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). Traffic coming from the same
39 * group ID as the effective group ID that this process is running
40 * as is not intercepted.
42 * The code first sets up the virtual interface, then begins to
43 * redirect the DNS traffic to it, and then on errors or SIGTERM shuts
44 * down the virtual interface and removes the rules for the traffic
48 * Note that having this binary SUID is only partially safe: it will
49 * allow redirecting (and intercepting / mangling) of all DNS traffic
50 * originating from this system by any user who is able to run it.
51 * Furthermore, this code will make it trivial to DoS all DNS traffic
52 * originating from the current system, simply by sending it to
53 * nowhere (redirect stdout to /dev/null).
55 * Naturally, neither of these problems can be helped as this is the
56 * fundamental purpose of the binary. Certifying that this code is
57 * "safe" thus only means that it doesn't allow anything else (such
58 * as local priv. escalation, etc.).
60 * The following list of people have reviewed this code and considered
61 * it safe (within specifications) since the last modification (if you
62 * reviewed it, please have your name added to the list):
64 * - Christian Grothoff
68 #include <linux/if_tun.h>
71 * Need 'struct GNUNET_MessageHeader'.
73 #include "gnunet_common.h"
76 * Need DNS message types.
78 #include "gnunet_protocols.h"
81 * Maximum size of a GNUnet message (GNUNET_SERVER_MAX_MESSAGE_SIZE)
83 #define MAX_SIZE 65536
87 * This is in linux/include/net/ipv6.h, but not always exported...
91 struct in6_addr ifr6_addr;
92 uint32_t ifr6_prefixlen;
93 unsigned int ifr6_ifindex;
98 * Name and full path of IPTABLES binary.
100 static const char *sbin_iptables;
103 * Name and full path of IPTABLES binary.
105 static const char *sbin_ip;
108 * Port for DNS traffic.
110 #define DNS_PORT "53"
113 * Marker we set for our hijacked DNS traffic. We use GNUnet's
114 * port (2086) plus the DNS port (53) in HEX to make a 32-bit mark
115 * (which is hopefully long enough to not collide); so
116 * 0x08260035 = 136708149 (hopefully unique enough...).
118 #define DNS_MARK "136708149"
121 * Table we use for our DNS rules. 0-255 is the range and
122 * 0, 253, 254 and 255 are already reserved. As this is about
123 * DNS and as "53" is likely (fingers crossed!) high enough to
124 * not usually conflict with a normal user's setup, we use 53
125 * to give a hint that this has something to do with DNS.
127 #define DNS_TABLE "53"
131 * Control pipe for shutdown via signal. [0] is the read end,
132 * [1] is the write end.
138 * Signal handler called to initiate "nice" shutdown. Signals select
139 * loop via non-bocking pipe 'cpipe'.
141 * @param signal signal number of the signal (not used)
144 signal_handler (int signal)
146 /* ignore return value, as the signal handler could theoretically
147 be called many times before the shutdown can actually happen */
148 (void) write (cpipe[1], "K", 1);
153 * Run the given command and wait for it to complete.
