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Use SHA-256 to provide security against DNS cache poisoning.

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Use the SHA-256 hash function to verify that DNS answers
received are for the questions originally asked. This replaces
the slightly insecure SHA-1 (when compiled with DNSSEC) or
the very insecure CRC32 (otherwise). Refer: CERT VU#434904.
This commit is contained in:
Simon Kelley
2020-11-12 22:06:07 +00:00
parent 257ac0c5f7
commit 2d765867c5
8 changed files with 301 additions and 124 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
src/dnsmasq.h
View File

@@ -655,11 +655,7 @@ struct hostsfile {
#define FREC_TEST_PKTSZ 256
#define FREC_HAS_EXTRADATA 512
#ifdef HAVE_DNSSEC
#define HASH_SIZE 20 /* SHA-1 digest size */
#else
#define HASH_SIZE sizeof(int)
#endif
#define HASH_SIZE 32 /* SHA-256 digest size */
struct frec {
union mysockaddr source;
@@ -1229,7 +1225,6 @@ int check_for_bogus_wildcard(struct dns_header *header, size_t qlen, char *name,
struct bogus_addr *baddr, time_t now);
int check_for_ignored_address(struct dns_header *header, size_t qlen, struct bogus_addr *baddr);
int check_for_local_domain(char *name, time_t now);
unsigned int questions_crc(struct dns_header *header, size_t plen, char *name);
size_t resize_packet(struct dns_header *header, size_t plen,
unsigned char *pheader, size_t hlen);
int add_resource_record(struct dns_header *header, char *limit, int *truncp,
@@ -1254,9 +1249,11 @@ int dnssec_validate_reply(time_t now, struct dns_header *header, size_t plen, ch
int check_unsigned, int *neganswer, int *nons, int *nsec_ttl);
int dnskey_keytag(int alg, int flags, unsigned char *key, int keylen);
size_t filter_rrsigs(struct dns_header *header, size_t plen);
unsigned char* hash_questions(struct dns_header *header, size_t plen, char *name);
int setup_timestamp(void);
/* hash_questions.c */
unsigned char *hash_questions(struct dns_header *header, size_t plen, char *name);
/* crypto.c */
const struct nettle_hash *hash_find(char *name);
int hash_init(const struct nettle_hash *hash, void **ctxp, unsigned char **digestp);

31
src/dnssec.c
View File

@@ -2087,35 +2087,4 @@ size_t dnssec_generate_query(struct dns_header *header, unsigned char *end, char
return ret;
}
unsigned char* hash_questions(struct dns_header *header, size_t plen, char *name)
{
int q;
unsigned int len;
unsigned char *p = (unsigned char *)(header+1);
const struct nettle_hash *hash;
void *ctx;
unsigned char *digest;
if (!(hash = hash_find("sha1")) || !hash_init(hash, &ctx, &digest))
return NULL;
for (q = ntohs(header->qdcount); q != 0; q--)
{
if (!extract_name(header, plen, &p, name, 1, 4))
break; /* bad packet */
len = to_wire(name);
hash->update(ctx, len, (unsigned char *)name);
/* CRC the class and type as well */
hash->update(ctx, 4, p);
p += 4;
if (!CHECK_LEN(header, p, plen, 0))
break; /* bad packet */
}
hash->digest(ctx, hash->digest_size, digest);
return digest;
}
#endif /* HAVE_DNSSEC */

