Cannot get the correct IP version from packet

Cannot get the correct IP version from packet

[Alessandro Vesely]

Help me please,

In the queue callback (const struct nlmsghdr *nlh, void *data), I do:

     struct nlattr *attr[NFQA_MAX+1] = {};
     if (nfq_nlmsg_parse(nlh, attr) < 0)
         return MNL_CB_ERROR;

     char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);
     if (payload)
     {
         int const af = (payload[0] & 0xf);

And here I get af == 5.  It should be 4 or 6.

That's using libmnl0:amd64 1.0.4-3.  What's happening?


TIA for any hint
Best
Ale

Re: Cannot get the correct IP version from packet

[Alessandro Vesely]

On Sat 16/Mar/2024 19:11:58 +0100 Alessandro Vesely wrote:
> 
>      char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);
>      if (payload)
>      {
>          int const af = (payload[0] & 0xf);

Heck, should be af = (payload[0] &0xf0)>>4 = 0x6

third time I change it...


Best
Ale

Re: Cannot get the correct IP version from packet

[Meiyong Yu]

> 在 2024年3月17日，02:51，Alessandro Vesely <vesely@tana.it> 写道：
> 
> On Sat 16/Mar/2024 19:11:58 +0100 Alessandro Vesely wrote:
>>     char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);
>>     if (payload)
>>     {
>>         int const af = (payload[0] & 0xf);
> 
> Heck, should be af = (payload[0] &0xf0)>>4 = 0x6
> 
> third time I change it...
> 
> 
> Best
> Ale
> 

char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);

 before this line, you must check if attr[NFQA_PAYLOAD] is NULL pointer，may be it is NULL pointer

Re: Cannot get the correct IP version from packet

[Alessandro Vesely]

On Sat 16/Mar/2024 23:21:31 +0100 Meiyong Yu wrote:
>> 在 2024年3月17日，02:51，Alessandro Vesely <vesely@tana.it> 写道：
>> On Sat 16/Mar/2024 19:11:58 +0100 Alessandro Vesely wrote:
>>>     char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);
>>>     if (payload)
>>>     {
>>>         int const af = (payload[0] & 0xf);
>> 
>> Heck, should be af = (payload[0] &0xf0)>>4 = 0x6
>> 
>> third time I change it...
> 
> char *payload = mnl_attr_get_payload(attr[NFQA_PAYLOAD]);
> 
>   before this line, you must check if attr[NFQA_PAYLOAD] is NULL pointer，may be it is NULL pointer


Good point, I'll add that check.  Thank you.


Best
Ale