extract IP from a buffer of bytes

Question

If I have a buffer containing these bytes:
510175126-94-51080

How can I extract the 75126-94-51 part from it(which is the IP) and print the correct IP?

for(int i = 0; i < bytes_recv; i++) { cout << static_cast<int>(temp[i]); } cout << endl;
edit: this is my output: 5 1 0 1 75 126 -94 -51 0 80

Answer 1

A lot of information is missing from the question. So, you are reading the server reply from the SOCKS protocol. First, the buffer should not and will not have a fixed size of 10 bytes. It has 10 bytes if your address is IPv4 (which coincidently was exhausted a few days ago, time to think about IPv6). If the origin has a IPv6 address, the size of the server response is different.

From RFC 1928, section 6, the server reply has this format:

+----+-----+-------+------+----------+----------+
|VER | REP |  RSV  | ATYP | BND.ADDR | BND.PORT |
+----+-----+-------+------+----------+----------+
| 1  |  1  | X'00' |  1   | Variable |    2     |
+----+-----+-------+------+----------+----------+

Note that the address field has variable size. For IPv4, specifically, ATYP == 0x01 and BND.ADDR has size 4, which makes 1+1+1+1+4+2 = 10 bytes. But you should consider that other sizes are possible, specially if ATYP == 0x03, which makes BND.ADDR truly variable in length.

So, answering your question, considering that you have these bytes in a char buffer[] array (or pointer), you must first check the type of the address, then extract it like this:

#include <arpa/inet.h>

switch (buffer[3]) {
  case 0x01: {    /* IPv4 address */
    char result[INET_ADDRSTRLEN];
    inet_ntop(AF_INET, (void*)(&buffer[4]), result, sizeof result);
    std::cout << "IPv4: " << result << "\n";
    break;
  }

  case 0x04: {    /* IPv6 address */
    char result[INET6_ADDRSTRLEN];
    inet_ntop(AF_INET6, (void*)(&buffer[4]), result, sizeof result);
    std::cout << "IPv6: " << result << "\n";
    break;
  }

  default:
    std::cout << "Unsupported format.\n";
    break;
}

Answer 2

In Python:

input = '510175126-94-51080'
output = input[4:-3]

print output

Answer 3

How are you getting this buffer? Is it part of a struct sockaddr_in?

Are you showing a decimal dump of bytes starting at the lowest address of the buffer?

What you want to do is get a pointer to the base (memory) address of the IP addr part and use ntohl() to translate it to host byte order, and likewise use ntohs to translate the port addr.

edit

Suppose you have

stuct sockaddr_in foo;
char str[16];
// do some stuff that initializes foo, e.g. a call to accept() returns

You can extract the IP address in dotted-quad form with

inet_ntop(AF_INET, &(foo.sin_addr), str, sizeof(str));

Answer 4

If you have the following buffer:

buffer[10] = {5, 1, 0, 1, 75, 126, 162, 205, 80, 00};

And you want it in C/C++. I would define a struct, like this:

#pragma pack(1)
struct buffer_s {
    uint8_t  version;
    uint8_t  reserve_1;
    uint8_t  atype;
    uint32_t ip;
    uint16_t port;
};
#pragma pack()

Then put it in a union:

union buffer_u {
    char buffer[10];
    struct buffer_s data;
};

Now you can declare union buffer_u buf and read bytes into buf.buffer. Then for the ip and port fields use ntohl and ntohs to convert network byte order to host byte order.