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 (void) execv (file, cmd);
177 /* can only get here on error */
179 "exec `%s' failed: %s\n",
184 /* keep running waitpid as long as the only error we get is 'EINTR' */
185 while ( (-1 == (ret = waitpid (pid, &status, 0))) &&
190 "waitpid failed: %s\n",
194 if (! (WIFEXITED (status) && (0 == WEXITSTATUS (status))))
196 /* child process completed and returned success, we're happy */
202 * Creates a tun-interface called dev;
204 * @param dev is asumed to point to a char[IFNAMSIZ]
205 * if *dev == '\\0', uses the name supplied by the kernel;
206 * @return the fd to the tun or -1 on error
220 if (-1 == (fd = open ("/dev/net/tun", O_RDWR)))
222 fprintf (stderr, "Error opening `%s': %s\n", "/dev/net/tun",
227 if (fd >= FD_SETSIZE)
229 fprintf (stderr, "File descriptor to large: %d", fd);
233 memset (&ifr, 0, sizeof (ifr));
234 ifr.ifr_flags = IFF_TUN;
237 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
239 if (-1 == ioctl (fd, TUNSETIFF, (void *) &ifr))
241 fprintf (stderr, "Error with ioctl on `%s': %s\n", "/dev/net/tun",
246 strcpy (dev, ifr.ifr_name);
252 * @brief Sets the IPv6-Address given in address on the interface dev
254 * @param dev the interface to configure
255 * @param address the IPv6-Address
256 * @param prefix_len the length of the network-prefix
259 set_address6 (const char *dev, const char *address, unsigned long prefix_len)
262 struct in6_ifreq ifr6;
263 struct sockaddr_in6 sa6;
267 * parse the new address
269 memset (&sa6, 0, sizeof (struct sockaddr_in6));
270 sa6.sin6_family = AF_INET6;
271 if (1 != inet_pton (AF_INET6, address, sa6.sin6_addr.s6_addr))
273 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
278 if (-1 == (fd = socket (PF_INET6, SOCK_DGRAM, 0)))
280 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
284 memset (&ifr, 0, sizeof (struct ifreq));
286 * Get the index of the if
288 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
289 if (-1 == ioctl (fd, SIOGIFINDEX, &ifr))
291 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
296 memset (&ifr6, 0, sizeof (struct in6_ifreq));
297 ifr6.ifr6_addr = sa6.sin6_addr;
298 ifr6.ifr6_ifindex = ifr.ifr_ifindex;
299 ifr6.ifr6_prefixlen = prefix_len;
304 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr6))
306 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
315 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
317 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
324 * Add the UP and RUNNING flags
326 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
327 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
329 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
337 fprintf (stderr, "close failed: %s\n", strerror (errno));
344 * @brief Sets the IPv4-Address given in address on the interface dev
346 * @param dev the interface to configure
347 * @param address the IPv4-Address
348 * @param mask the netmask
351 set_address4 (const char *dev, const char *address, const char *mask)
354 struct sockaddr_in *addr;
357 memset (&ifr, 0, sizeof (struct ifreq));
358 addr = (struct sockaddr_in *) &(ifr.ifr_addr);
359 addr->sin_family = AF_INET;
364 if (1 != inet_pton (AF_INET, address, &addr->sin_addr.s_addr))
366 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
371 if (-1 == (fd = socket (PF_INET, SOCK_DGRAM, 0)))
373 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
377 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
382 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr))
384 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
392 addr = (struct sockaddr_in *) &(ifr.ifr_netmask);
393 if (1 != inet_pton (AF_INET, mask, &addr->sin_addr.s_addr))
395 fprintf (stderr, "Failed to parse address `%s': %s\n", mask,
404 if (-1 == ioctl (fd, SIOCSIFNETMASK, &ifr))
406 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
415 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
417 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
424 * Add the UP and RUNNING flags