43
src/forward.c
View File

@@ -256,19 +256,16 @@ static int forward_query(int udpfd, union mysockaddr *udpaddr,
union all_addr *addrp = NULL;
unsigned int flags = 0;
struct server *start = NULL;
#ifdef HAVE_DNSSEC
void *hash = hash_questions(header, plen, daemon->namebuff);
#ifdef HAVE_DNSSEC
int do_dnssec = 0;
#else
unsigned int crc = questions_crc(header, plen, daemon->namebuff);
void *hash = &crc;
#endif
unsigned int gotname = extract_request(header, plen, daemon->namebuff, NULL);
unsigned char *oph = find_pseudoheader(header, plen, NULL, NULL, NULL, NULL);
(void)do_bit;
/* may be no servers available. */
if (forward || (hash && (forward = lookup_frec_by_sender(ntohs(header->id), udpaddr, hash))))
if (forward || (forward = lookup_frec_by_sender(ntohs(header->id), udpaddr, hash)))
{
/* If we didn't get an answer advertising a maximal packet in EDNS,
fall back to 1280, which should work everywhere on IPv6.
@@ -769,9 +766,6 @@ void reply_query(int fd, int family, time_t now)
size_t nn;
struct server *server;
void *hash;
#ifndef HAVE_DNSSEC
unsigned int crc;
#endif
/* packet buffer overwritten */
daemon->srv_save = NULL;
@@ -798,12 +792,7 @@ void reply_query(int fd, int family, time_t now)
if (difftime(now, server->pktsz_reduced) > UDP_TEST_TIME)
server->edns_pktsz = daemon->edns_pktsz;
#ifdef HAVE_DNSSEC
hash = hash_questions(header, n, daemon->namebuff);
#else
hash = &crc;
crc = questions_crc(header, n, daemon->namebuff);
#endif
if (!(forward = lookup_frec(ntohs(header->id), fd, family, hash)))
return;
@@ -1115,8 +1104,7 @@ void reply_query(int fd, int family, time_t now)
log_query(F_NOEXTRA | F_DNSSEC | F_IPV6, daemon->keyname, (union all_addr *)&(server->addr.in6.sin6_addr),
querystr("dnssec-query", querytype));
if ((hash = hash_questions(header, nn, daemon->namebuff)))
memcpy(new->hash, hash, HASH_SIZE);
memcpy(new->hash, hash_questions(header, nn, daemon->namebuff), HASH_SIZE);
new->new_id = get_id();
header->id = htons(new->new_id);
/* Save query for retransmission */
@@ -1970,15 +1958,9 @@ unsigned char *tcp_request(int confd, time_t now,
if (!flags && last_server)
{
struct server *firstsendto = NULL;
#ifdef HAVE_DNSSEC
unsigned char *newhash, hash[HASH_SIZE];
if ((newhash = hash_questions(header, (unsigned int)size, daemon->namebuff)))
memcpy(hash, newhash, HASH_SIZE);
else
memset(hash, 0, HASH_SIZE);
#else
unsigned int crc = questions_crc(header, (unsigned int)size, daemon->namebuff);
#endif
unsigned char hash[HASH_SIZE];
memcpy(hash, hash_questions(header, (unsigned int)size, daemon->namebuff), HASH_SIZE);
/* Loop round available servers until we succeed in connecting to one.
Note that this code subtly ensures that consecutive queries on this connection
which can go to the same server, do so. */
@@ -2117,20 +2099,11 @@ unsigned char *tcp_request(int confd, time_t now,
/* If the crc of the question section doesn't match the crc we sent, then
someone might be attempting to insert bogus values into the cache by
sending replies containing questions and bogus answers. */
#ifdef HAVE_DNSSEC
newhash = hash_questions(header, (unsigned int)m, daemon->namebuff);
if (!newhash || memcmp(hash, newhash, HASH_SIZE) != 0)
if (memcmp(hash, hash_questions(header, (unsigned int)m, daemon->namebuff), HASH_SIZE) != 0)
{
m = 0;
break;
}
#else
if (crc != questions_crc(header, (unsigned int)m, daemon->namebuff))
{
m = 0;
break;
}
#endif
m = process_reply(header, now, last_server, (unsigned int)m,
option_bool(OPT_NO_REBIND) && !norebind, no_cache_dnssec, cache_secure, bogusanswer,
@@ -2345,7 +2318,7 @@ static struct frec *lookup_frec(unsigned short id, int fd, int family, void *has
for(f = daemon->frec_list; f; f = f->next)
if (f->sentto && f->new_id == id &&
(!hash || memcmp(hash, f->hash, HASH_SIZE) == 0))
(memcmp(hash, f->hash, HASH_SIZE) == 0))
{
/* sent from random port */
if (family == AF_INET && f->rfd4 && f->rfd4->fd == fd)