155 * @param file name of the binary to run
156 * @param cmd command line arguments (as given to 'execv')
157 * @return 0 on success, 1 on any error
160 fork_and_exec (const char *file,
177 /* we are the child process */
178 /* close stdin/stdout to not cause interference
179 with the helper's main protocol! */
182 (void) execv (file, cmd);
183 /* can only get here on error */
185 "exec `%s' failed: %s\n",
190 /* keep running waitpid as long as the only error we get is 'EINTR' */
191 while ( (-1 == (ret = waitpid (pid, &status, 0))) &&
196 "waitpid failed: %s\n",
200 if (! (WIFEXITED (status) && (0 == WEXITSTATUS (status))))
202 /* child process completed and returned success, we're happy */
208 * Creates a tun-interface called dev;
210 * @param dev is asumed to point to a char[IFNAMSIZ]
211 * if *dev == '\\0', uses the name supplied by the kernel;
212 * @return the fd to the tun or -1 on error
226 if (-1 == (fd = open ("/dev/net/tun", O_RDWR)))
228 fprintf (stderr, "Error opening `%s': %s\n", "/dev/net/tun",
233 if (fd >= FD_SETSIZE)
235 fprintf (stderr, "File descriptor to large: %d", fd);
239 memset (&ifr, 0, sizeof (ifr));
240 ifr.ifr_flags = IFF_TUN;
243 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
245 if (-1 == ioctl (fd, TUNSETIFF, (void *) &ifr))
247 fprintf (stderr, "Error with ioctl on `%s': %s\n", "/dev/net/tun",
252 strcpy (dev, ifr.ifr_name);
258 * @brief Sets the IPv6-Address given in address on the interface dev
260 * @param dev the interface to configure
261 * @param address the IPv6-Address
262 * @param prefix_len the length of the network-prefix
265 set_address6 (const char *dev, const char *address, unsigned long prefix_len)
268 struct in6_ifreq ifr6;
269 struct sockaddr_in6 sa6;
273 * parse the new address
275 memset (&sa6, 0, sizeof (struct sockaddr_in6));
276 sa6.sin6_family = AF_INET6;
277 if (1 != inet_pton (AF_INET6, address, sa6.sin6_addr.s6_addr))
279 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
284 if (-1 == (fd = socket (PF_INET6, SOCK_DGRAM, 0)))
286 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
290 memset (&ifr, 0, sizeof (struct ifreq));
292 * Get the index of the if
294 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
295 if (-1 == ioctl (fd, SIOGIFINDEX, &ifr))
297 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
302 memset (&ifr6, 0, sizeof (struct in6_ifreq));
303 ifr6.ifr6_addr = sa6.sin6_addr;
304 ifr6.ifr6_ifindex = ifr.ifr_ifindex;
305 ifr6.ifr6_prefixlen = prefix_len;
310 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr6))
312 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
321 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
323 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
330 * Add the UP and RUNNING flags
332 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
333 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
335 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
343 fprintf (stderr, "close failed: %s\n", strerror (errno));
350 * @brief Sets the IPv4-Address given in address on the interface dev
352 * @param dev the interface to configure
353 * @param address the IPv4-Address
354 * @param mask the netmask
357 set_address4 (const char *dev, const char *address, const char *mask)
360 struct sockaddr_in *addr;
363 memset (&ifr, 0, sizeof (struct ifreq));
364 addr = (struct sockaddr_in *) &(ifr.ifr_addr);
365 addr->sin_family = AF_INET;
370 if (1 != inet_pton (AF_INET, address, &addr->sin_addr.s_addr))
372 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
377 if (-1 == (fd = socket (PF_INET, SOCK_DGRAM, 0)))
379 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
383 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
388 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr))
390 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
398 addr = (struct sockaddr_in *) &(ifr.ifr_netmask);
399 if (1 != inet_pton (AF_INET, mask, &addr->sin_addr.s_addr))
401 fprintf (stderr, "Failed to parse address `%s': %s\n", mask,
410 if (-1 == ioctl (fd, SIOCSIFNETMASK, &ifr))
412 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
