, be
it a host, network or broadcast address.

Regards,

Chris