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);
240 memset (&ifr, 0, sizeof (ifr));
241 ifr.ifr_flags = IFF_TUN;
244 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
246 if (-1 == ioctl (fd, TUNSETIFF, (void *) &ifr))
248 fprintf (stderr, "Error with ioctl on `%s': %s\n", "/dev/net/tun",
253 strcpy (dev, ifr.ifr_name);
259 * @brief Sets the IPv6-Address given in address on the interface dev
261 * @param dev the interface to configure
262 * @param address the IPv6-Address
263 * @param prefix_len the length of the network-prefix
266 set_address6 (const char *dev, const char *address, unsigned long prefix_len)
269 struct in6_ifreq ifr6;
270 struct sockaddr_in6 sa6;
274 * parse the new address
276 memset (&sa6, 0, sizeof (struct sockaddr_in6));
277 sa6.sin6_family = AF_INET6;
278 if (1 != inet_pton (AF_INET6, address, sa6.sin6_addr.s6_addr))
280 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
285 if (-1 == (fd = socket (PF_INET6, SOCK_DGRAM, 0)))
287 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
291 memset (&ifr, 0, sizeof (struct ifreq));
293 * Get the index of the if
295 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
296 if (-1 == ioctl (fd, SIOGIFINDEX, &ifr))
298 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
303 memset (&ifr6, 0, sizeof (struct in6_ifreq));
304 ifr6.ifr6_addr = sa6.sin6_addr;
305 ifr6.ifr6_ifindex = ifr.ifr_ifindex;
306 ifr6.ifr6_prefixlen = prefix_len;
311 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr6))
313 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
322 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
324 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
331 * Add the UP and RUNNING flags
333 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
334 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
336 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
344 fprintf (stderr, "close failed: %s\n", strerror (errno));
351 * @brief Sets the IPv4-Address given in address on the interface dev
353 * @param dev the interface to configure
354 * @param address the IPv4-Address
355 * @param mask the netmask
358 set_address4 (const char *dev, const char *address, const char *mask)
361 struct sockaddr_in *addr;
364 memset (&ifr, 0, sizeof (struct ifreq));
365 addr = (struct sockaddr_in *) &(ifr.ifr_addr);
366 addr->sin_family = AF_INET;
371 if (1 != inet_pton (AF_INET, address, &addr->sin_addr.s_addr))
373 fprintf (stderr, "Failed to parse address `%s': %s\n", address,
378 if (-1 == (fd = socket (PF_INET, SOCK_DGRAM, 0)))
380 fprintf (stderr, "Error creating socket: %s\n", strerror (errno));
384 strncpy (ifr.ifr_name, dev, IFNAMSIZ);
389 if (-1 == ioctl (fd, SIOCSIFADDR, &ifr))
391 fprintf (stderr, "ioctl failed at %d: %s\n", __LINE__, strerror (errno));
399 addr = (struct sockaddr_in *) &(ifr.ifr_netmask);
400 if (1 != inet_pton (AF_INET, mask, &addr->sin_addr.s_addr))
402 fprintf (stderr, "Failed to parse address `%s': %s\n", mask,
411 if (-1 == ioctl (fd, SIOCSIFNETMASK, &ifr))
413 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
422 if (-1 == ioctl (fd, SIOCGIFFLAGS, &ifr))
424 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
431 * Add the UP and RUNNING flags
433 ifr.ifr_flags |= IFF_UP | IFF_RUNNING;
434 if (-1 == ioctl (fd, SIOCSIFFLAGS, &ifr))
436 fprintf (stderr, "ioctl failed at line %d: %s\n", __LINE__,
444 fprintf (stderr, "close failed: %s\n", strerror (errno));
452 * Start forwarding to and from the tunnel. This function runs with
453 * "reduced" priviledges (saved UID is still 0, but effective UID is
