在下面的代码中，有人可以向我解释一下下面的代码是如何工作的吗？
/*
* sniffex.c
*
* Sniffer example of TCP/IP packet capture using libpcap.
*
* Version 0.1.1 (2005-07-05)
* Copyright (c) 2005 The Tcpdump Group
*
* This software is intended to be used as a practical example and
* demonstration of the libpcap library; available at:
* http://www.tcpdump.org/
*
****************************************************************************
*
* This software is a modification of Tim Carstens' "sniffer.c"
* demonstration source code, released as follows:
*
* sniffer.c
* Copyright (c) 2002 Tim Carstens
* 2002-01-07
* Demonstration of using libpcap
* timcarst -at- yahoo -dot- com
*
* "sniffer.c" is distributed under these terms:
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 4. The name "Tim Carstens" may not be used to endorse or promote
* products derived from this software without prior written permission
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
* <end of "sniffer.c" terms>
*
* This software, "sniffex.c", is a derivative work of "sniffer.c" and is
* covered by the following terms:
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Because this is a derivative work, you must comply with the "sniffer.c"
* terms reproduced above.
* 2. Redistributions of source code must retain the Tcpdump Group copyright
* notice at the top of this source file, this list of conditions and the
* following disclaimer.
* 3. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 4. The names "tcpdump" or "libpcap" may not be used to endorse or promote
* products derived from this software without prior written permission.
*
* THERE IS ABSOLUTELY NO WARRANTY FOR THIS PROGRAM.
* BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
* FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
* OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
* PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
* OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
* TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
* PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
* REPAIR OR CORRECTION.
*
* IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
* WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
* REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
* INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
* OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
* TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
* YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
* PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGES.
* <end of "sniffex.c" terms>
*
****************************************************************************
*
* Below is an excerpt from an email from Guy Harris on the tcpdump-workers
* mail list when someone asked, "How do I get the length of the TCP
* payload?" Guy Harris' slightly snipped response (edited by him to
* speak of the IPv4 header length and TCP data offset without referring
* to bitfield structure members) is reproduced below:
*
* The Ethernet size is always 14 bytes.
*
* <snip>...</snip>
*
* In fact, you *MUST* assume the Ethernet header is 14 bytes, *and*, if
* you're using structures, you must use structures where the members
* always have the same size on all platforms, because the sizes of the
* fields in Ethernet - and IP, and TCP, and... - headers are defined by
* the protocol specification, not by the way a particular platform's C
* compiler works.)
*
* The IP header size, in bytes, is the value of the IP header length,
* as extracted from the "ip_vhl" field of "struct sniff_ip" with
* the "IP_HL()" macro, times 4 ("times 4" because it's in units of
* 4-byte words). If that value is less than 20 - i.e., if the value
* extracted with "IP_HL()" is less than 5 - you have a malformed
* IP datagram.
*
* The TCP header size, in bytes, is the value of the TCP data offset,
* as extracted from the "th_offx2" field of "struct sniff_tcp" with
* the "TH_OFF()" macro, times 4 (for the same reason - 4-byte words).
* If that value is less than 20 - i.e., if the value extracted with
* "TH_OFF()" is less than 5 - you have a malformed TCP segment.
*
* So, to find the IP header in an Ethernet packet, look 14 bytes after
* the beginning of the packet data. To find the TCP header, look
* "IP_HL(ip)*4" bytes after the beginning of the IP header. To find the
* TCP payload, look "TH_OFF(tcp)*4" bytes after the beginning of the TCP
* header.
*
* To find out how much payload there is:
*
* Take the IP *total* length field - "ip_len" in "struct sniff_ip"
* - and, first, check whether it's less than "IP_HL(ip)*4" (after
* you've checked whether "IP_HL(ip)" is >= 5). If it is, you have
* a malformed IP datagram.
*
* Otherwise, subtract "IP_HL(ip)*4" from it; that gives you the length
* of the TCP segment, including the TCP header. If that's less than
* "TH_OFF(tcp)*4" (after you've checked whether "TH_OFF(tcp)" is >= 5),
* you have a malformed TCP segment.
*
* Otherwise, subtract "TH_OFF(tcp)*4" from it; that gives you the
* length of the TCP payload.
