2 This file is part of GNUnet.
3 (C) 2010, 2011, 2012 Christian Grothoff
5 GNUnet is free software; you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published
7 by the Free Software Foundation; either version 3, or (at your
8 option) any later version.
10 GNUnet is distributed in the hope that it will be useful, but
11 WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with GNUnet; see the file COPYING. If not, write to the
17 Free Software Foundation, Inc., 59 Temple Place - Suite 330,
18 Boston, MA 02111-1307, USA.
22 * @file dns/gnunet-helper-dns.c
23 * @brief helper to install firewall rules to hijack all DNS traffic
24 * and send it to our virtual interface (except for DNS traffic
25 * that originates on the specified port). We then
26 * allow interacting with our virtual interface via stdin/stdout.
27 * @author Philipp Tölke
28 * @author Christian Grothoff
30 * This program alters the Linux firewall rules so that DNS traffic
31 * that ordinarily exits the system can be intercepted and managed by
32 * a virtual interface. In order to achieve this, DNS traffic is
33 * marked with the DNS_MARK given in below and re-routed to a custom
34 * table with the DNS_TABLE ID given below. Systems and
35 * administrators must take care to not cause conflicts with these
36 * values (it was deemed safest to hardcode them as passing these
37 * values as arguments might permit messing with arbitrary firewall
38 * rules, which would be dangerous).
40 * The code first sets up the virtual interface, then begins to
41 * redirect the DNS traffic to it, and then on errors or SIGTERM shuts
42 * down the virtual interface and removes the rules for the traffic
46 * Note that having this binary SUID is only partially safe: it will
47 * allow redirecting (and intercepting / mangling) of all DNS traffic
48 * originating from this system by any user who is able to run it.
49 * Furthermore, this code will make it trivial to DoS all DNS traffic
50 * originating from the current system, simply by sending it to
51 * nowhere (redirect stdout to /dev/null).
53 * Naturally, neither of these problems can be helped as this is the
54 * fundamental purpose of the binary. Certifying that this code is
55 * "safe" thus only means that it doesn't allow anything else (such
56 * as local priv. escalation, etc.).
58 * The following list of people have reviewed this code and considered
59 * it safe (within specifications) since the last modification (if you
60 * reviewed it, please have your name added to the list):
62 * - Christian Grothoff
66 #include <linux/if_tun.h>
69 * Need 'struct GNUNET_MessageHeader'.
71 #include "gnunet_common.h"
74 * Need DNS message types.
76 #include "gnunet_protocols.h"
79 * Maximum size of a GNUnet message (GNUNET_SERVER_MAX_MESSAGE_SIZE)
81 #define MAX_SIZE 65536
85 * This is in linux/include/net/ipv6.h, but not always exported...
89 struct in6_addr ifr6_addr;
90 uint32_t ifr6_prefixlen;
91 unsigned int ifr6_ifindex;
96 * Name and full path of IPTABLES binary.
98 static const char *sbin_iptables;
101 * Name and full path of IPTABLES binary.
103 static const char *sbin_ip;
106 * Port for DNS traffic.
108 #define DNS_PORT "53"
111 * Marker we set for our hijacked DNS traffic. We use GNUnet's
112 * port (2086) plus the DNS port (53) in HEX to make a 32-bit mark
113 * (which is hopefully long enough to not collide); so
114 * 0x08260035 = 136708149 (hopefully unique enough...).
116 #define DNS_MARK "136708149"
119 * Table we use for our DNS rules. 0-255 is the range and
120 * 0, 253, 254 and 255 are already reserved. As this is about
121 * DNS and as "53" is likely (fingers crossed!) high enough to
122 * not usually conflict with a normal user's setup, we use 53
123 * to give a hint that this has something to do with DNS.
125 #define DNS_TABLE "53"
129 * Control pipe for shutdown via signal. [0] is the read end,
130 * [1] is the write end.
136 * Signal handler called to initiate "nice" shutdown. Signals select
137 * loop via non-bocking pipe 'cpipe'.
139 * @param signal signal number of the signal (not used)
142 signal_handler (int signal)
144 /* ignore return value, as the signal handler could theoretically
145 be called many times before the shutdown can actually happen */
146 (void) write (cpipe[1], "K", 1);
151 * Run the given command and wait for it to complete.
