Joshua.Hu | Joshua Rogers' Scribbles

Fuzzing glibc's libresolv's res_init()

Looking back at the issue I had with libresolv nearly 10 years ago, I decided it might be interesting to fuzz glibc’s res_init() to see if we can find any bugs.

Most of the processing of res_init() happens when /etc/resolv.conf is parsed, so we’re just going to make a program that creates /etc/resolv.conf and fills it with data and calls res_init().

No need to trash our harddrive, so let’s use a ram disk:

mkdir -p /tmp/fuzz
mount -t tmpfs -o size=100M tmpfs /tmp/fuzz

We’re going to chroot into /tmp/fuzz/* in order to not destroy anything on our server, too,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <resolv.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

int main(int argc, char **argv) {
  char chroot_dir_base[] = "/tmp/fuzz/";
  char resolv_conf_path[] = "/etc/resolv.conf";

  int dir_suffix = 1;
  char chroot_dir[256];

  while (1) { // find a new /tmp/fuzz/[number]/ directory to use for our chroot.
    snprintf(chroot_dir, sizeof(chroot_dir), "%s%d/", chroot_dir_base, dir_suffix);
    if (access(chroot_dir, F_OK) != 0) {
      if (mkdir(chroot_dir, 0755) != 0) {
      } else {


  if (chroot(chroot_dir) != 0) { //chroot into /tmp/fuzz/[number]/
    return 1;

  if (access("/etc/", F_OK) != 0) { //create /etc/
    if (mkdir("/etc/", 0755) != 0) {
      perror("mkdir 2");
      return 1;

  unsigned char *buf = __AFL_FUZZ_TESTCASE_BUF;
  while (__AFL_LOOP(1000000)) {
    int len = __AFL_FUZZ_TESTCASE_LEN;
    FILE *resolv_conf = fopen(resolv_conf_path, "w"); //create /etc/resolv.conf
    if (resolv_conf == NULL) {

    fprintf(resolv_conf, "%s\n", buf);

    if (res_init() != 0) { //res_init should always return 0t

  return 0;

We create different numbers /tmp/fuzz/ folders for each chroot because there will be a race condition if all of the fuzzers are writing /tmp/fuzz/etc/resolv.conf at the same time.

Compiling it with afl-fuzz, we find two unique issues within about 5 seconds.

Infinite loop in sortlist handling

The first issue is related to the sortlist directive. sortlist is an obsolete directive which allows for networks or subnets to be prefered if multiple addresses are received from a dns query. The C code for parsing this directive looks like:

while (true)
    while (*cp == ' ' || *cp == '\t')
    if (*cp == '\0' || *cp == '\n' || *cp == ';')
    char *net = cp;
    while (*cp && !is_sort_mask (*cp) && *cp != ';'
           && isascii (*cp) && !isspace (*cp))
    char separator = *cp;
    *cp = 0;
    struct resolv_sortlist_entry e;
    if (__inet_aton (net, &a))
    *cp = separator;

Cycling through the line until the value for for the sortlist directive is found, separator is any character which is not: ; [space] [ascii] & or /. Once a separator (or the end of the buffer) is found, everything before then is assumed to be some type of address and a null byte is placed at the separator. However, if __inet_aton() fails, the “separator” is placed back where it was in the cp buffer, meaning the loop will continue. The loop will continue forever in the case of the character being not the aforementioned ; [space] [ascii] & or /.

Therefore, if we print, say, \x321 into a value for sortlist, this will loop forever. printf "sortlist 192.0\3212.0" >> /etc/resolv.conf ; ping will hang forever. I can imagine embedded devices which allow you to set /etc/resolv.conf could be completely bricked by this, as anything using glibc’s libresolv when any function that calls res_init() (AKA all libresolv functions) will simply hang forever. Reported in 31025.

Reachable assertion in resolv_conf.c:570: update_from_conf: Assertion `resolv_conf_matches (resp, conf)’ failed.

Another less interesting issue was an assert in the search directive. The technical details are boring, but I’m surprised nobody has noticed this before:

echo "search corp.corp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.examcorp.exam" >> /etc/resolv.conf
# ping
ping: resolv_conf.c:570: update_from_conf: Assertion `resolv_conf_matches (resp, conf)' failed.

Basically if the directive for search is greater than 255 characters, an assertion. Reported in 31026.

Has nobody fuzzed glibc before? How strange.