426 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
427 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
429 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
437 fprintf (stderr, "close failed: %s\n", strerror (errno));
445 * Start forwarding to and from the tunnel. This function runs with
446 * "reduced" priviledges (saved UID is still 0, but effective UID is
449 * @param fd_tun tunnel FD
455 * The buffer filled by reading from fd_tun
457 unsigned char buftun[MAX_SIZE];
458 ssize_t buftun_size = 0;
459 unsigned char *buftun_read = NULL;
462 * The buffer filled by reading from stdin
464 unsigned char bufin[MAX_SIZE];
465 ssize_t bufin_size = 0;
466 size_t bufin_rpos = 0;
467 unsigned char *bufin_read = NULL;
478 * We are supposed to read and the buffer is empty
479 * -> select on read from tun
481 if (0 == buftun_size)
482 FD_SET (fd_tun, &fds_r);
485 * We are supposed to read and the buffer is not empty
486 * -> select on write to stdout
488 if (0 != buftun_size)
492 * We are supposed to write and the buffer is empty
493 * -> select on read from stdin
495 if (NULL == bufin_read)
499 * We are supposed to write and the buffer is not empty
500 * -> select on write to tun
502 if (NULL != bufin_read)
503 FD_SET (fd_tun, &fds_w);
505 FD_SET (cpipe[0], &fds_r);
506 max = (fd_tun > cpipe[0]) ? fd_tun : cpipe[0];
508 int r = select (max + 1, &fds_r, &fds_w, NULL, NULL);
514 fprintf (stderr, "select failed: %s\n", strerror (errno));
520 if (FD_ISSET (cpipe[0], &fds_r))
521 return; /* aborted by signal */
523 if (FD_ISSET (fd_tun, &fds_r))
526 read (fd_tun, buftun + sizeof (struct GNUNET_MessageHeader),
527 MAX_SIZE - sizeof (struct GNUNET_MessageHeader));
528 if (-1 == buftun_size)
530 if ( (errno == EINTR) ||
533 fprintf (stderr, "read-error: %s\n", strerror (errno));
536 if (0 == buftun_size)
538 fprintf (stderr, "EOF on tun\n");
541 buftun_read = buftun;
543 struct GNUNET_MessageHeader *hdr =
544 (struct GNUNET_MessageHeader *) buftun;
545 buftun_size += sizeof (struct GNUNET_MessageHeader);
546 hdr->type = htons (GNUNET_MESSAGE_TYPE_DNS_HELPER);
547 hdr->size = htons (buftun_size);
550 else if (FD_ISSET (1, &fds_w))
552 ssize_t written = write (1, buftun_read, buftun_size);
556 if ( (errno == EINTR) ||
559 fprintf (stderr, "write-error to stdout: %s\n", strerror (errno));
564 fprintf (stderr, "write returned 0\n");
567 buftun_size -= written;
568 buftun_read += written;
571 if (FD_ISSET (0, &fds_r))
573 bufin_size = read (0, bufin + bufin_rpos, MAX_SIZE - bufin_rpos);
574 if (-1 == bufin_size)
576 if ( (errno == EINTR) ||
579 fprintf (stderr, "read-error: %s\n", strerror (errno));
584 fprintf (stderr, "EOF on stdin\n");
588 struct GNUNET_MessageHeader *hdr;
591 bufin_rpos += bufin_size;
592 if (bufin_rpos < sizeof (struct GNUNET_MessageHeader))
594 hdr = (struct GNUNET_MessageHeader *) bufin;
595 if (ntohs (hdr->type) != GNUNET_MESSAGE_TYPE_DNS_HELPER)
597 fprintf (stderr, "protocol violation!\n");
600 if (ntohs (hdr->size) > bufin_rpos)
602 bufin_read = bufin + sizeof (struct GNUNET_MessageHeader);
603 bufin_size = ntohs (hdr->size) - sizeof (struct GNUNET_MessageHeader);
604 bufin_rpos -= bufin_size + sizeof (struct GNUNET_MessageHeader);
607 else if (FD_ISSET (fd_tun, &fds_w))
609 ssize_t written = write (fd_tun, bufin_read, bufin_size);
613 if ( (errno == EINTR) ||
616 fprintf (stderr, "write-error to tun: %s\n", strerror (errno));
621 fprintf (stderr, "write returned 0\n");
625 bufin_size -= written;
626 bufin_read += written;
629 memmove (bufin, bufin_read, bufin_rpos);
630 bufin_read = NULL; /* start reading again */
642 * Main function of "gnunet-helper-dns", which opens a VPN tunnel interface,
643 * redirects all outgoing DNS traffic (except from the specified port) to that
644 * interface and then passes traffic from and to the interface via stdin/stdout.
646 * Once stdin/stdout close or have other errors, the tunnel is closed and the
647 * DNS traffic redirection is stopped.