456 * @param fd_tun tunnel FD
462 * The buffer filled by reading from fd_tun
464 unsigned char buftun[MAX_SIZE];
465 ssize_t buftun_size = 0;
466 unsigned char *buftun_read = NULL;
469 * The buffer filled by reading from stdin
471 unsigned char bufin[MAX_SIZE];
472 ssize_t bufin_size = 0;
473 size_t bufin_rpos = 0;
474 unsigned char *bufin_read = NULL;
485 * We are supposed to read and the buffer is empty
486 * -> select on read from tun
488 if (0 == buftun_size)
489 FD_SET (fd_tun, &fds_r);
492 * We are supposed to read and the buffer is not empty
493 * -> select on write to stdout
495 if (0 != buftun_size)
499 * We are supposed to write and the buffer is empty
500 * -> select on read from stdin
502 if (NULL == bufin_read)
506 * We are supposed to write and the buffer is not empty
507 * -> select on write to tun
509 if (NULL != bufin_read)
510 FD_SET (fd_tun, &fds_w);
512 FD_SET (cpipe[0], &fds_r);
513 max = (fd_tun > cpipe[0]) ? fd_tun : cpipe[0];
515 int r = select (max + 1, &fds_r, &fds_w, NULL, NULL);
521 fprintf (stderr, "select failed: %s\n", strerror (errno));
527 if (FD_ISSET (cpipe[0], &fds_r))
528 return; /* aborted by signal */
530 if (FD_ISSET (fd_tun, &fds_r))
533 read (fd_tun, buftun + sizeof (struct GNUNET_MessageHeader),
534 MAX_SIZE - sizeof (struct GNUNET_MessageHeader));
535 if (-1 == buftun_size)
537 if ( (errno == EINTR) ||
540 fprintf (stderr, "read-error: %s\n", strerror (errno));
543 if (0 == buftun_size)
545 fprintf (stderr, "EOF on tun\n");
548 buftun_read = buftun;
550 struct GNUNET_MessageHeader *hdr =
551 (struct GNUNET_MessageHeader *) buftun;
552 buftun_size += sizeof (struct GNUNET_MessageHeader);
553 hdr->type = htons (GNUNET_MESSAGE_TYPE_DNS_HELPER);
554 hdr->size = htons (buftun_size);
557 else if (FD_ISSET (1, &fds_w))
559 ssize_t written = write (1, buftun_read, buftun_size);
563 if ( (errno == EINTR) ||
566 fprintf (stderr, "write-error to stdout: %s\n", strerror (errno));
571 fprintf (stderr, "write returned 0\n");
574 buftun_size -= written;
575 buftun_read += written;
578 if (FD_ISSET (0, &fds_r))
580 bufin_size = read (0, bufin + bufin_rpos, MAX_SIZE - bufin_rpos);
581 if (-1 == bufin_size)
584 if ( (errno == EINTR) ||
587 fprintf (stderr, "read-error: %s\n", strerror (errno));
593 fprintf (stderr, "EOF on stdin\n");
597 struct GNUNET_MessageHeader *hdr;
600 bufin_rpos += bufin_size;
601 if (bufin_rpos < sizeof (struct GNUNET_MessageHeader))
603 hdr = (struct GNUNET_MessageHeader *) bufin;
604 if (ntohs (hdr->type) != GNUNET_MESSAGE_TYPE_DNS_HELPER)
606 fprintf (stderr, "protocol violation!\n");
609 if (ntohs (hdr->size) > bufin_rpos)
611 bufin_read = bufin + sizeof (struct GNUNET_MessageHeader);
612 bufin_size = ntohs (hdr->size) - sizeof (struct GNUNET_MessageHeader);
613 bufin_rpos -= bufin_size + sizeof (struct GNUNET_MessageHeader);
616 else if (FD_ISSET (fd_tun, &fds_w))
618 ssize_t written = write (fd_tun, bufin_read, bufin_size);
622 if ( (errno == EINTR) ||
625 fprintf (stderr, "write-error to tun: %s\n", strerror (errno));
630 fprintf (stderr, "write returned 0\n");
634 bufin_size -= written;
635 bufin_read += written;
638 memmove (bufin, bufin_read, bufin_rpos);
639 bufin_read = NULL; /* start reading again */
651 * Main function of "gnunet-helper-dns", which opens a VPN tunnel interface,
652 * redirects all outgoing DNS traffic (except from the specified port) to that
653 * interface and then passes traffic from and to the interface via stdin/stdout.
655 * Once stdin/stdout close or have other errors, the tunnel is closed and the
656 * DNS traffic redirection is stopped.
