How to timestamp incomming IP packets in kernel

[jituk]

Hi,

I am trying to timestamp all incomming IP packets. For this I am making changes to ip_rcv_finish() function, in Linux/net/ipv4/ip_input.c. Before ip header is checked (iph->ihl > 5).

If the incomming IP packet header lenght is 5, i.e their is no option field available in IP header. How can I allocate memory in IP header to add timestamp infomation.

I have tried using sbk_push(), to allocate extra memory for timestamp. But I am not sure weather this is the right way to timestamp IP packets.

[fernape]

I am trying to timestamp all incomming IP packets. For this I am making changes to ip_rcv_finish() function, in Linux/net/ipv4/ip_input.c. Before ip header is checked (iph->ihl > 5).

But I am not sure weather this is the right way to timestamp IP packets.

I strongly recommnd that u use the Linux Trace Toolkit facility or netfilter hooks to do this.

Best Regards

[jituk]

Thank's for your comments..

I strongly recommnd that u use the Linux Trace Toolkit facility.

Are you refering to use gdb or something like that..

Regards,
Jituk

[fernape]

No.

Netfilter allows you to hook some IP stack functions (maybe to do some pre and post processing). So with it, u should be able to take some timestamps.

On the ohter hand, Linux Trace Toolkit allows to define some "checkpoints" inside the linux kernel, so u can do some work "before" a certain function is executed.

Best Regards

[jituk]

Thanks for replying.

Netfilter allows you to hook some IP stack functions (maybe to do some pre and post processing). So with it, u should be able to take some timestamps.

I am new to netfilter, but as far as my understanding goes, with netfilter I will create a hook in linux kernel code, and while doing so I will be specifing a function that will add timestamp to ip packet.. So still my problem for allocating extra memory in IP header (skbuff) persist. Correct me if I am wrong.

On the ohter hand, Linux Trace Toolkit allows to define some "checkpoints" inside the linux kernel, so u can do some work "before" a certain function is executed.

Okey, I will chk on this one.

[fernape]

Maybe I didn't understand your problem... do you want to change the IP header allocating more memory? I mean, do you want to make it bigger?

Best Regards

[jituk]

Maybe I didn't understand your problem... do you want to change the IP header allocating more memory? I mean, do you want to make it bigger?
Best Regards

Yes, exactly I need to make incomming IP header bigger. Since all most all the incomming IP packets do not have IP option field in the header (IP header lenght field is 5 (i.e. 20 bytes)), I need to allocated extra memory so that I can incorporate timestamp option field in IP header (extra 8 bytes.. to make it 28 bytes. ) ..

thanks

[fernape]

Did you try with kmalloc()? Didn't work for u?

I think you could allocate memory for your new IP packet and then copy the old into the new plus your custom field.

Best Regards

[jituk]

Did you try with kmalloc()? Didn't work for u?
I think you could allocate memory for your new IP packet and then copy the old into the new plus your custom field.
Best Regards

Yea I tried tow ways to allocate memory one useng kmalloc and using skb_push... Here is my code that is written in ip_rcv_finish, just before ip header length is checked (

if (iph->ihl > 5)

)

// Check it the "sysctl" variable is set or not
if (sysctl_ip_opt_timestamps == 1) {
struct iphdr *new_iph;
unsigned char *new_ip_opt;

if (iph->ihl == 5) {
// Allocate a new memory buffer of the IP header
// new_iph = kmalloc(sizeof(struct iphdr) + , GFP_ATOMIC);
new_iph = (struct iphdr *)skb_push(skb,sizeof(struct iphdr) + ;

// Copy the contents of previous IP header to the new one.
memcpy(new_iph, iph, sizeof(struct iphdr));

// Now a assign the new IP header pointer to the local IP header pointer
// We are doing this so that we dont have to modify the code after **END**
iph = new_iph;

// We are allocation the new IP header to soket buffer, raw sub field.
// NOTE: we are not assigining the new pointer to "skb->nh.iph"
skb->nh.raw = iph;

// Modifing the lenght field to new IP header to 7 (5 for IP header + 2 for (8 byte option field)).
iph->ihl = 7;

// Creating a local pointer to point the IP option field
new_ip_opt = (unsigned char *)(new_iph + sizeof(struct iphdr));

// Adding data in the option field as per client requirment
*(new_ip_opt + 0) = 68; // Adding data to IP timestamp option type field (1st byte of option)
*(new_ip_opt + 1) = 8; // Adding data to IP timestamp option leng field (2nd byte of option)
*(new_ip_opt + 2) = 5; // Adding data to IP timestamp option pointer field (3rd byte of option)
*(new_ip_opt + 3) = 0; // Adding data to IP timestamp option overflow + flag field (4th byte of option)

// NOTE: Timestamp is not added at this point. But when the control goes to "ip_options_compile" function
// See 22 lines below this comment.
// We have just set the control for time stamp to be added.
}
}

#endif /*CONFIG_IP_PACKET_TIMESTAMP*/

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