421 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
423 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
430 * Add the UP and RUNNING flags
432 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
433 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
435 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
443 fprintf (stderr, "close failed: %s\n", strerror (errno));
451 * Start forwarding to and from the tunnel. This function runs with
452 * "reduced" priviledges (saved UID is still 0, but effective UID is
455 * @param fd_tun tunnel FD
461 * The buffer filled by reading from fd_tun
463 unsigned char buftun[MAX_SIZE];
464 ssize_t buftun_size = 0;
465 unsigned char *buftun_read = NULL;
468 * The buffer filled by reading from stdin
470 unsigned char bufin[MAX_SIZE];
471 ssize_t bufin_size = 0;
472 size_t bufin_rpos = 0;
473 unsigned char *bufin_read = NULL;
484 * We are supposed to read and the buffer is empty
485 * -> select on read from tun
487 if (0 == buftun_size)
488 FD_SET (fd_tun, &fds_r);
491 * We are supposed to read and the buffer is not empty
492 * -> select on write to stdout
494 if (0 != buftun_size)
498 * We are supposed to write and the buffer is empty
499 * -> select on read from stdin
501 if (NULL == bufin_read)
505 * We are supposed to write and the buffer is not empty
506 * -> select on write to tun
508 if (NULL != bufin_read)
509 FD_SET (fd_tun, &fds_w);
511 FD_SET (cpipe[0], &fds_r);
512 max = (fd_tun > cpipe[0]) ? fd_tun : cpipe[0];
514 int r = select (max + 1, &fds_r, &fds_w, NULL, NULL);
520 fprintf (stderr, "select failed: %s\n", strerror (errno));
526 if (FD_ISSET (cpipe[0], &fds_r))
527 return; /* aborted by signal */
529 if (FD_ISSET (fd_tun, &fds_r))
532 read (fd_tun, buftun + sizeof (struct GNUNET_MessageHeader),
533 MAX_SIZE - sizeof (struct GNUNET_MessageHeader));
534 if (-1 == buftun_size)
536 if ( (errno == EINTR) ||
539 fprintf (stderr, "read-error: %s\n", strerror (errno));
542 if (0 == buftun_size)
544 fprintf (stderr, "EOF on tun\n");
547 buftun_read = buftun;
549 struct GNUNET_MessageHeader *hdr =
550 (struct GNUNET_MessageHeader *) buftun;
551 buftun_size += sizeof (struct GNUNET_MessageHeader);
552 hdr->type = htons (GNUNET_MESSAGE_TYPE_DNS_HELPER);
553 hdr->size = htons (buftun_size);
556 else if (FD_ISSET (1, &fds_w))
558 ssize_t written = write (1, buftun_read, buftun_size);
562 if ( (errno == EINTR) ||
565 fprintf (stderr, "write-error to stdout: %s\n", strerror (errno));
570 fprintf (stderr, "write returned 0\n");
573 buftun_size -= written;
574 buftun_read += written;
577 if (FD_ISSET (0, &fds_r))
579 bufin_size = read (0, bufin + bufin_rpos, MAX_SIZE - bufin_rpos);
580 if (-1 == bufin_size)
583 if ( (errno == EINTR) ||
586 fprintf (stderr, "read-error: %s\n", strerror (errno));
592 fprintf (stderr, "EOF on stdin\n");
596 struct GNUNET_MessageHeader *hdr;
599 bufin_rpos += bufin_size;
600 if (bufin_rpos < sizeof (struct GNUNET_MessageHeader))
602 hdr = (struct GNUNET_MessageHeader *) bufin;
603 if (ntohs (hdr->type) != GNUNET_MESSAGE_TYPE_DNS_HELPER)
605 fprintf (stderr, "protocol violation!\n");
608 if (ntohs (hdr->size) > bufin_rpos)
610 bufin_read = bufin + sizeof (struct GNUNET_MessageHeader);
611 bufin_size = ntohs (hdr->size) - sizeof (struct GNUNET_MessageHeader);
612 bufin_rpos -= bufin_size + sizeof (struct GNUNET_MessageHeader);
615 else if (FD_ISSET (fd_tun, &fds_w))
617 ssize_t written = write (fd_tun, bufin_read, bufin_size);
621 if ( (errno == EINTR) ||
624 fprintf (stderr, "write-error to tun: %s\n", strerror (errno));
629 fprintf (stderr, "write returned 0\n");
633 bufin_size -= written;
634 bufin_read += written;
637 memmove (bufin, bufin_read, bufin_rpos);
638 bufin_read = NULL; /* start reading again */
650 * Main function of "gnunet-helper-dns", which opens a VPN tunnel interface,
651 * redirects all outgoing DNS traffic (except from the specified port) to that
652 * interface and then passes traffic from and to the interface via stdin/stdout.
654 * Once stdin/stdout close or have other errors, the tunnel is closed and the
655 * DNS traffic redirection is stopped.
