Hello All,

I am trying to modify the linux ip stack to get the avg/min/max of the TTL value (in trasport layer) from the received UDP packet. I have modifed the 2.6.28 kernel for this and added new setsockopt flags as IP_RECVTTL_AVG, IP_RECVTLL_MIN, IP_RECVTTL_MAX.

Please see below changes I have made to do this:

1. Inside "in.h" file Added following macros
#define IP_RECVTTL_AVG 20
#define IP_RECVTTL_MIN 21
#define IP_RECVTTL_MAX 22

2. Inside "inet_sock.h" in struct inet_sock {
added 3 variables
__s16 uc_ttl_avg;
__s16 uc_ttl_min;
__s16 uc_ttl_max;

3. In ipv4/udp.c added following peice of code
sk = __udp4_lib_lookup_skb(skb, uh->source, uh->dest, udptable);
if (sk != NULL) {

#if 1 // Changes to get TTL stats..Start
struct iphdr *iph;
struct inet_sock *inet;
inet = inet_sk(sk);
iph = ip_hdr(skb);
inet->uc_ttl_avg = (iph->ttl + inet->uc_ttl_avg)>>1;
if (inet->uc_ttl_max < iph->ttl)
inet->uc_ttl_max = iph->ttl;
if (inet->uc_ttl_min > iph->ttl)
inet->uc_ttl_min = iph->ttl;
#endif // Changes to get TTL stats..Start

int ret = udp_queue_rcv_skb(sk, skb);
sock_put(sk);

4. Changes inside "ip_sockglue.c" file

static void ip_cmsg_recv_ttl_avg(struct msghdr *msg, struct sk_buff *skb)
{
struct inet_sock *inet = inet_sk(skb->sk);
int ttl = inet->uc_ttl_avg;
printk(KERN_DEBUG "***<recv ttl avg () is called -- value = %d***\n", ttl);
put_cmsg(msg, SOL_IP, IP_TTL, sizeof(int), &ttl);
}

static void ip_cmsg_recv_ttl_max(struct msghdr *msg, struct sk_buff *skb)
{
struct inet_sock *inet = inet_sk(skb->sk);
int ttl = inet->uc_ttl_max;
printk(KERN_DEBUG "***<recv ttl max () is called -- value = %d***\n", ttl);
put_cmsg(msg, SOL_IP, IP_TTL, sizeof(int), &ttl);
}

static void ip_cmsg_recv_ttl_min(struct msghdr *msg, struct sk_buff *skb)
{
struct inet_sock *inet = inet_sk(skb->sk);
int ttl = inet->uc_ttl_min;
printk(KERN_DEBUG "***<recv ttl min () is called -- value = %d***\n", ttl);
put_cmsg(msg, SOL_IP, IP_TTL, sizeof(int), &ttl);
}

void ip_cmsg_recv(struct msghdr *msg, struct sk_buff *skb)
{
.
.
.
.
#if 1 // Changes to get TTL stats..Start
if (flags & 1)
ip_cmsg_recv_ttl_avg(msg, skb);
if ((flags>>=1) == 0)
return;
if (flags & 1)
ip_cmsg_recv_ttl_min(msg, skb);
if ((flags>>=1) == 0)
return;
if (flags & 1)
ip_cmsg_recv_ttl_max(msg, skb);
#endif // Changes to get TTL stats..Start

}

inside do_ip_setsockopt() added following code

#if 1 // Changes to get TTL stats..Start
case IP_RECVTTL_AVG:
if (val)
inet->cmsg_flags |= IP_CMSG_TTL_AVG;
else
inet->cmsg_flags &= ~IP_CMSG_TTL_AVG;
break;
case IP_RECVTTL_MIN:
if (val)
inet->cmsg_flags |= IP_CMSG_TTL_MIN;
else
inet->cmsg_flags &= ~IP_CMSG_TTL_MIN;
break;
case IP_RECVTTL_MAX:
if (val)
inet->cmsg_flags |= IP_CMSG_TTL_MAX;
else
inet->cmsg_flags &= ~IP_CMSG_TTL_MAX;
break;
#endif // Changes to get TTL stats..End

================================================== ===============

I have compiled this and written the udp client server to test this.


In my client application i am sending 64 packets with some dummy message. In server I have used the recvmsg() and setsockopt() to test these changes as below:

setsockopt(serverfd, IPPROTO_IP, IP_OPTIONS, NULL, 0);
if (setsockopt (serverfd,IPPROTO_IP, IP_RECVTTL_AVG, &ttl,ttlSize)<0) {

printf ("Error occured in setsockopt\n");
exit(1);
}

now if i do the recvmsg 64 times after recvfrom in my server code it gives me proper ttl avg values.. but i do not want to seek this values for every packet i receive.. i want them once for all sent 64 udp packets.. the same recvmsg fails if i doit single recvmsg after 64 received packets..


Can anyone tell about what i am doing wrong in the present scenario?

Thanks in advance
mpk