281
src/hash_questions.c Normal file
View File

@@ -0,0 +1,281 @@
/* Copyright (c) 2012-2020 Simon Kelley
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 dated June, 1991, or
(at your option) version 3 dated 29 June, 2007.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
/* Hash the question section. This is used to safely detect query
retransmission and to detect answers to questions we didn't ask, which
might be poisoning attacks. Note that we decode the name rather
than CRC the raw bytes, since replies might be compressed differently.
We ignore case in the names for the same reason.
The hash used is SHA-256. If we're building with DNSSEC support,
we use the Nettle cypto library. If not, we prefer not to
add a dependency on Nettle, and use a stand-alone implementaion.
*/
#include "dnsmasq.h"
#ifdef HAVE_DNSSEC
unsigned char *hash_questions(struct dns_header *header, size_t plen, char *name)
{
int q;
unsigned char *p = (unsigned char *)(header+1);
const struct nettle_hash *hash;
void *ctx;
unsigned char *digest;
if (!(hash = hash_find("sha256")) || !hash_init(hash, &ctx, &digest))
{
/* don't think this can ever happen. */
static unsigned char dummy[HASH_SIZE];
static int warned = 0;
if (warned)
my_syslog(LOG_ERR, _("Failed to create SHA-256 hash object"));
warned = 1;
return dummy;
}
for (q = ntohs(header->qdcount); q != 0; q--)
{
char *cp, c;
if (!extract_name(header, plen, &p, name, 1, 4))
break; /* bad packet */
for (cp = name; (c = *cp); cp++)
if (c >= 'A' && c <= 'Z')
*cp += 'a' - 'A';
hash->update(ctx, cp - name, (unsigned char *)name);
/* CRC the class and type as well */
hash->update(ctx, 4, p);
p += 4;
if (!CHECK_LEN(header, p, plen, 0))
break; /* bad packet */
}
hash->digest(ctx, hash->digest_size, digest);
return digest;
}
#else /* HAVE_DNSSEC */
#define SHA256_BLOCK_SIZE 32 // SHA256 outputs a 32 byte digest
typedef unsigned char BYTE; // 8-bit byte
typedef unsigned int WORD; // 32-bit word, change to "long" for 16-bit machines
typedef struct {
BYTE data[64];
WORD datalen;
unsigned long long bitlen;
WORD state[8];
} SHA256_CTX;
static void sha256_init(SHA256_CTX *ctx);
static void sha256_update(SHA256_CTX *ctx, const BYTE data[], size_t len);
static void sha256_final(SHA256_CTX *ctx, BYTE hash[]);
unsigned char *hash_questions(struct dns_header *header, size_t plen, char *name)
{
int q;
unsigned char *p = (unsigned char *)(header+1);
SHA256_CTX ctx;
static BYTE digest[SHA256_BLOCK_SIZE];
sha256_init(&ctx);
for (q = ntohs(header->qdcount); q != 0; q--)
{
char *cp, c;
if (!extract_name(header, plen, &p, name, 1, 4))
break; /* bad packet */
for (cp = name; (c = *cp); cp++)
if (c >= 'A' && c <= 'Z')
*cp += 'a' - 'A';
sha256_update(&ctx, (BYTE *)name, cp - name);
/* CRC the class and type as well */
sha256_update(&ctx, (BYTE *)p, 4);
p += 4;
if (!CHECK_LEN(header, p, plen, 0))
break; /* bad packet */
}
sha256_final(&ctx, digest);
return (unsigned char *)digest;
}
/* Code from here onwards comes from https://github.com/B-Con/crypto-algorithms
and was written by Brad Conte (brad@bradconte.com), to whom all credit is given.
This code is in the public domain, and the copyright notice at the head of this
file does not apply to it.
*/
/****************************** MACROS ******************************/
#define ROTLEFT(a,b) (((a) << (b)) | ((a) >> (32-(b))))
#define ROTRIGHT(a,b) (((a) >> (b)) | ((a) << (32-(b))))
#define CH(x,y,z) (((x) & (y)) ^ (~(x) & (z)))
#define MAJ(x,y,z) (((x) & (y)) ^ ((x) & (z)) ^ ((y) & (z)))
#define EP0(x) (ROTRIGHT(x,2) ^ ROTRIGHT(x,13) ^ ROTRIGHT(x,22))
#define EP1(x) (ROTRIGHT(x,6) ^ ROTRIGHT(x,11) ^ ROTRIGHT(x,25))
#define SIG0(x) (ROTRIGHT(x,7) ^ ROTRIGHT(x,18) ^ ((x) >> 3))
#define SIG1(x) (ROTRIGHT(x,17) ^ ROTRIGHT(x,19) ^ ((x) >> 10))
