Question On how to Fill icmp_data

The following code is from UNP

Code:

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1 #include    "ping.h"

2 void
3 send_v4 (void)
4 {
5    int    len;
6    struct icmp *icmp;

7    icmp = (struct icmp *) sendbuf;
8    icmp->icmp_type = ICMP_ECHO;
9    icmp->icmp_code = 0;
10    icmp->icmp_id = pid;
11    icmp->icmp_seq = nsent++;
12    memset (icmp->icmp_data, 0xa5, datalen); /* fill with pattern */
13    Gettimeofday ((struct timeval *) icmp->icmp_data, NULL);

14    len = 8 + datalen;          /* checksum ICMP header and data */
15    icmp->icmp_cksum = 0;
16    icmp->icmp_cksum = in_cksum ((u_short *) icmp, len);

17    Sendto (sockfd, sendbuf, len, 0, pr->sasend, pr->salen);
18 }

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At line 12, why 0xa5 is used to fill icmp_data? Any good reason for doing so? Can I use something else to fill icmp_data

You can fill it with anything you like, it's just payload to pad out the length of the message.

Various tools use the known content of such padding to identify the sender.

A5 is just one of those alternating 0,1 patterns at the bit level.

What if I don't fill it with anything? That is, I don't write the following code

Code:

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12    memset (icmp->icmp_data, 0xa5, datalen); /* fill with pattern */

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Quote:

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Originally Posted by Antigloss

What if I don't fill it with anything?

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Then you end up sending whatever bytes happened to be sitting around in sendbuf. Maybe it matters, maybe it doesn't. In this case, it probably doesn't. But why not initialize it? It's painless.