649 * @param argc number of arguments
650 * @param argv 0: binary name (should be "gnunet-helper-vpn")
651 * 1: tunnel interface name (typically "gnunet-dns")
652 * 2: IPv6 address for the tunnel ("FE80::1")
653 * 3: IPv6 netmask length in bits ("64")
654 * 4: IPv4 address for the tunnel ("1.2.3.4")
655 * 5: IPv4 netmask ("255.255.0.0")
656 * 6: PORT to not hijack ("55533")
657 * @return 0 on success, otherwise code indicating type of error:
658 * 1 wrong number of arguments
659 * 2 invalid arguments (i.e. port number / prefix length wrong)
660 * 3 iptables not executable
661 * 4 ip not executable
662 * 5 failed to initialize tunnel interface
663 * 6 failed to initialize control pipe
664 * 8 failed to change routing table, cleanup successful
665 * 9-23 failed to change routing table and failed to undo some changes to routing table
666 * 24 failed to drop privs
667 * 25-39 failed to drop privs and then failed to undo some changes to routing table
668 * 40 failed to regain privs
669 * 41-55 failed to regain prisv and then failed to undo some changes to routing table
670 * 255 failed to handle kill signal properly
673 main (int argc, char *const*argv)
683 fprintf (stderr, "Fatal: must supply 6 arguments!\n");
687 /* verify that the binaries were care about are executable */
688 if (0 == access ("/sbin/iptables", X_OK))
689 sbin_iptables = "/sbin/iptables";
690 else if (0 == access ("/usr/sbin/iptables", X_OK))
691 sbin_iptables = "/usr/sbin/iptables";
695 "Fatal: executable iptables not found in approved directories: %s\n",
699 if (0 == access ("/sbin/ip", X_OK))
700 sbin_ip = "/sbin/ip";
701 else if (0 == access ("/usr/sbin/ip", X_OK))
702 sbin_ip = "/usr/sbin/ip";
706 "Fatal: executable ip not found in approved directories: %s\n",
711 /* validate port number */
712 port = atoi (argv[6]);
713 if ( (port == 0) || (port >= 65536) )
716 "Port `%u' is invalid\n",
720 /* print port number to string for command-line use*/
721 (void) snprintf (localport,
726 /* do not die on SIGPIPE */
727 if (SIG_ERR == signal (SIGPIPE, SIG_IGN))
729 fprintf (stderr, "Failed to protect against SIGPIPE: %s\n",
734 /* setup pipe to shutdown nicely on SIGINT */
735 if (0 != pipe (cpipe))
738 "Fatal: could not setup control pipe: %s\n",
742 if (cpipe[0] >= FD_SETSIZE)
744 fprintf (stderr, "Pipe file descriptor to large: %d", cpipe[0]);
745 (void) close (cpipe[0]);
746 (void) close (cpipe[1]);
750 /* make pipe non-blocking, as we theoretically could otherwise block
751 in the signal handler */
752 int flags = fcntl (cpipe[1], F_GETFL);
755 fprintf (stderr, "Failed to read flags for pipe: %s", strerror (errno));
756 (void) close (cpipe[0]);
757 (void) close (cpipe[1]);
761 if (0 != fcntl (cpipe[1], F_SETFL, flags))
763 fprintf (stderr, "Failed to make pipe non-blocking: %s", strerror (errno));
764 (void) close (cpipe[0]);
765 (void) close (cpipe[1]);
769 if ( (SIG_ERR == signal (SIGTERM, &signal_handler)) ||
770 (SIG_ERR == signal (SIGINT, &signal_handler)) ||
771 (SIG_ERR == signal (SIGHUP, &signal_handler)) )
774 "Fatal: could not initialize signal handler: %s\n",
776 (void) close (cpipe[0]);
777 (void) close (cpipe[1]);
782 /* get interface name */
783 strncpy (dev, argv[1], IFNAMSIZ);
784 dev[IFNAMSIZ - 1] = '\0';
786 /* now open virtual interface (first part that requires root) */
787 if (-1 == (fd_tun = init_tun (dev)))
789 fprintf (stderr, "Fatal: could not initialize tun-interface\n");
790 (void) signal (SIGTERM, SIG_IGN);
791 (void) signal (SIGINT, SIG_IGN);
792 (void) signal (SIGHUP, SIG_IGN);
793 (void) close (cpipe[0]);
794 (void) close (cpipe[1]);
798 /* now set interface addresses */
800 const char *address = argv[2];
801 long prefix_len = atol (argv[3]);
803 if ((prefix_len < 1) || (prefix_len > 127))
805 fprintf (stderr, "Fatal: prefix_len out of range\n");