/**************************** VARIABLES *****************************/
static const WORD k[64] = {
0x428a2f98,0x71374491,0xb5c0fbcf,0xe9b5dba5,0x3956c25b,0x59f111f1,0x923f82a4,0xab1c5ed5,
0xd807aa98,0x12835b01,0x243185be,0x550c7dc3,0x72be5d74,0x80deb1fe,0x9bdc06a7,0xc19bf174,
0xe49b69c1,0xefbe4786,0x0fc19dc6,0x240ca1cc,0x2de92c6f,0x4a7484aa,0x5cb0a9dc,0x76f988da,
0x983e5152,0xa831c66d,0xb00327c8,0xbf597fc7,0xc6e00bf3,0xd5a79147,0x06ca6351,0x14292967,
0x27b70a85,0x2e1b2138,0x4d2c6dfc,0x53380d13,0x650a7354,0x766a0abb,0x81c2c92e,0x92722c85,
0xa2bfe8a1,0xa81a664b,0xc24b8b70,0xc76c51a3,0xd192e819,0xd6990624,0xf40e3585,0x106aa070,
0x19a4c116,0x1e376c08,0x2748774c,0x34b0bcb5,0x391c0cb3,0x4ed8aa4a,0x5b9cca4f,0x682e6ff3,
0x748f82ee,0x78a5636f,0x84c87814,0x8cc70208,0x90befffa,0xa4506ceb,0xbef9a3f7,0xc67178f2
};
/*********************** FUNCTION DEFINITIONS ***********************/
static void sha256_transform(SHA256_CTX *ctx, const BYTE data[])
{
WORD a, b, c, d, e, f, g, h, i, j, t1, t2, m[64];
for (i = 0, j = 0; i < 16; ++i, j += 4)
m[i] = (data[j] << 24) | (data[j + 1] << 16) | (data[j + 2] << 8) | (data[j + 3]);
for ( ; i < 64; ++i)
m[i] = SIG1(m[i - 2]) + m[i - 7] + SIG0(m[i - 15]) + m[i - 16];
a = ctx->state[0];
b = ctx->state[1];
c = ctx->state[2];
d = ctx->state[3];
e = ctx->state[4];
f = ctx->state[5];
g = ctx->state[6];
h = ctx->state[7];
for (i = 0; i < 64; ++i)
{
t1 = h + EP1(e) + CH(e,f,g) + k[i] + m[i];
t2 = EP0(a) + MAJ(a,b,c);
h = g;
g = f;
f = e;
e = d + t1;
d = c;
c = b;
b = a;
a = t1 + t2;
}
ctx->state[0] += a;
ctx->state[1] += b;
ctx->state[2] += c;
ctx->state[3] += d;
ctx->state[4] += e;
ctx->state[5] += f;
ctx->state[6] += g;
ctx->state[7] += h;
}
static void sha256_init(SHA256_CTX *ctx)
{
ctx->datalen = 0;
ctx->bitlen = 0;
ctx->state[0] = 0x6a09e667;
ctx->state[1] = 0xbb67ae85;
ctx->state[2] = 0x3c6ef372;
ctx->state[3] = 0xa54ff53a;
ctx->state[4] = 0x510e527f;
ctx->state[5] = 0x9b05688c;
ctx->state[6] = 0x1f83d9ab;
ctx->state[7] = 0x5be0cd19;
}
static void sha256_update(SHA256_CTX *ctx, const BYTE data[], size_t len)
{
WORD i;
for (i = 0; i < len; ++i)
{
ctx->data[ctx->datalen] = data[i];
ctx->datalen++;
if (ctx->datalen == 64) {
sha256_transform(ctx, ctx->data);
ctx->bitlen += 512;
ctx->datalen = 0;
}
}
}
static void sha256_final(SHA256_CTX *ctx, BYTE hash[])
{
WORD i;
i = ctx->datalen;
// Pad whatever data is left in the buffer.
if (ctx->datalen < 56)
{
ctx->data[i++] = 0x80;
while (i < 56)
ctx->data[i++] = 0x00;
}
else
{
ctx->data[i++] = 0x80;
while (i < 64)
ctx->data[i++] = 0x00;
sha256_transform(ctx, ctx->data);
memset(ctx->data, 0, 56);
}
// Append to the padding the total message's length in bits and transform.
ctx->bitlen += ctx->datalen * 8;
ctx->data[63] = ctx->bitlen;
ctx->data[62] = ctx->bitlen >> 8;
ctx->data[61] = ctx->bitlen >> 16;
ctx->data[60] = ctx->bitlen >> 24;
ctx->data[59] = ctx->bitlen >> 32;
ctx->data[58] = ctx->bitlen >> 40;
ctx->data[57] = ctx->bitlen >> 48;
ctx->data[56] = ctx->bitlen >> 56;
sha256_transform(ctx, ctx->data);
// Since this implementation uses little endian byte ordering and SHA uses big endian,
// reverse all the bytes when copying the final state to the output hash.
for (i = 0; i < 4; ++i)
{
hash[i] = (ctx->state[0] >> (24 - i * 8)) & 0x000000ff;
hash[i + 4] = (ctx->state[1] >> (24 - i * 8)) & 0x000000ff;
hash[i + 8] = (ctx->state[2] >> (24 - i * 8)) & 0x000000ff;
hash[i + 12] = (ctx->state[3] >> (24 - i * 8)) & 0x000000ff;
hash[i + 16] = (ctx->state[4] >> (24 - i * 8)) & 0x000000ff;
hash[i + 20] = (ctx->state[5] >> (24 - i * 8)) & 0x000000ff;
hash[i + 24] = (ctx->state[6] >> (24 - i * 8)) & 0x000000ff;
hash[i + 28] = (ctx->state[7] >> (24 - i * 8)) & 0x000000ff;
}
}
#endif

