I have problem with my C program, everything is fine until I start capturing packets. It should log, but the handler void in pcap_loop() isn't. Also I have another problem with the input in fgets on line 98 when it gets passed into the pcap create void it will not find the interface by the name. When I hardcode the string it works.
/*
Compile and run it using GCC ->
gcc main.c -o output -L/usr/include -lpcap && ./output
*/
#include <stdio.h>
#include <pcap.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#define __NEWLINE__ "\r\r\n"
#define __SEPARATOR__ "------------------------"
/*
Print all flags in human readable string
instead of hexadecimal number
*/
char* get_interface_flags(bpf_u_int32* decimal_flags) {
char* flag_string = (char*)malloc(sizeof(char) * 30);
strcpy(flag_string, "\0");
const int flag_names[9] = {
PCAP_IF_LOOPBACK,
PCAP_IF_UP,
PCAP_IF_RUNNING,
PCAP_IF_WIRELESS
};
const char* flag_values[9] = {
"LOOPBACK",
"UP",
"RUNNING",
"WIRELESS"
};
for (size_t i = 0; i < (sizeof(flag_names) / sizeof(flag_names[0])); i )
{
if(flag_names[i] & *decimal_flags) {
if(strlen(flag_string) != 0) strcat(flag_string, ", ");
strcat(flag_string, flag_values[i]);
}
}
return flag_string;
}
/*
Print available information about network interface
This recursive function can be called again
if there is more than one interface present
*/
void print_interface(pcap_if_t* device) {
// Device parameters
bpf_u_int32* flags = &device->flags;
char* device_name = device->name;
char* device_desc = device->description;
char* flags_string = get_interface_flags(flags);
// Device addresses
pcap_addr_t* addr = device->addresses;
// We'll print devices with only 1 flag or more to avoid interfaces that aren't up
if(strlen(flags_string) != 0) {
fprintf(stdout, "%s: FLAGS(%lu)<%s>%s", device_name, strlen(flags_string), flags_string, __NEWLINE__);
fprintf(stdout, "%s%s", device_desc, __NEWLINE__);
fprintf(stdout, "%s%s", __SEPARATOR__, __NEWLINE__);
}
// Check if there is another device to print, if so call the recursive function again
if(device[0].next != NULL) print_interface(device->next);
free(flags_string);
}
// Let user select the network interface that will capture packets
char* get_selected_interface() {
char* selected_interface = (char*)malloc(sizeof(char) * 20);
char* device_name;
char* ip_addr;
char errbuf[PCAP_ERRBUF_SIZE];
pcap_if_t* alldevs;
// Get all available network interfaces, returns PCAP_ERROR on failure or 0 on success
int devices_succefully_returned = pcap_findalldevs(&alldevs, errbuf);
// If there was an error while fetching interfaces, return empty char
if(devices_succefully_returned == PCAP_ERROR) {
printf("%s", errbuf);
return '\0';
}
print_interface(alldevs);
fprintf(stdout, "Name of the network interface to use: ");
fgets(selected_interface, 20, stdin);
return selected_interface;
}
void captured_packet(u_char *args, const struct pcap_pkthdr *hdr, const u_char *pkt) {
fprintf(stdout, "log");
}
void capture(char* device) {
char* errbuf[PCAP_ERRBUF_SIZE];
/*
Will create a packet capture handle
Returns NULL if handle can't be created, in this case
we'll print out the error buffer and exit the program with 1 status code
*/
pcap_t* created = pcap_create("eth0", *errbuf);
if(created == NULL) {
fprintf(stderr, "%s%s", *errbuf, __NEWLINE__);
return exit(EXIT_FAILURE);
};
int activation_status = pcap_activate(created);
if(activation_status == PCAP_ERROR || activation_status != 0) {
fprintf(stderr, "%s", pcap_geterr(created));
exit(EXIT_FAILURE);
}
/*
Interface was activated so start capturing incoming packets
capture_packet will handle incoming packets.