806 (void) signal (SIGTERM, SIG_IGN);
807 (void) signal (SIGINT, SIG_IGN);
808 (void) signal (SIGHUP, SIG_IGN);
809 (void) close (cpipe[0]);
810 (void) close (cpipe[1]);
813 set_address6 (dev, address, prefix_len);
817 const char *address = argv[4];
818 const char *mask = argv[5];
820 set_address4 (dev, address, mask);
823 /* update routing tables -- next part why we need SUID! */
824 /* Forward everything from the given local port (with destination
825 to port 53, and only for UDP) without hijacking */
826 r = 8; /* failed to fully setup routing table */
828 char *const mangle_args[] =
830 "iptables", "-t", "mangle", "-I", "OUTPUT", "1", "-p",
831 "udp", "--sport", localport, "--dport", DNS_PORT, "-j",
834 if (0 != fork_and_exec (sbin_iptables, mangle_args))
837 /* Mark all of the other DNS traffic using our mark DNS_MARK */
839 char *const mark_args[] =
841 "iptables", "-t", "mangle", "-I", "OUTPUT", "2", "-p",
842 "udp", "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK,
845 if (0 != fork_and_exec (sbin_iptables, mark_args))
846 goto cleanup_mangle_1;
848 /* Forward all marked DNS traffic to our DNS_TABLE */
850 char *const forward_args[] =
852 "ip", "rule", "add", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
854 if (0 != fork_and_exec (sbin_ip, forward_args))
857 /* Finally, add rule in our forwarding table to pass to our virtual interface */
859 char *const route_args[] =
861 "ip", "route", "add", "default", "dev", dev,
862 "table", DNS_TABLE, NULL
864 if (0 != fork_and_exec (sbin_ip, route_args))
865 goto cleanup_forward_3;
868 /* drop privs *except* for the saved UID; this is not perfect, but better
869 than doing nothing */
870 uid_t uid = getuid ();
871 #ifdef HAVE_SETRESUID
872 if (0 != setresuid (uid, uid, 0))
874 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
876 goto cleanup_route_4;
879 /* Note: no 'setuid' here as we must keep our saved UID as root */
880 if (0 != seteuid (uid))
882 fprintf (stderr, "Failed to seteuid: %s\n", strerror (errno));
884 goto cleanup_route_4;
888 r = 0; /* did fully setup routing table (if nothing else happens, we were successful!) */
890 /* now forward until we hit a problem */
893 /* now need to regain privs so we can remove the firewall rules we added! */
894 #ifdef HAVE_SETRESUID
895 if (0 != setresuid (uid, 0, 0))
897 fprintf (stderr, "Failed to setresuid back to root: %s\n", strerror (errno));
899 goto cleanup_route_4;
902 if (0 != seteuid (0))
904 fprintf (stderr, "Failed to seteuid back to root: %s\n", strerror (errno));
906 goto cleanup_route_4;
910 /* update routing tables again -- this is why we could not fully drop privs */
911 /* now undo updating of routing tables; normal exit or clean-up-on-error case */
914 char *const route_clean_args[] =
916 "ip", "route", "del", "default", "dev", dev,
917 "table", DNS_TABLE, NULL
919 if (0 != fork_and_exec (sbin_ip, route_clean_args))
924 char *const forward_clean_args[] =
926 "ip", "rule", "del", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
928 if (0 != fork_and_exec (sbin_ip, forward_clean_args))
933 char *const mark_clean_args[] =
935 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
936 "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK, NULL
938 if (0 != fork_and_exec (sbin_iptables, mark_clean_args))
943 char *const mangle_clean_args[] =
945 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
946 "--sport", localport, "--dport", DNS_PORT, "-j", "ACCEPT",
949 if (0 != fork_and_exec (sbin_iptables, mangle_clean_args))
954 /* close virtual interface */
955 (void) close (fd_tun);
956 /* remove signal handler so we can close the pipes */
957 (void) signal (SIGTERM, SIG_IGN);
958 (void) signal (SIGINT, SIG_IGN);
959 (void) signal (SIGHUP, SIG_IGN);
960 (void) close (cpipe[0]);
961 (void) close (cpipe[1]);
965 /* end of gnunet-helper-dns.c */