[Bridge] bridge-utils and ppp

[Bridge] bridge-utils and ppp

[Lou Steinberg]

Hi Folks-

Forgive the newbie question, but my attempts to answer this through 
google have left me confused. 

I'm trying to bridge between a pair of ppp and eth interfaces in an 
embedded linux distribution called SlugOS.  The kernel is 2.6.21 and my 
understanding was that the 2.6 kernels have the mods to support ppp 
interfaces.  When I try to add my ppp0 interface to a bridge, I get:
> $ brctl addif test ppp0
> can't add ppp0 to bridge test: Invalid argument

MTUs are set the same for all interfaces.  Is there a specific version 
of the bridge-utils package I need?  The version packed for my 
distribution claims to be 1.2-r0

Thanks in advance,
Lou

Re: [Bridge] bridge-utils and ppp

[richardvoigt]

On Thu, Mar 6, 2008 at 9:00 PM, Lou Steinberg <lspam5@operations.com> wrote:
> Hi Folks-
>
>  Forgive the newbie question, but my attempts to answer this through
>  google have left me confused.
>
>  I'm trying to bridge between a pair of ppp and eth interfaces in an
>  embedded linux distribution called SlugOS.  The kernel is 2.6.21 and my
>  understanding was that the 2.6 kernels have the mods to support ppp
>  interfaces.  When I try to add my ppp0 interface to a bridge, I get:
>  > $ brctl addif test ppp0
>  > can't add ppp0 to bridge test: Invalid argument

Does a bridge "test" exist?  What is the output of brctl show?

>
>  MTUs are set the same for all interfaces.  Is there a specific version
>  of the bridge-utils package I need?  The version packed for my
>  distribution claims to be 1.2-r0
>
>  Thanks in advance,
>  Lou
>
>
>  _______________________________________________
>  Bridge mailing list
>  Bridge@lists.linux-foundation.org
>  https://lists.linux-foundation.org/mailman/listinfo/bridge
>

Re: [Bridge] bridge-utils and ppp

[Lou Steinberg]

Hi Richard-

[snip]
> 
> Does a bridge "test" exist?  What is the output of brctl show?
> 
[snip]

Yes, the bridge exists.  brctl shows:

 > root@wan-bridge:~$ brctl addif test ppp0
 > can't add ppp0 to bridge test: Invalid argument
 > root@wan-bridge:~$ echo $?
 > 1
 > root@wan-bridge:~$ brctl show
 > bridge name     bridge id               STP enabled     interfaces
 > test            8000.000000000000       no

Thanks,
Lou

Re: [Bridge] bridge-utils and ppp

[Stephen Hemminger]

On Sun, 09 Mar 2008 09:58:57 -0400
Lou Steinberg <lspam5@operations.com> wrote:

> Hi Richard-
> 
> [snip]
> > 
> > Does a bridge "test" exist?  What is the output of brctl show?
> > 
> [snip]
> 
> Yes, the bridge exists.  brctl shows:
> 
>  > root@wan-bridge:~$ brctl addif test ppp0
>  > can't add ppp0 to bridge test: Invalid argument
>  > root@wan-bridge:~$ echo $?
>  > 1
>  > root@wan-bridge:~$ brctl show
>  > bridge name     bridge id               STP enabled     interfaces
>  > test            8000.000000000000       no
> 

The bridging code only works for devices that have Ethernet headers.
That is the way the bridging 802.2 standard works, it is a Layer 2 bridge
and it uses the address information from Ethernet to do routing.

PPP does not use Ethernet level framing so it can't be bridged.
What were you trying to do? there are plenty of other ways to get the
same effect.

Re: [Bridge] bridge-utils and ppp

[Lou Steinberg]

Stephen-

Stephen Hemminger wrote:

> 
> The bridging code only works for devices that have Ethernet headers.
> That is the way the bridging 802.2 standard works, it is a Layer 2 bridge
> and it uses the address information from Ethernet to do routing.
> 
> PPP does not use Ethernet level framing so it can't be bridged.
> What were you trying to do? there are plenty of other ways to get the
> same effect.

Makes sense, I suppose (The forwarding is complicated by a p-p link 
without a destination mac addr that looks like ethernet).  I expected 
that someone had built a table to do the mappings because the docs I 
referenced in the original post specifically say that ppp can be bridged.

What I'm trying to do is a simple/low latency forward from a vpn 
concentrator/firewall/router out to a cellular WAN modem that attaches 
to a USB.  My firewall lacks the USB interface so I simply need to 
forward packets between the USB and ethernet.  Am doing it now at layer 
3 (using iptables to do NAT) but that adds a lot of complexity and my 
firewall already does NAT (so it's happening twice).    I haven't yet 
tested an ipsec tunnel over the link, but am concerned that I may also 
need to add ipsec passthrough if I do this at layer 3.

All in all, a simple layer 2 forward between eth0 and ttyUSB0 is what I 
really need.

/Lou

Re: [Bridge] bridge-utils and ppp

[Stephen Hemminger]

On Thu, 21 Aug 2008 17:44:59 -0700
"Kelvin Chan" <kelvinc@positronics.com> wrote:

> Hi Stephen,
> 
> --god, I hate outlook. Sorry, was trying to insert an line break
> but instead sent the email. --
> 
> I'm trying to create an multicast enabled Ethernet bridge using ppp
> and brctl. 
> 
> I tried:
> brctl addbr br0
> brctl addif eth0
> brctl addif ppp0
> 
> after last command, I get:
> can't add ppp0 to bridge br0: Invalid argument
> 
> both eth0 and ppp0 are up and have an address. I googled around 
> and found this an issue with the kernel bridge code. However, in 
> Linux Foundation's bridge-utils documentation, it didn't mention
> the required kernel version. I'm running 2.6.18 from Debian Etch.
> 
> Can you give me any hints to bridge an ppp to eth device?
> 

Won't work because PPP is a tunneling interface so it doesn't
have an ethernet header required for bridging.  Layer 2 versus Layer 3 issue.