658 * @param argc number of arguments
659 * @param argv 0: binary name (should be "gnunet-helper-vpn")
660 * 1: tunnel interface name (typically "gnunet-dns")
661 * 2: IPv6 address for the tunnel ("FE80::1")
662 * 3: IPv6 netmask length in bits ("64")
663 * 4: IPv4 address for the tunnel ("1.2.3.4")
664 * 5: IPv4 netmask ("255.255.0.0")
665 * @return 0 on success, otherwise code indicating type of error:
666 * 1 wrong number of arguments
667 * 2 invalid arguments (i.e. port number / prefix length wrong)
668 * 3 iptables not executable
669 * 4 ip not executable
670 * 5 failed to initialize tunnel interface
671 * 6 failed to initialize control pipe
672 * 8 failed to change routing table, cleanup successful
673 * 9-23 failed to change routing table and failed to undo some changes to routing table
674 * 24 failed to drop privs
675 * 25-39 failed to drop privs and then failed to undo some changes to routing table
676 * 40 failed to regain privs
677 * 41-55 failed to regain prisv and then failed to undo some changes to routing table
678 * 255 failed to handle kill signal properly
681 main (int argc, char *const*argv)
690 fprintf (stderr, "Fatal: must supply 6 arguments!\n");
694 /* verify that the binaries were care about are executable */
695 if (0 == access ("/sbin/iptables", X_OK))
696 sbin_iptables = "/sbin/iptables";
697 else if (0 == access ("/usr/sbin/iptables", X_OK))
698 sbin_iptables = "/usr/sbin/iptables";
702 "Fatal: executable iptables not found in approved directories: %s\n",
706 if (0 == access ("/sbin/ip", X_OK))
707 sbin_ip = "/sbin/ip";
708 else if (0 == access ("/usr/sbin/ip", X_OK))
709 sbin_ip = "/usr/sbin/ip";
713 "Fatal: executable ip not found in approved directories: %s\n",
718 /* setup 'mygid' string */
719 snprintf (mygid, sizeof (mygid), "%d", (int) getegid());
721 /* do not die on SIGPIPE */
722 if (SIG_ERR == signal (SIGPIPE, SIG_IGN))
724 fprintf (stderr, "Failed to protect against SIGPIPE: %s\n",
729 /* setup pipe to shutdown nicely on SIGINT */
730 if (0 != pipe (cpipe))
733 "Fatal: could not setup control pipe: %s\n",
737 if (cpipe[0] >= FD_SETSIZE)
739 fprintf (stderr, "Pipe file descriptor to large: %d", cpipe[0]);
740 (void) close (cpipe[0]);
741 (void) close (cpipe[1]);
745 /* make pipe non-blocking, as we theoretically could otherwise block
746 in the signal handler */
747 int flags = fcntl (cpipe[1], F_GETFL);
750 fprintf (stderr, "Failed to read flags for pipe: %s", strerror (errno));
751 (void) close (cpipe[0]);
752 (void) close (cpipe[1]);
756 if (0 != fcntl (cpipe[1], F_SETFL, flags))
758 fprintf (stderr, "Failed to make pipe non-blocking: %s", strerror (errno));
759 (void) close (cpipe[0]);
760 (void) close (cpipe[1]);
764 if ( (SIG_ERR == signal (SIGTERM, &signal_handler)) ||
765 (SIG_ERR == signal (SIGINT, &signal_handler)) ||
766 (SIG_ERR == signal (SIGHUP, &signal_handler)) )
769 "Fatal: could not initialize signal handler: %s\n",
771 (void) close (cpipe[0]);
772 (void) close (cpipe[1]);
777 /* get interface name */
778 strncpy (dev, argv[1], IFNAMSIZ);
779 dev[IFNAMSIZ - 1] = '\0';
781 /* now open virtual interface (first part that requires root) */
782 if (-1 == (fd_tun = init_tun (dev)))
784 fprintf (stderr, "Fatal: could not initialize tun-interface\n");
785 (void) signal (SIGTERM, SIG_IGN);
786 (void) signal (SIGINT, SIG_IGN);
787 (void) signal (SIGHUP, SIG_IGN);
788 (void) close (cpipe[0]);
789 (void) close (cpipe[1]);
793 /* now set interface addresses */
795 const char *address = argv[2];
796 long prefix_len = atol (argv[3]);
798 if ((prefix_len < 1) || (prefix_len > 127))
800 fprintf (stderr, "Fatal: prefix_len out of range\n");
801 (void) signal (SIGTERM, SIG_IGN);
802 (void) signal (SIGINT, SIG_IGN);
803 (void) signal (SIGHUP, SIG_IGN);
804 (void) close (cpipe[0]);