*
* Note that you also need to make sure that you don't go past the end
* of the captured data in the packet - you might, for example, have a
* 15-byte Ethernet packet that claims to contain an IP datagram, but if
* it's 15 bytes, it has only one byte of Ethernet payload, which is too
* small for an IP header. The length of the captured data is given in
* the "caplen" field in the "struct pcap_pkthdr"; it might be less than
* the length of the packet, if you're capturing with a snapshot length
* other than a value >= the maximum packet size.
* <end of response>
*
****************************************************************************
*
* Example compiler command-line for GCC:
* gcc -Wall -o sniffex sniffex.c -lpcap
*
****************************************************************************
*
* Code Comments
*
* This section contains additional information and explanations regarding
* comments in the source code. It serves as documentaion and rationale
* for why the code is written as it is without hindering readability, as it
* might if it were placed along with the actual code inline. References in
* the code appear as footnote notation (e.g. [1]).
*
* 1. Ethernet headers are always exactly 14 bytes, so we define this
* explicitly with "#define". Since some compilers might pad structures to a
* multiple of 4 bytes - some versions of GCC for ARM may do this -
* "sizeof (struct sniff_ethernet)" isn't used.
*
* 2. Check the link-layer type of the device that's being opened to make
* sure it's Ethernet, since that's all we handle in this example. Other
* link-layer types may have different length headers (see [1]).
*
* 3. This is the filter expression that tells libpcap which packets we're
* interested in (i.e. which packets to capture). Since this source example
* focuses on IP and TCP, we use the expression "ip", so we know we'll only
* encounter IP packets. The capture filter syntax, along with some
* examples, is documented in the tcpdump man page under "expression."
* Below are a few simple examples:
*
* Expression Description
* ---------- -----------
* ip Capture all IP packets.
* tcp Capture only TCP packets.
* tcp port 80 Capture only TCP packets with a port equal to 80.
* ip host 10.1.2.3 Capture all IP packets to or from host 10.1.2.3.
*
****************************************************************************
*
*/
#define APP_NAME "sniffex"
#define APP_DESC "Sniffer example using libpcap"
#define APP_COPYRIGHT "Copyright (c) 2005 The Tcpdump Group"
#define APP_DISCLAIMER "THERE IS ABSOLUTELY NO WARRANTY FOR THIS PROGRAM."
#include <pcap.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/* default snap length (maximum bytes per packet to capture) */
#define SNAP_LEN 1518
/* ethernet headers are always exactly 14 bytes [1] */
#define SIZE_ETHERNET 14
/* Ethernet addresses are 6 bytes */
#define ETHER_ADDR_LEN 6
/* Ethernet header */
struct sniff_ethernet {
u_char ether_dhost[ETHER_ADDR_LEN]; /* destination host address */
u_char ether_shost[ETHER_ADDR_LEN]; /* source host address */
u_short ether_type; /* IP? ARP? RARP? etc */
};
/* IP header */
struct sniff_ip {
u_char ip_vhl; /* version << 4 | header length >> 2 */
u_char ip_tos; /* type of service */
u_short ip_len; /* total length */
u_short ip_id; /* identification */
u_short ip_off; /* fragment offset field */
#define IP_RF 0x8000 /* reserved fragment flag */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_char ip_ttl; /* time to live */
u_char ip_p; /* protocol */
u_short ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */
};
#define IP_HL(ip) (((ip)->ip_vhl) & 0x0f)
#define IP_V(ip) (((ip)->ip_vhl) >> 4)
/* TCP header */
typedef u_int tcp_seq;
struct sniff_tcp {
u_short th_sport; /* source port */
u_short th_dport; /* destination port */
tcp_seq th_seq; /* sequence number */
tcp_seq th_ack; /* acknowledgement number */
u_char th_offx2; /* data offset, rsvd */
#define TH_OFF(th) (((th)->th_offx2 & 0xf0) >> 4)
u_char th_flags;
#define TH_FIN 0x01
#define TH_SYN 0x02
#define TH_RST 0x04
#define TH_PUSH 0x08
#define TH_ACK 0x10
#define TH_URG 0x20
#define TH_ECE 0x40
#define TH_CWR 0x80
#define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR)
u_short th_win; /* window */
u_short th_sum; /* checksum */
u_short th_urp; /* urgent pointer */
};
void
got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet);
void
print_payload(const u_char *payload, int len);
void
print_hex_ascii_line(const u_char *payload, int len, int offset);
void
print_app_banner(void);
void
print_app_usage(void);
/*
* app name/banner
*/
void
print_app_banner(void)
{
printf("%s - %s\n", APP_NAME, APP_DESC);
printf("%s\n", APP_COPYRIGHT);
printf("%s\n", APP_DISCLAIMER);
printf("\n");
return;
}
/*
* print help text
*/
void
print_app_usage(void)
{
printf("Usage: %s [interface]\n", APP_NAME);
printf("\n");
printf("Options:\n");
printf(" interface Listen on <interface> for packets.\n");
printf("\n");
return;
}
/*
* print data in rows of 16 bytes: offset hex ascii
*
* 00000 47 45 54 20 2f 20 48 54 54 50 2f 31 2e 31 0d 0a GET / HTTP/1.1..