49
src/rfc1035.c
View File

@@ -333,55 +333,6 @@ unsigned char *skip_section(unsigned char *ansp, int count, struct dns_header *h
return ansp;
}
/* CRC the question section. This is used to safely detect query
retransmission and to detect answers to questions we didn't ask, which
might be poisoning attacks. Note that we decode the name rather
than CRC the raw bytes, since replies might be compressed differently.
We ignore case in the names for the same reason. Return all-ones
if there is not question section. */
#ifndef HAVE_DNSSEC
unsigned int questions_crc(struct dns_header *header, size_t plen, char *name)
{
int q;
unsigned int crc = 0xffffffff;
unsigned char *p1, *p = (unsigned char *)(header+1);
for (q = ntohs(header->qdcount); q != 0; q--)
{
if (!extract_name(header, plen, &p, name, 1, 4))
return crc; /* bad packet */
for (p1 = (unsigned char *)name; *p1; p1++)
{
int i = 8;
char c = *p1;
if (c >= 'A' && c <= 'Z')
c += 'a' - 'A';
crc ^= c << 24;
while (i--)
crc = crc & 0x80000000 ? (crc << 1) ^ 0x04c11db7 : crc << 1;
}
/* CRC the class and type as well */
for (p1 = p; p1 < p+4; p1++)
{
int i = 8;
crc ^= *p1 << 24;
while (i--)
crc = crc & 0x80000000 ? (crc << 1) ^ 0x04c11db7 : crc << 1;
}
p += 4;
if (!CHECK_LEN(header, p, plen, 0))
return crc; /* bad packet */
}
return crc;
}
#endif
size_t resize_packet(struct dns_header *header, size_t plen, unsigned char *pheader, size_t hlen)
{
unsigned char *ansp = skip_questions(header, plen);