805 (void) close (cpipe[1]);
808 set_address6 (dev, address, prefix_len);
812 const char *address = argv[4];
813 const char *mask = argv[5];
815 set_address4 (dev, address, mask);
818 /* update routing tables -- next part why we need SUID! */
819 /* Forward everything from our EGID (which should only be held
820 by the 'gnunet-service-dns') and with destination
821 to port 53 on UDP, without hijacking */
822 r = 8; /* failed to fully setup routing table */
824 char *const mangle_args[] =
826 "iptables", "-m", "owner", "-t", "mangle", "-I", "OUTPUT", "1", "-p",
827 "udp", "--gid-owner", mygid, "--dport", DNS_PORT, "-j",
830 if (0 != fork_and_exec (sbin_iptables, mangle_args))
833 /* Mark all of the other DNS traffic using our mark DNS_MARK */
835 char *const mark_args[] =
837 "iptables", "-t", "mangle", "-I", "OUTPUT", "2", "-p",
838 "udp", "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK,
841 if (0 != fork_and_exec (sbin_iptables, mark_args))
842 goto cleanup_mangle_1;
844 /* Forward all marked DNS traffic to our DNS_TABLE */
846 char *const forward_args[] =
848 "ip", "rule", "add", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
850 if (0 != fork_and_exec (sbin_ip, forward_args))
853 /* Finally, add rule in our forwarding table to pass to our virtual interface */
855 char *const route_args[] =
857 "ip", "route", "add", "default", "dev", dev,
858 "table", DNS_TABLE, NULL
860 if (0 != fork_and_exec (sbin_ip, route_args))
861 goto cleanup_forward_3;
864 /* drop privs *except* for the saved UID; this is not perfect, but better
865 than doing nothing */
866 uid_t uid = getuid ();
867 #ifdef HAVE_SETRESUID
868 if (0 != setresuid (uid, uid, 0))
870 fprintf (stderr, "Failed to setresuid: %s\n", strerror (errno));
872 goto cleanup_route_4;
875 /* Note: no 'setuid' here as we must keep our saved UID as root */
876 if (0 != seteuid (uid))
878 fprintf (stderr, "Failed to seteuid: %s\n", strerror (errno));
880 goto cleanup_route_4;
884 r = 0; /* did fully setup routing table (if nothing else happens, we were successful!) */
886 /* now forward until we hit a problem */
889 /* now need to regain privs so we can remove the firewall rules we added! */
890 #ifdef HAVE_SETRESUID
891 if (0 != setresuid (uid, 0, 0))
893 fprintf (stderr, "Failed to setresuid back to root: %s\n", strerror (errno));
895 goto cleanup_route_4;
898 if (0 != seteuid (0))
900 fprintf (stderr, "Failed to seteuid back to root: %s\n", strerror (errno));
902 goto cleanup_route_4;
906 /* update routing tables again -- this is why we could not fully drop privs */
907 /* now undo updating of routing tables; normal exit or clean-up-on-error case */
910 char *const route_clean_args[] =
912 "ip", "route", "del", "default", "dev", dev,
913 "table", DNS_TABLE, NULL
915 if (0 != fork_and_exec (sbin_ip, route_clean_args))
920 char *const forward_clean_args[] =
922 "ip", "rule", "del", "fwmark", DNS_MARK, "table", DNS_TABLE, NULL
924 if (0 != fork_and_exec (sbin_ip, forward_clean_args))
929 char *const mark_clean_args[] =
931 "iptables", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
932 "--dport", DNS_PORT, "-j", "MARK", "--set-mark", DNS_MARK, NULL
934 if (0 != fork_and_exec (sbin_iptables, mark_clean_args))
939 char *const mangle_clean_args[] =
941 "iptables", "-m", "owner", "-t", "mangle", "-D", "OUTPUT", "-p", "udp",
942 "--gid-owner", mygid, "--dport", DNS_PORT, "-j", "ACCEPT",
945 if (0 != fork_and_exec (sbin_iptables, mangle_clean_args))
950 /* close virtual interface */
951 (void) close (fd_tun);
952 /* remove signal handler so we can close the pipes */
953 (void) signal (SIGTERM, SIG_IGN);
954 (void) signal (SIGINT, SIG_IGN);
955 (void) signal (SIGHUP, SIG_IGN);
956 (void) close (cpipe[0]);
957 (void) close (cpipe[1]);
961 /* end of gnunet-helper-dns.c */