*/
void
print_hex_ascii_line(const u_char *payload, int len, int offset)
{
int i;
int gap;
const u_char *ch;
/* offset */
printf("%05d ", offset);
/* hex */
ch = payload;
for(i = 0; i < len; i++) {
printf("%02x ", *ch);
ch++;
/* print extra space after 8th byte for visual aid */
if (i == 7)
printf(" ");
}
/* print space to handle line less than 8 bytes */
if (len < 8)
printf(" ");
/* fill hex gap with spaces if not full line */
if (len < 16) {
gap = 16 - len;
for (i = 0; i < gap; i++) {
printf(" ");
}
}
printf(" ");
/* ascii (if printable) */
ch = payload;
for(i = 0; i < len; i++) {
if (isprint(*ch))
printf("%c", *ch);
else
printf(".");
ch++;
}
printf("\n");
return;
}
/*
* print packet payload data (avoid printing binary data)
*/
void
print_payload(const u_char *payload, int len)
{
int len_rem = len;
int line_width = 16; /* number of bytes per line */
int line_len;
int offset = 0; /* zero-based offset counter */
const u_char *ch = payload;
if (len <= 0)
return;
/* data fits on one line */
if (len <= line_width) {
print_hex_ascii_line(ch, len, offset);
return;
}
/* data spans multiple lines */
for ( ;; ) {
/* compute current line length */
line_len = line_width % len_rem;
/* print line */
print_hex_ascii_line(ch, line_len, offset);
/* compute total remaining */
len_rem = len_rem - line_len;
/* shift pointer to remaining bytes to print */
ch = ch + line_len;
/* add offset */
offset = offset + line_width;
/* check if we have line width chars or less */
if (len_rem <= line_width) {
/* print last line and get out */
print_hex_ascii_line(ch, len_rem, offset);
break;
}
}
return;
}
/*
* dissect/print packet
*/
void
got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet)
{
static int count = 1; /* packet counter */
/* declare pointers to packet headers */
const struct sniff_ethernet *ethernet; /* The ethernet header [1] */
const struct sniff_ip *ip; /* The IP header */
const struct sniff_tcp *tcp; /* The TCP header */
const char *payload; /* Packet payload */
int size_ip;
int size_tcp;
int size_payload;
printf("\nPacket number %d:\n", count);
count++;
/* define ethernet header */
ethernet = (struct sniff_ethernet*)(packet);
/* define/compute ip header offset */
ip = (struct sniff_ip*)(packet + SIZE_ETHERNET);
size_ip = IP_HL(ip)*4;
if (size_ip < 20) {
printf(" * Invalid IP header length: %u bytes\n", size_ip);
return;
}
/* print source and destination IP addresses */
printf(" From: %s\n", inet_ntoa(ip->ip_src));
printf(" To: %s\n", inet_ntoa(ip->ip_dst));
/* determine protocol */
switch(ip->ip_p) {
case IPPROTO_TCP:
printf(" Protocol: TCP\n");
break;
case IPPROTO_UDP:
printf(" Protocol: UDP\n");
return;
case IPPROTO_ICMP:
printf(" Protocol: ICMP\n");
return;
case IPPROTO_IP:
printf(" Protocol: IP\n");
return;
default:
printf(" Protocol: unknown\n");
return;
}
/*
* OK, this packet is TCP.
