[lwip-users] Forwarding IP packets from PPP to Ethernet fails - no room for Ethernet header

[Jan Ulvesten]

Hi everybody

 

I’m trying out the lwip and accomplished to have both a Ethernet interface and a PPP interface up and running

 

The IP forwarding does however fail, because the PPP header (8 bytes)  is smaller than the Ethernet header (14 bytes)

 

The pbuf from the PPP.c flows to etharp_output. At this point, the PPP header has been removed,

etharp_output fails to insert a Ethernet header since the pbuf is  6 bytes too short (14-8)  (the deviation between the pbuf pointer and the payload is only 0x18 bytes while 0x1E is required)

 

So the question is: How can I insert 6 dummy bytes into the beginning of a pbuf without too much processing?

 

Thanks in advance!

 

Jan Ulvesten

Senior Software Engineer
SICOM  AS