We'll capture infinite amount of packets unless user provides a count
*/
fprintf(stdout, "Interface activated!%s", __NEWLINE__);
int loop_status = pcap_loop(created, -1, captured_packet, NULL);
switch (loop_status)
{
case PCAP_ERROR_BREAK:
fprintf(stderr, "Loop was finished beacause of breakloop that was called.");
break;
case PCAP_ERROR_NOT_ACTIVATED:
fprintf(stderr, "Device wasn't activated before it started capturing.");
break;
case 0:
fprintf(stderr, "Loop was terminated due to exhaustion of count");
break;
default:
fprintf(stderr, "Some error happened while trying to loop through packets -> %s", pcap_geterr(created));
break;
}
// Program was successfully closed, exit..
pcap_close(created);
exit(0);
}
int main(int argc, char **argv) {
if(getuid() != 0) {
fprintf(stderr, "Please make sure to run this tool as root!%s", __NEWLINE__);
return 1;
}
char* selected = get_selected_interface();
capture(selected);
free(selected);
return 0;
}
I tried everything, it should log unlimited number of packets on the "eth0" interface but nothing happens.
CodePudding user response：
I have problem with my C program, everything is fine until I start capturing packets. It should log, but the handler void in pcap_loop() isn't.
To quote the pcap(3PCAP) man page:
packet buffer timeout
If, when capturing, packets are delivered as soon as they
arrive, the application capturing the packets will be woken up
for each packet as it arrives, and might have to make one or
more calls to the operating system to fetch each packet.
If, instead, packets are not delivered as soon as they arrive,
but are delivered after a short delay (called a "packet buffer
timeout"), more than one packet can be accumulated before the
packets are delivered, so that a single wakeup would be done for
multiple packets, and each set of calls made to the operating
system would supply multiple packets, rather than a single
packet. This reduces the per-packet CPU overhead if packets are
arriving at a high rate, increasing the number of packets per
second that can be captured.
The packet buffer timeout is required so that an application
won't wait for the operating system's capture buffer to fill up
before packets are delivered; if packets are arriving slowly,
that wait could take an arbitrarily long period of time.
Not all platforms support a packet buffer timeout; on platforms
that don't, the packet buffer timeout is ignored. A zero value
for the timeout, on platforms that support a packet buffer time-
out, will cause a read to wait forever to allow enough packets
to arrive, with no timeout. A negative value is invalid; the
result of setting the timeout to a negative value is unpre-
dictable.
NOTE: the packet buffer timeout cannot be used to cause calls
that read packets to return within a limited period of time,
because, on some platforms, the packet buffer timeout isn't sup-
ported, and, on other platforms, the timer doesn't start until
at least one packet arrives. This means that the packet buffer
timeout should NOT be used, for example, in an interactive
application to allow the packet capture loop to ``poll'' for
user input periodically, as there's no guarantee that a call
reading packets will return after the timeout expires even if no
packets have arrived.
The packet buffer timeout is set with pcap_set_timeout().
and the pcap_set_timeout(3PCAP) man page:
pcap_set_timeout() sets the packet buffer timeout that will be used on
a capture handle when the handle is activated to to_ms, which is in
units of milliseconds. (See pcap(3PCAP) for an explanation of the
packet buffer timeout.)
The behavior, if the timeout isn't specified, is undefined, as is the
behavior if the timeout is set to zero or to a negative value. We rec-
ommend always setting the timeout to a non-zero value unless immediate
mode is set, in which case the timeout has no effect.
You haven't called pcap_set_timeout(); on some platforms, that can cause pcap_loop() to wait until it gets an entire buffer full of packet, which could take a significant amount of time.
Set the timeout to 100, which means 100 milliseconds or a tenth of a second.
Also I have another problem with the input in fgets on line 98 when it gets passed into the pcap create void it will not find the interface by the name. When I hardcode the string it works.
To quote the fgets(3) man page (this is on macOS, and you're probably working on Linux, given the "eth0" name, but all of the stuff I'm quoting from the man pages applies to macOS, Linux, and other UN*Xes):
The fgets() function reads at most one less than the number of characters
specified by size from the given stream and stores them in the string
str. Reading stops when a newline character is found, at end-of-file or
error. The newline, if any, is retained. If any characters are read and
there is no error, a `\0' character is appended to end the string.
The important part here is "The newline, if any, is retained." - this means that, if the user types "eth0" and then hits the Return key, the string read in will be "eth0\n", complete with a newline character. The name of the interface is "eth0", not "eth0\n"; you will have to remove any trailing newline from the string before using it.