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Re: [lwip-users] Add new header & tail

From: address@hidden
Subject: Re: [lwip-users] Add new header & tail
Date: Thu, 13 Sep 2018 21:34:39 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 11.09.2018 12:29, dhosa wrote:

Hi, I'm trying to add & delete custom header ESP header 8 bytes before IP header, I used pbuf_header(p,8) in ip_output_if_opt before if (pbuf_header(p, IP_HLEN))

 *                              |room for new header
 *     ________________________\/________________________________________________
 *    |          ¦       ¦      ¦                             ¦ padd       ¦ ev. |
 *    | Ethernet ¦  IP   ¦ ESP  ¦          Payload(TCP,UDP    ¦ next-proto ¦ ICV |
 *    |__________¦_______¦______¦_____________________________¦____________¦_____|
 *    ¦                         ¦                             ¦                  ¦ 
 *                                                            ¦<-   room tail  ->¦ 

struct esp_hdr {
PACK_STRUCT_FIELD(u32_t sequence_number);
esphdr = (struct esp_hdr *) q->payload;
esphdr->spi = spi;
esphdr->sequence_number = sequence_number;

That doesn't seem completely correct. Per your later image, it seems you need the IP header to contain proto = ESP, not proto = TCP...

is that correct? How can I delete esp header for an incoming packet only and keep IP header?
in ip_input :

But you must also check it, not only remove it, don't you?

// remove ip and esp header ... 20 for ip + 8 byte for esp

esphdr = (struct esp_hdr *)p->payload;


// generate IP header
pbuf_header(p, IP_HLEN);

again is that correct?

No, that doesn't seem correct. pbuf_header only moves the payload pointer, the data is still where it was before. You'll need to fiddle around with the data or create new pbufs to achieve what you want.

If you wanted to implement this without changing the stack, you'd need some kind of new hook (as Sergio mentioned) to add the header and change the IP protocol.

For RX, it might be enough to implement a raw pcb that detects the IP proto = ESP, checks and hides the header and sends the remaining pbuf up to tcp_input().


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