153 * @param file name of the binary to run
154 * @param cmd command line arguments (as given to 'execv')
155 * @return 0 on success, 1 on any error
158 fork_and_exec (const char *file,
175 /* we are the child process */
176 /* close stdin/stdout to not cause interference
177 with the helper's main protocol! */
180 (void) execv (file, cmd);
181 /* can only get here on error */
183 "exec `%s' failed: %s\n",
188 /* keep running waitpid as long as the only error we get is 'EINTR' */
189 while ( (-1 == (ret = waitpid (pid, &status, 0))) &&
194 "waitpid failed: %s\n",
198 if (! (WIFEXITED (status) && (0 == WEXITSTATUS (status))))
200 /* child process completed and returned success, we're happy */
206 * Creates a tun-interface called dev;
208 * @param dev is asumed to point to a char[IFNAMSIZ]
209 * if *dev == '\\0', uses the name supplied by the kernel;
210 * @return the fd to the tun or -1 on error
224 if (-1 == (fd = open ("/dev/net/tun", O_RDWR)))
226 fprintf (stderr, "Error opening `%s': %s\n", "/dev/net/tun",
231 if (fd >= FD_SETSIZE)
233 fprintf (stderr, "File descriptor to large: %d", fd);
237 memset (&ifr, 0, sizeof (ifr));
238 ifr.ifr_flags = IFF_TUN;
241 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
243 if (-1 == ioctl (fd, TUNSETIFF, (void *) &ifr))
245 fprintf (stderr, "Error with ioctl on `%s': %s\n", "/dev/net/tun",
250 strcpy (dev, ifr.ifr_name);
256 * @brief Sets the IPv6-Address given in address on the interface dev
258 * @param dev the interface to configure
259 * @param address the IPv6-Address
260 * @param prefix_len the length of the network-prefix
263 set_address6 (const char *dev, const char *address, unsigned long prefix_len)
266 struct in6_ifreq ifr6;
267 struct sockaddr_in6 sa6;
271 * parse the new address
273 memset (&sa6, 0, sizeof (struct sockaddr_in6));
274 sa6.sin6_family = AF_INET6;
275 if (1 != inet_pton (AF_INET6, address, sa6.sin6_addr.s6_addr))
277 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
282 if (-1 == (fd = socket (PF_INET6, SOCK_DGRAM, 0)))
284 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
288 memset (&ifr, 0, sizeof (struct ifreq));
290 * Get the index of the if
292 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
293 if (-1 == ioctl (fd, SIOGIFINDEX, &ifr))
295 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
300 memset (&ifr6, 0, sizeof (struct in6_ifreq));
301 ifr6.ifr6_addr = sa6.sin6_addr;
302 ifr6.ifr6_ifindex = ifr.ifr_ifindex;
303 ifr6.ifr6_prefixlen = prefix_len;
308 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr6))
310 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
319 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
321 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
328 * Add the UP and RUNNING flags
330 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
331 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
333 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
341 fprintf (stderr, "close failed: %s\n", strerror (errno));
348 * @brief Sets the IPv4-Address given in address on the interface dev
350 * @param dev the interface to configure
351 * @param address the IPv4-Address
352 * @param mask the netmask
355 set_address4 (const char *dev, const char *address, const char *mask)
358 struct sockaddr_in *addr;
361 memset (&ifr, 0, sizeof (struct ifreq));
362 addr = (struct sockaddr_in *) &(ifr.ifr_addr);
363 addr->sin_family = AF_INET;
368 if (1 != inet_pton (AF_INET, address, &addr->sin_addr.s_addr))
370 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
375 if (-1 == (fd = socket (PF_INET, SOCK_DGRAM, 0)))
377 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
381 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
386 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr))
388 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
396 addr = (struct sockaddr_in *) &(ifr.ifr_netmask);
397 if (1 != inet_pton (AF_INET, mask, &addr->sin_addr.s_addr))
399 fprintf (stderr, "Failed to parse address `%s': %s\n", mask,
408 if (-1 == ioctl (fd, SIOCSIFNETMASK, &ifr))
410 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
