Re: [lwip-users] ppp-new IP forwarding only works one direction (Ethernet to PPP)

[LMao]

Hi Sylvain,

Thank you for your reply. I really appreciate your help. The below are a few 
comments and more observations.

>Well, ip_forward() should call netif->output(), which in our case is 
>ppp_netif_output_ip4(), could you first check that ?
Sorry, I may confuse you about which direction the problem is. Actually, the 
problem is it doesn't output the corresponding Ethernet packets based on the 
incoming PPP packets.

>You can also enable PPP_DEBUG in your lwipopts.h, this will display discarded 
>frames in ppp_netif_output_ip4() if any.
The following logging messages are captured with PPP_DEBUG enabled. I also 
enabled ETHARP_DEBUG & NETIF_DEBUG.

From the newly captured messages below(last line), it seems there's a memory 
issue.
etharp_timer
etharp_timer
ethernet_input: dest:00:bd:33:06:68:22, src:c8:d7:19:ee:1f:fc, type:800
ip_input: iphdr->dest 0x6a01a8c0 netif->ip_addr 0x3200a8c0 (0x1a8c0, 0xa8c0, 
0x6a000000)
ip_input: iphdr->dest 0x6a01a8c0 netif->ip_addr 0x6901a8c0 (0x6a01a8c0, 
0x6901a8c0, 0x0)
ip_input: packet not for us.
ip_forward: forward packets to interface.pp
ip_forward: forwarding packet to 192.168.1.106
ip_input: iphdr->dest 0xd300a8c0 netif->ip_addr 0x6901a8c0 (0xd300a8c0, 
0x6901a8c0, 0x0)
ip_input: iphdr->dest 0xd300a8c0 netif->ip_addr 0x3200a8c0 (0xa8c0, 0xa8c0, 
0xd3000000)
ip_input: packet not for us.
ip_forward: forward packets to interface.em
ip_forward: forwarding packet to 192.168.0.211
etharp_output: could not allocate room for header.

I looked at the code of etharp_output function, it seems pbuf_header function 
cannot find room for Ethernet header (code snippet is shown below).
  /* make room for Ethernet header - should not fail */
#if ETHARP_SUPPORT_VLAN && defined(LWIP_HOOK_VLAN_SET)
  if (pbuf_header(q, sizeof(struct eth_hdr) + SIZEOF_VLAN_HDR) != 0) {
#else /* ETHARP_SUPPORT_VLAN && defined(LWIP_HOOK_VLAN_SET) */
  if (pbuf_header(q, sizeof(struct eth_hdr)) != 0) {
#endif /* ETHARP_SUPPORT_VLAN && defined(LWIP_HOOK_VLAN_SET) */
    /* bail out */
    LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS,
      ("etharp_output: could not allocate room for header.\n"));
    LINK_STATS_INC(link.lenerr);
    return ERR_BUF;

However, the non-forwarded IP packets have no problem to be sent out. For 
example, if I ping 192.168.1.105 which is the PPP interface on the gateway 
device, we can IP packet is sent out without problem (log messages are shown 
below).
etharp_timer
etharp_timer
ethernet_input: dest:ff:ff:ff:ff:ff:ff, src:c8:d7:19:ee:1f:fc, type:806
etharp_update_arp_entry: 192.168.0.211 - c8:d7:19:ee:1f:fc
etharp_find_entry: found matching entry 0
etharp_update_arp_entry: updating stable entry 0
etharp_arp_input: incoming ARP request
etharp_arp_input: replying to ARP request for our IP address
ethernet_input: dest:00:bd:33:06:68:22, src:c8:d7:19:ee:1f:fc, type:800
ip_input: iphdr->dest 0x6901a8c0 netif->ip_addr 0x3200a8c0 (0x1a8c0, 0xa8c0, 
0x69000000)
ip_input: iphdr->dest 0x6901a8c0 netif->ip_addr 0x6901a8c0 (0x6901a8c0, 
0x6901a8c0, 0x0)
ip_input: packet accepted on interface pp
ip_input: 
IP header:
+-------------------------------+
| 4 | 5 |  0x00 |        60     | (v, hl, tos, len)
+-------------------------------+
|     3005      |000|       0   | (id, flags, offset)
+-------------------------------+
|  128  |    1  |    0xab77     | (ttl, proto, chksum)
+-------------------------------+
|  192  |  168  |    0  |  211  | (src)
+-------------------------------+
|  192  |  168  |    1  |  105  | (dest)
+-------------------------------+
ip_input: p->len 60 p->tot_len 60
ip_output_if: em0
IP header:
+-------------------------------+
| 4 | 5 |  0x00 |        60     | (v, hl, tos, len)
+-------------------------------+
|     3005      |000|       0   | (id, flags, offset)
+-------------------------------+
|  255  |    1  |    0x2c77     | (ttl, proto, chksum)
+-------------------------------+
|  192  |  168  |    1  |  105  | (src)
+-------------------------------+
|  192  |  168  |    0  |  211  | (dest)
+-------------------------------+
netif->output()
etharp_send_ip: sending packet 0020cf34
etharp_timer
etharp_timer

Now, what I really don't understand is the forwarded and non-forwarded IP 
packets both need room for Ethernet header before sending out, why one 
succeeded in making room for Ethernet header and the other failed. Is it 
because these two type packets use different types of PBUF?

Charles