657 * @param argc number of arguments
658 * @param argv 0: binary name (should be "gnunet-helper-vpn")
659 * 1: tunnel interface name (typically "gnunet-dns")
660 * 2: IPv6 address for the tunnel ("FE80::1")
661 * 3: IPv6 netmask length in bits ("64")
662 * 4: IPv4 address for the tunnel ("1.2.3.4")
663 * 5: IPv4 netmask ("255.255.0.0")
664 * @return 0 on success, otherwise code indicating type of error:
665 * 1 wrong number of arguments
666 * 2 invalid arguments (i.e. port number / prefix length wrong)
667 * 3 iptables not executable
668 * 4 ip not executable
669 * 5 failed to initialize tunnel interface
670 * 6 failed to initialize control pipe
671 * 8 failed to change routing table, cleanup successful
672 * 9-23 failed to change routing table and failed to undo some changes to routing table
673 * 24 failed to drop privs
674 * 25-39 failed to drop privs and then failed to undo some changes to routing table
675 * 40 failed to regain privs
676 * 41-55 failed to regain prisv and then failed to undo some changes to routing table
677 * 255 failed to handle kill signal properly
680 main (int argc, char *const*argv)
689 fprintf (stderr, "Fatal: must supply 6 arguments!\n");
693 /* verify that the binaries were care about are executable */
694 if (0 == access ("/sbin/iptables", X_OK))
695 sbin_iptables = "/sbin/iptables";
696 else if (0 == access ("/usr/sbin/iptables", X_OK))
697 sbin_iptables = "/usr/sbin/iptables";
701 "Fatal: executable iptables not found in approved directories: %s\n",
705 if (0 == access ("/sbin/ip", X_OK))
706 sbin_ip = "/sbin/ip";
707 else if (0 == access ("/usr/sbin/ip", X_OK))
708 sbin_ip = "/usr/sbin/ip";
712 "Fatal: executable ip not found in approved directories: %s\n",
717 /* setup 'mygid' string */
718 snprintf (mygid, sizeof (mygid), "%d", (int) getegid());
720 /* do not die on SIGPIPE */
721 if (SIG_ERR == signal (SIGPIPE, SIG_IGN))
723 fprintf (stderr, "Failed to protect against SIGPIPE: %s\n",
728 /* setup pipe to shutdown nicely on SIGINT */
729 if (0 != pipe (cpipe))
732 "Fatal: could not setup control pipe: %s\n",
736 if (cpipe[0] >= FD_SETSIZE)
738 fprintf (stderr, "Pipe file descriptor to large: %d", cpipe[0]);
739 (void) close (cpipe[0]);
740 (void) close (cpipe[1]);
744 /* make pipe non-blocking, as we theoretically could otherwise block
745 in the signal handler */
746 int flags = fcntl (cpipe[1], F_GETFL);
749 fprintf (stderr, "Failed to read flags for pipe: %s", strerror (errno));
750 (void) close (cpipe[0]);
751 (void) close (cpipe[1]);
755 if (0 != fcntl (cpipe[1], F_SETFL, flags))
757 fprintf (stderr, "Failed to make pipe non-blocking: %s", strerror (errno));
758 (void) close (cpipe[0]);
759 (void) close (cpipe[1]);
763 if ( (SIG_ERR == signal (SIGTERM, &signal_handler)) ||
764 (SIG_ERR == signal (SIGINT, &signal_handler)) ||
765 (SIG_ERR == signal (SIGHUP, &signal_handler)) )
768 "Fatal: could not initialize signal handler: %s\n",
770 (void) close (cpipe[0]);
771 (void) close (cpipe[1]);
776 /* get interface name */
777 strncpy (dev, argv[1], IFNAMSIZ);
778 dev[IFNAMSIZ - 1] = '\0';
780 /* now open virtual interface (first part that requires root) */
781 if (-1 == (fd_tun = init_tun (dev)))
783 fprintf (stderr, "Fatal: could not initialize tun-interface\n");
784 (void) signal (SIGTERM, SIG_IGN);
785 (void) signal (SIGINT, SIG_IGN);
786 (void) signal (SIGHUP, SIG_IGN);
787 (void) close (cpipe[0]);
788 (void) close (cpipe[1]);
792 /* now set interface addresses */
794 const char *address = argv[2];
795 long prefix_len = atol (argv[3]);
797 if ((prefix_len < 1) || (prefix_len > 127))
799 fprintf (stderr, "Fatal: prefix_len out of range\n");
800 (void) signal (SIGTERM, SIG_IGN);
801 (void) signal (SIGINT, SIG_IGN);
802 (void) signal (SIGHUP, SIG_IGN);
803 (void) close (cpipe[0]);