419 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
421 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
428 * Add the UP and RUNNING flags
430 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
431 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
433 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
441 fprintf (stderr, "close failed: %s\n", strerror (errno));
449 * Start forwarding to and from the tunnel. This function runs with
450 * "reduced" priviledges (saved UID is still 0, but effective UID is
453 * @param fd_tun tunnel FD
459 * The buffer filled by reading from fd_tun
461 unsigned char buftun[MAX_SIZE];
462 ssize_t buftun_size = 0;
463 unsigned char *buftun_read = NULL;
466 * The buffer filled by reading from stdin
468 unsigned char bufin[MAX_SIZE];
469 ssize_t bufin_size = 0;
470 size_t bufin_rpos = 0;
471 unsigned char *bufin_read;
483 * We are supposed to read and the buffer is empty
484 * -> select on read from tun
486 if (0 == buftun_size)
487 FD_SET (fd_tun, &fds_r);
490 * We are supposed to read and the buffer is not empty
491 * -> select on write to stdout
493 if (0 != buftun_size)
497 * We are supposed to write and the buffer is empty
498 * -> select on read from stdin
500 if (NULL == bufin_read)
504 * We are supposed to write and the buffer is not empty
505 * -> select on write to tun
507 if (NULL != bufin_read)
508 FD_SET (fd_tun, &fds_w);
510 FD_SET (cpipe[0], &fds_r);
511 max = (fd_tun > cpipe[0]) ? fd_tun : cpipe[0];
513 int r = select (max + 1, &fds_r, &fds_w, NULL, NULL);
519 fprintf (stderr, "select failed: %s\n", strerror (errno));
525 if (FD_ISSET (cpipe[0], &fds_r))
526 return; /* aborted by signal */
528 if (FD_ISSET (fd_tun, &fds_r))
531 read (fd_tun, buftun + sizeof (struct GNUNET_MessageHeader),
532 MAX_SIZE - sizeof (struct GNUNET_MessageHeader));
533 if (-1 == buftun_size)
535 if ( (errno == EINTR) ||
538 fprintf (stderr, "read-error: %s\n", strerror (errno));
541 if (0 == buftun_size)
543 fprintf (stderr, "EOF on tun\n");
546 buftun_read = buftun;
548 struct GNUNET_MessageHeader *hdr =
549 (struct GNUNET_MessageHeader *) buftun;
550 buftun_size += sizeof (struct GNUNET_MessageHeader);
551 hdr->type = htons (GNUNET_MESSAGE_TYPE_DNS_HELPER);
552 hdr->size = htons (buftun_size);
555 else if (FD_ISSET (1, &fds_w))
557 ssize_t written = write (1, buftun_read, buftun_size);
561 if ( (errno == EINTR) ||
564 fprintf (stderr, "write-error to stdout: %s\n", strerror (errno));
569 fprintf (stderr, "write returned 0\n");
572 buftun_size -= written;
573 buftun_read += written;
576 if (FD_ISSET (0, &fds_r))
578 bufin_size = read (0, bufin + bufin_rpos, MAX_SIZE - bufin_rpos);
579 if (-1 == bufin_size)
581 if ( (errno == EINTR) ||
584 fprintf (stderr, "read-error: %s\n", strerror (errno));
589 fprintf (stderr, "EOF on stdin\n");
593 struct GNUNET_MessageHeader *hdr;
596 bufin_rpos += bufin_size;
597 if (bufin_rpos < sizeof (struct GNUNET_MessageHeader))
599 hdr = (struct GNUNET_MessageHeader *) bufin;
600 if (ntohs (hdr->type) != GNUNET_MESSAGE_TYPE_DNS_HELPER)
602 fprintf (stderr, "protocol violation!\n");
605 if (ntohs (hdr->size) > bufin_rpos)
607 bufin_read = bufin + sizeof (struct GNUNET_MessageHeader);
608 bufin_size = ntohs (hdr->size) - sizeof (struct GNUNET_MessageHeader);
609 bufin_rpos -= bufin_size + sizeof (struct GNUNET_MessageHeader);
612 else if (FD_ISSET (fd_tun, &fds_w))
614 ssize_t written = write (fd_tun, bufin_read, bufin_size);
618 if ( (errno == EINTR) ||
621 fprintf (stderr, "write-error to tun: %s\n", strerror (errno));
626 fprintf (stderr, "write returned 0\n");
630 bufin_size -= written;
631 bufin_read += written;
634 memmove (bufin, bufin_read, bufin_rpos);
635 bufin_read = NULL; /* start reading again */
647 * Main function of "gnunet-helper-dns", which opens a VPN tunnel interface,
648 * redirects all outgoing DNS traffic (except from the specified port) to that
649 * interface and then passes traffic from and to the interface via stdin/stdout.
651 * Once stdin/stdout close or have other errors, the tunnel is closed and the
652 * DNS traffic redirection is stopped.
