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/*
* Copyright (C) 2014-2023 Firejail Authors
*
* This file is part of firejail project
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#include "fnettrace_dns.h"
#include <sys/ioctl.h>
#include <time.h>
#include <linux/filter.h>
#include <linux/if_ether.h>
#include <sys/prctl.h>
#include <signal.h>
#define MAX_BUF_SIZE (64 * 1024)
static char last[512] = {'\0'};
// pkt - start of DNS layer
void print_dns(uint32_t ip_src, unsigned char *pkt) {
assert(pkt);
char ip[30];
sprintf(ip, "%d.%d.%d.%d", PRINT_IP(ip_src));
time_t seconds = time(NULL);
struct tm *t = localtime(&seconds);
int nxdomain = ((*(pkt + 3) & 0x03) == 0x03)? 1: 0;
// expecting a single question count
if (pkt[4] != 0 || pkt[5] != 1)
goto errout;
// check cname
unsigned char *ptr = pkt + 12;
int len = 0;
while (*ptr != 0 && len < 255) { // 255 is the maximum length of a domain name including multiple '.'
if (*ptr > 63) // the name left of a '.' is 63 length maximum
goto errout;
int delta = *ptr + 1;
*ptr = '.';
len += delta;;
ptr += delta;
}
if (*ptr != 0)
goto errout;
ptr++;
uint16_t type;
memcpy(&type, ptr, 2);
type = ntohs(type);
// filter output
char tmp[sizeof(last)];
snprintf(tmp, sizeof(last), "%02d:%02d:%02d %-15s %s (type %u)%s",
t->tm_hour, t->tm_min, t->tm_sec, ip, pkt + 12 + 1,
type, (nxdomain)? " NXDOMAIN": "");
if (strcmp(tmp, last)) {
printf("%s\n", tmp);
fflush(0);
strcpy(last, tmp);
}
return;
errout:
printf("%02d:%02d:%02d %15s Error: invalid DNS packet\n", t->tm_hour, t->tm_min, t->tm_sec, ip);
fflush(0);
}
// https://www.kernel.org/doc/html/latest/networking/filter.html
static void custom_bpf(int sock) {
struct sock_filter code[] = {
// sudo tcpdump ip and udp and src port 53 -dd
{ 0x28, 0, 0, 0x0000000c },
{ 0x15, 0, 8, 0x00000800 },
{ 0x30, 0, 0, 0x00000017 },
{ 0x15, 0, 6, 0x00000011 },
{ 0x28, 0, 0, 0x00000014 },
{ 0x45, 4, 0, 0x00001fff },
{ 0xb1, 0, 0, 0x0000000e },
{ 0x48, 0, 0, 0x0000000e },
{ 0x15, 0, 1, 0x00000035 },
{ 0x6, 0, 0, 0x00040000 },
{ 0x6, 0, 0, 0x00000000 },
};
struct sock_fprog bpf = {
.len = (unsigned short) sizeof(code) / sizeof(code[0]),
.filter = code,
};
int rv = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
if (rv < 0) {
fprintf(stderr, "Error: cannot attach BPF filter\n");
exit(1);
}
}
static void print_date(void) {
static int day = -1;
time_t now = time(NULL);
struct tm *t = localtime(&now);
if (day != t->tm_yday) {
printf("\nDNS trace for %s", ctime(&now));
day = t->tm_yday;
}
fflush(0);
}
static void run_trace(void) {
// grab all Ethernet packets and use a custom BPF filter to get only UDP from source port 53
int s = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
if (s < 0)
errExit("socket");
custom_bpf(s);
struct timeval tv;
tv.tv_sec = 10;
tv.tv_usec = 0;
unsigned char buf[MAX_BUF_SIZE];
while (1) {
fd_set rfds;
FD_ZERO(&rfds);
FD_SET(s, &rfds);
int rv = select(s + 1, &rfds, NULL, NULL, &tv);
if (rv < 0)
errExit("select");
else if (rv == 0) {
print_date();
tv.tv_sec = 10;
tv.tv_usec = 0;
continue;
}
unsigned bytes = recvfrom(s, buf, MAX_BUF_SIZE, 0, NULL, NULL);
if (bytes >= (14 + 20 + 8)) { // size of MAC + IP + UDP headers
uint8_t ip_hlen = (buf[14] & 0x0f) * 4;
uint16_t port_src;
memcpy(&port_src, buf + 14 + ip_hlen, 2);
port_src = ntohs(port_src);
uint8_t protocol = buf[14 + 9];
uint32_t ip_src;
memcpy(&ip_src, buf + 14 + 12, 4);
ip_src = ntohl(ip_src);
// if DNS packet, extract the query
if (port_src == 53 && protocol == 0x11) // UDP protocol
print_dns(ip_src, buf + 14 + ip_hlen + 8); // IP and UDP header len
}
}
close(s);
}
static void usage(void) {
printf("Usage: fnettrace-dns [OPTIONS]\n");
printf("Options:\n");
printf(" --help, -? - this help screen\n");
printf("\n");
}
int main(int argc, char **argv) {
int i;
for (i = 1; i < argc; i++) {
if (strcmp(argv[i], "--help") == 0 || strcmp(argv[i], "-?") == 0) {
usage();
return 0;
}
else {
fprintf(stderr, "Error: invalid argument\n");
return 1;
}
}
if (getuid() != 0) {
fprintf(stderr, "Error: you need to be root to run this program\n");
return 1;
}
// kill the process if the parent died
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
print_date();
run_trace();
return 0;
}
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