804 (void) close (cpipe[1]);
807 set_address6 (dev, address, prefix_len);
811 const char *address = argv[4];
812 const char *mask = argv[5];
814 set_address4 (dev, address, mask);
817 /* update routing tables -- next part why we need SUID! */
818 /* Forward everything from our EGID (which should only be held
819 by the 'gnunet-service-dns') and with destination
820 to port 53 on UDP, without hijacking */
821 r = 8; /* failed to fully setup routing table */
823 char *const mangle_args[] =
825 "iptables", "-m", "owner", "-t", "mangle", "-I", "OUTPUT", "1", "-p",
826 "udp", "--gid-owner", mygid, "--dport", DNS_PORT, "-j",
829 if (0 != fork_and_exec (sbin_iptables, mangle_args))
832 /* Mark all of the other DNS traffic using our mark DNS_MARK */
834 char *const mark_args[] =
836 "iptables", "-t", "mangle", "-I", "OUTPUT", "2", "-p",
837 "udp", "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK,
840 if (0 != fork_and_exec (sbin_iptables, mark_args))
841 goto cleanup_mangle_1;
843 /* Forward all marked DNS traffic to our DNS_TABLE */
845 char *const forward_args[] =
847 "ip", "rule", "add", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
849 if (0 != fork_and_exec (sbin_ip, forward_args))
852 /* Finally, add rule in our forwarding table to pass to our virtual interface */
854 char *const route_args[] =
856 "ip", "route", "add", "default", "dev", dev,
857 "table", DNS_TABLE, NULL
859 if (0 != fork_and_exec (sbin_ip, route_args))
860 goto cleanup_forward_3;
863 /* drop privs *except* for the saved UID; this is not perfect, but better
864 than doing nothing */
865 uid_t uid = getuid ();
866 #ifdef HAVE_SETRESUID
867 if (0 != setresuid (uid, uid, 0))
869 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
871 goto cleanup_route_4;
874 /* Note: no 'setuid' here as we must keep our saved UID as root */
875 if (0 != seteuid (uid))
877 fprintf (stderr, "Failed to seteuid: %s\n", strerror (errno));
879 goto cleanup_route_4;
883 r = 0; /* did fully setup routing table (if nothing else happens, we were successful!) */
885 /* now forward until we hit a problem */
888 /* now need to regain privs so we can remove the firewall rules we added! */
889 #ifdef HAVE_SETRESUID
890 if (0 != setresuid (uid, 0, 0))
892 fprintf (stderr, "Failed to setresuid back to root: %s\n", strerror (errno));
894 goto cleanup_route_4;
897 if (0 != seteuid (0))
899 fprintf (stderr, "Failed to seteuid back to root: %s\n", strerror (errno));
901 goto cleanup_route_4;
905 /* update routing tables again -- this is why we could not fully drop privs */
906 /* now undo updating of routing tables; normal exit or clean-up-on-error case */
909 char *const route_clean_args[] =
911 "ip", "route", "del", "default", "dev", dev,
912 "table", DNS_TABLE, NULL
914 if (0 != fork_and_exec (sbin_ip, route_clean_args))
919 char *const forward_clean_args[] =
921 "ip", "rule", "del", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
923 if (0 != fork_and_exec (sbin_ip, forward_clean_args))
928 char *const mark_clean_args[] =
930 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
931 "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK, NULL
933 if (0 != fork_and_exec (sbin_iptables, mark_clean_args))
938 char *const mangle_clean_args[] =
940 "iptables", "-m", "owner", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
941 "--gid-owner", mygid, "--dport", DNS_PORT, "-j", "ACCEPT",
944 if (0 != fork_and_exec (sbin_iptables, mangle_clean_args))
949 /* close virtual interface */
950 (void) close (fd_tun);
951 /* remove signal handler so we can close the pipes */
952 (void) signal (SIGTERM, SIG_IGN);
953 (void) signal (SIGINT, SIG_IGN);
954 (void) signal (SIGHUP, SIG_IGN);
955 (void) close (cpipe[0]);
956 (void) close (cpipe[1]);
960 /* end of gnunet-helper-dns.c */