654 * @param argc number of arguments
655 * @param argv 0: binary name (should be "gnunet-helper-vpn")
656 * 1: tunnel interface name (typically "gnunet-dns")
657 * 2: IPv6 address for the tunnel ("FE80::1")
658 * 3: IPv6 netmask length in bits ("64")
659 * 4: IPv4 address for the tunnel ("1.2.3.4")
660 * 5: IPv4 netmask ("255.255.0.0")
661 * 6: PORT to not hijack ("55533")
662 * @return 0 on success, otherwise code indicating type of error:
663 * 1 wrong number of arguments
664 * 2 invalid arguments (i.e. port number / prefix length wrong)
665 * 3 iptables not executable
666 * 4 ip not executable
667 * 5 failed to initialize tunnel interface
668 * 6 failed to initialize control pipe
669 * 8 failed to change routing table, cleanup successful
670 * 9-23 failed to change routing table and failed to undo some changes to routing table
671 * 24 failed to drop privs
672 * 25-39 failed to drop privs and then failed to undo some changes to routing table
673 * 40 failed to regain privs
674 * 41-55 failed to regain prisv and then failed to undo some changes to routing table
675 * 255 failed to handle kill signal properly
678 main (int argc, char *const*argv)
688 fprintf (stderr, "Fatal: must supply 6 arguments!\n");
692 /* verify that the binaries were care about are executable */
693 if (0 == access ("/sbin/iptables", X_OK))
694 sbin_iptables = "/sbin/iptables";
695 else if (0 == access ("/usr/sbin/iptables", X_OK))
696 sbin_iptables = "/usr/sbin/iptables";
700 "Fatal: executable iptables not found in approved directories: %s\n",
704 if (0 == access ("/sbin/ip", X_OK))
705 sbin_ip = "/sbin/ip";
706 else if (0 == access ("/usr/sbin/ip", X_OK))
707 sbin_ip = "/usr/sbin/ip";
711 "Fatal: executable ip not found in approved directories: %s\n",
716 /* validate port number */
717 port = atoi (argv[6]);
718 if ( (port == 0) || (port >= 65536) )
721 "Port `%u' is invalid\n",
725 /* print port number to string for command-line use*/
726 (void) snprintf (localport,
731 /* do not die on SIGPIPE */
732 if (SIG_ERR == signal (SIGPIPE, SIG_IGN))
734 fprintf (stderr, "Failed to protect against SIGPIPE: %s\n",
739 /* setup pipe to shutdown nicely on SIGINT */
740 if (0 != pipe (cpipe))
743 "Fatal: could not setup control pipe: %s\n",
747 if (cpipe[0] >= FD_SETSIZE)
749 fprintf (stderr, "Pipe file descriptor to large: %d", cpipe[0]);
750 (void) close (cpipe[0]);
751 (void) close (cpipe[1]);
755 /* make pipe non-blocking, as we theoretically could otherwise block
756 in the signal handler */
757 int flags = fcntl (cpipe[1], F_GETFL);
760 fprintf (stderr, "Failed to read flags for pipe: %s", strerror (errno));
761 (void) close (cpipe[0]);
762 (void) close (cpipe[1]);
766 if (0 != fcntl (cpipe[1], F_SETFL, flags))
768 fprintf (stderr, "Failed to make pipe non-blocking: %s", strerror (errno));
769 (void) close (cpipe[0]);
770 (void) close (cpipe[1]);
774 if ( (SIG_ERR == signal (SIGTERM, &signal_handler)) ||
775 (SIG_ERR == signal (SIGINT, &signal_handler)) ||
776 (SIG_ERR == signal (SIGHUP, &signal_handler)) )
779 "Fatal: could not initialize signal handler: %s\n",
781 (void) close (cpipe[0]);
782 (void) close (cpipe[1]);
787 /* get interface name */
788 strncpy (dev, argv[1], IFNAMSIZ);
789 dev[IFNAMSIZ - 1] = '\0';
791 /* now open virtual interface (first part that requires root) */
792 if (-1 == (fd_tun = init_tun (dev)))
794 fprintf (stderr, "Fatal: could not initialize tun-interface\n");
795 (void) signal (SIGTERM, SIG_IGN);
796 (void) signal (SIGINT, SIG_IGN);
797 (void) signal (SIGHUP, SIG_IGN);
798 (void) close (cpipe[0]);
799 (void) close (cpipe[1]);
803 /* now set interface addresses */
805 const char *address = argv[2];
806 long prefix_len = atol (argv[3]);
808 if ((prefix_len < 1) || (prefix_len > 127))
810 fprintf (stderr, "Fatal: prefix_len out of range\n");