*/
/* define/compute tcp header offset */
tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip);
size_tcp = TH_OFF(tcp)*4;
if (size_tcp < 20) {
printf(" * Invalid TCP header length: %u bytes\n", size_tcp);
return;
}
printf(" Src port: %d\n", ntohs(tcp->th_sport));
printf(" Dst port: %d\n", ntohs(tcp->th_dport));
/* define/compute tcp payload (segment) offset */
payload = (u_char *)(packet + SIZE_ETHERNET + size_ip + size_tcp);
/* compute tcp payload (segment) size */
size_payload = ntohs(ip->ip_len) - (size_ip + size_tcp);
/*
* Print payload data; it might be binary, so don't just
* treat it as a string.
*/
if (size_payload > 0) {
printf(" Payload (%d bytes):\n", size_payload);
print_payload(payload, size_payload);
}
return;
}
int main(int argc, char **argv)
{
char *dev = NULL; /* capture device name */
char errbuf[PCAP_ERRBUF_SIZE]; /* error buffer */
pcap_t *handle; /* packet capture handle */
char filter_exp[] = "ip"; /* filter expression [3] */
struct bpf_program fp; /* compiled filter program (expression) */
bpf_u_int32 mask; /* subnet mask */
bpf_u_int32 net; /* ip */
int num_packets = 10; /* number of packets to capture */
print_app_banner();
/* check for capture device name on command-line */
if (argc == 2) {
dev = argv[1];
}
else if (argc > 2) {
fprintf(stderr, "error: unrecognized command-line options\n\n");
print_app_usage();
exit(EXIT_FAILURE);
}
else {
/* find a capture device if not specified on command-line */
dev = pcap_lookupdev(errbuf);
if (dev == NULL) {
fprintf(stderr, "Couldn't find default device: %s\n",
errbuf);
exit(EXIT_FAILURE);
}
}
/* get network number and mask associated with capture device */
if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
fprintf(stderr, "Couldn't get netmask for device %s: %s\n",
dev, errbuf);
net = 0;
mask = 0;
}
/* print capture info */
printf("Device: %s\n", dev);
printf("Number of packets: %d\n", num_packets);
printf("Filter expression: %s\n", filter_exp);
/* open capture device */
handle = pcap_open_live(dev, SNAP_LEN, 1, 1000, errbuf);
if (handle == NULL) {
fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
exit(EXIT_FAILURE);
}
/* make sure we're capturing on an Ethernet device [2] */
if (pcap_datalink(handle) != DLT_EN10MB) {
fprintf(stderr, "%s is not an Ethernet\n", dev);
exit(EXIT_FAILURE);
}
/* compile the filter expression */
if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
fprintf(stderr, "Couldn't parse filter %s: %s\n",
filter_exp, pcap_geterr(handle));
exit(EXIT_FAILURE);
}
/* apply the compiled filter */
if (pcap_setfilter(handle, &fp) == -1) {
fprintf(stderr, "Couldn't install filter %s: %s\n",
filter_exp, pcap_geterr(handle));
exit(EXIT_FAILURE);
}
/* now we can set our callback function */
pcap_loop(handle, num_packets, got_packet, NULL);
/* cleanup */
pcap_freecode(&fp);
pcap_close(handle);
printf("\nCapture complete.\n");
return 0;
}
问题:
(struct sniff_ethernet*)(packet) // <== A char casted to a struct = a struct with valid data?
(struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip); // <== A char plus a couple ints casted to a struct = a struct with valid data?
最佳答案
如果您查看 got_packet 的定义，您将看到 const u_char *packet。 packet 是一个指向 char 的指针(或者一般来说，指向内存中的某个位置)。
在这两种情况下，指针都会被转换为相应的struct sniff_ethernet或struct sniff_tcp指针，在第一种情况下无需操作(它从头开始访问数据包) ，在第二种情况下通过添加一些偏移量，即以太网 header 的大小和 ip 数据包的大小。它访问数据包中的 tcp 数据。
关于有人可以解释一下这些代码片段吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7549940/