Comparison of two IPs

Question

I am required to compare two IPs. One IP is stored in u8 format where I managed to convert it to char * which i stored in the variable arp_tbuf. However, the second IP to be compared is in u32 format (from ip.h). But every time I try to convert the IP in u32 format to char * as follows,

    unsigned int pkt_da = (unsigned int)ip_header->daddr;   
    char pkt_tbuf[16];
    char pkt_tbuf_tmp[4];

    pkt_tbuf_tmp[0] = pkt_da & 0x000000FF;
    pkt_tbuf_tmp[1] = (pkt_da & 0x0000FF00) >> 8;
    pkt_tbuf_tmp[2] = (pkt_da & 0x00FF0000) >> 16;
    pkt_tbuf_tmp[3] = (pkt_da & 0xFF000000) >> 24;  
    sprintf(pkt_tbuf, "%d.%d.%d.%d\n", pkt_tbuf_tmp[0], pkt_tbuf_tmp[1], pkt_tbuf_tmp[2], pkt_tbuf_tmp[3]);

I get a kernel panic error.

I am aware of the functionality memcmp to compare to characters.

It would be a great help if you experts would help me in converting this IP to char * and to compare the two IP's like memcmp(arp_tbuf, pkt_tbuf).

Thank you very much :)

EDIT

As @BobJarvis suggested, I ran the code again in my kernel. IT worked fine for converting the IPs in the LAN. However, When I loaded the web page, the kernel panic error occured. I there a cleaner way to perform this IP conversion from unsigned int into char * (dotted IP format) ?

Now, what do you mean by "u8 format"? Please show the declaration. Thanks. — Bob Jarvis - Слава Україні, May 24 '14 at 17:12
@JimGarrison ohh I will check on that asap..totally missed it. thank you very much :) — Hasitha Shan, May 24 '14 at 17:14
@JimGarrison putting `%s` also did not work. c an you suggest any alternative way to fix this? — Hasitha Shan, May 24 '14 at 17:20
@BobJarvis `u8` is the type defined by the header which i refer to when getting the first IP :) — Hasitha Shan, May 24 '14 at 17:22
Can you open up the header and post the definition of `u8`? I can think of several possible ways that something named `u8` might be defined... — Bob Jarvis - Слава Україні, May 24 '14 at 18:47
@JimGarrison - it's been a while, but I seem to recall that when arguments of a numeric type smaller than `int` (e.g. `char`) are passed to a variadic function in C/C++ the argument will be expanded to int on the stack. Or is this just my memory playing tricks on me? — Bob Jarvis - Слава Україні, May 24 '14 at 18:54
I just tested your code at [Ideone](https://ideone.com/yaCaSw) and it compiled and ran fine. (Note that I used a literal for the IP address). There must be something else in your code that's causing the kernel panic. Try taking the code I tested at Ideone, compile and run it at your site, and see what you get. Thanks. — Bob Jarvis - Слава Україні, May 24 '14 at 19:04
@BobJarvis ohh.. thank you sir for trying that out..I must then see to the rest of the code :) — Hasitha Shan, May 24 '14 at 20:04
Why are you putting `n` in your string? And why do you think the kernel panic occurs anywhere in the code you have posted? — n. m. could be an AI, May 24 '14 at 21:00
@JimGarrison Bob Jarvis is right: the `char` arguments will be automatically promoted to `int`s. — Steve Emmerson, May 24 '14 at 22:11
@SteveEmmerson thanks - it's been awhile since I wrote a C program :-( I should have double-checked. — Jim Garrison, May 25 '14 at 03:33
@n.m. thank you for the reply .. :) because all these error occurred after implementing the new code to convert to string. otherwise it worked just fine :) — Hasitha Shan, May 25 '14 at 04:22
"occurred after" does not entail "occured within". 16 characters are not enough to contain 4 3-character quads *and* three dots *and* a newline *and* the terminating null character. Just sayin'. — n. m. could be an AI, May 25 '14 at 04:54

score 2 · Answer 1 · edited May 23 '17 at 10:28

2

I believe you are running into problems with values (any 2 byte part of the IP) exceeding 127. The >> operator is an arithmetic shift (sign preserving) shift and not a logical shift. (see Shift operator in C ). You can see this in your code with simple tests of pkt_da = 0x7f7f7f7f and pkt_da = 0x80808080. Printing the values with:

for (it = 0; it < 4; it++)
    printf ("  pkt_tbuf_tmp[%2d]: %u\n", it, pkt_tbuf_tmp[it]);

Gives:

./bin/pktb 2139062143
pkt_tbuf_tmp[ 0]: 127
pkt_tbuf_tmp[ 1]: 127
pkt_tbuf_tmp[ 2]: 127
pkt_tbuf_tmp[ 3]: 127
Done - pkt_tbuf='127.127.127.127'

./bin/pktb 2155905152
pkt_tbuf_tmp[ 0]: 4294967168
pkt_tbuf_tmp[ 1]: 4294967168
pkt_tbuf_tmp[ 2]: 4294967168
pkt_tbuf_tmp[ 3]: 4294967168
Done - pkt_tbuf='-128.-128.-128.-128'

The right-shift behavior is compiler dependent. So what works on one box, may not on another system.

edited May 23 '17 at 10:28

Community

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answered May 24 '14 at 23:01

David C. Rankin

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Thank you very much for the reply..i will check post back :) – Hasitha Shan May 25 '14 at 04:24
Yes you absolutely right..what would be your solution to fix this sir? so that it could support all ip values? – Hasitha Shan May 25 '14 at 04:39
I am glad you found a solution. It was along what I was thinking. Take a look at the `inet_pton` function as well. – David C. Rankin May 25 '14 at 07:13
Yes sir, you suggestion helped me look for specific solutions for my problem.. thank you very much +1 – Hasitha Shan May 25 '14 at 07:18

score 0 · Accepted Answer · edited May 23 '17 at 10:28

With the explanation give by @DavidC.Rankin I was able to narrow down and look for more solution. Thus, I found a similar post which solved the question of Converting the int IP to dotted IP, Convert source IP address from struct iphdr* to string equivalent using Linux netfilter

Then I just performed a memcmp()

int cmp;
cmp=memcmp ( arp_tbuf, pkt_tbuf, sizeof(arp_tbuf) );

if((cmp > 0) || (cmp < 0)){
    printk(KERN_ALERT "Not matching");              
}

Thank you experts for the assistance in helping me understand the issue. :)

Comparison of two IPs

2 Answers2