811 (void) signal (SIGTERM, SIG_IGN);
812 (void) signal (SIGINT, SIG_IGN);
813 (void) signal (SIGHUP, SIG_IGN);
814 (void) close (cpipe[0]);
815 (void) close (cpipe[1]);
818 set_address6 (dev, address, prefix_len);
822 const char *address = argv[4];
823 const char *mask = argv[5];
825 set_address4 (dev, address, mask);
828 /* update routing tables -- next part why we need SUID! */
829 /* Forward everything from the given local port (with destination
830 to port 53, and only for UDP) without hijacking */
831 r = 8; /* failed to fully setup routing table */
833 char *const mangle_args[] =
835 "iptables", "-t", "mangle", "-I", "OUTPUT", "1", "-p",
836 "udp", "--sport", localport, "--dport", DNS_PORT, "-j",
839 if (0 != fork_and_exec (sbin_iptables, mangle_args))
842 /* Mark all of the other DNS traffic using our mark DNS_MARK */
844 char *const mark_args[] =
846 "iptables", "-t", "mangle", "-I", "OUTPUT", "2", "-p",
847 "udp", "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK,
850 if (0 != fork_and_exec (sbin_iptables, mark_args))
851 goto cleanup_mangle_1;
853 /* Forward all marked DNS traffic to our DNS_TABLE */
855 char *const forward_args[] =
857 "ip", "rule", "add", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
859 if (0 != fork_and_exec (sbin_ip, forward_args))
862 /* Finally, add rule in our forwarding table to pass to our virtual interface */
864 char *const route_args[] =
866 "ip", "route", "add", "default", "dev", dev,
867 "table", DNS_TABLE, NULL
869 if (0 != fork_and_exec (sbin_ip, route_args))
870 goto cleanup_forward_3;
873 /* drop privs *except* for the saved UID; this is not perfect, but better
874 than doing nothing */
875 uid_t uid = getuid ();
876 #ifdef HAVE_SETRESUID
877 if (0 != setresuid (uid, uid, 0))
879 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
881 goto cleanup_route_4;
884 /* Note: no 'setuid' here as we must keep our saved UID as root */
885 if (0 != seteuid (uid))
887 fprintf (stderr, "Failed to seteuid: %s\n", strerror (errno));
889 goto cleanup_route_4;
893 r = 0; /* did fully setup routing table (if nothing else happens, we were successful!) */
895 /* now forward until we hit a problem */
898 /* now need to regain privs so we can remove the firewall rules we added! */
899 #ifdef HAVE_SETRESUID
900 if (0 != setresuid (uid, 0, 0))
902 fprintf (stderr, "Failed to setresuid back to root: %s\n", strerror (errno));
904 goto cleanup_route_4;
907 if (0 != seteuid (0))
909 fprintf (stderr, "Failed to seteuid back to root: %s\n", strerror (errno));
911 goto cleanup_route_4;
915 /* update routing tables again -- this is why we could not fully drop privs */
916 /* now undo updating of routing tables; normal exit or clean-up-on-error case */
919 char *const route_clean_args[] =
921 "ip", "route", "del", "default", "dev", dev,
922 "table", DNS_TABLE, NULL
924 if (0 != fork_and_exec (sbin_ip, route_clean_args))
929 char *const forward_clean_args[] =
931 "ip", "rule", "del", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
933 if (0 != fork_and_exec (sbin_ip, forward_clean_args))
938 char *const mark_clean_args[] =
940 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
941 "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK, NULL
943 if (0 != fork_and_exec (sbin_iptables, mark_clean_args))
948 char *const mangle_clean_args[] =
950 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
951 "--sport", localport, "--dport", DNS_PORT, "-j", "ACCEPT",
954 if (0 != fork_and_exec (sbin_iptables, mangle_clean_args))
959 /* close virtual interface */
960 (void) close (fd_tun);
961 /* remove signal handler so we can close the pipes */
962 (void) signal (SIGTERM, SIG_IGN);
963 (void) signal (SIGINT, SIG_IGN);
964 (void) signal (SIGHUP, SIG_IGN);
965 (void) close (cpipe[0]);
966 (void) close (cpipe[1]);
970 /* end of gnunet-helper-dns.c */