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ntp: detect MS-SNTP packets

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When a received packet fails to authenticate, check if the digest
contains zeroes and treat it as an MS-SNTP packet with authenticator or
extended authenticator field. For now, discard these packets, i.e. don't
respond with a crypto-NAK.
This commit is contained in:
Miroslav Lichvar 2016-07-27 13:28:01 +02:00
parent 61dd4e0ccb
commit 2a8ce63fc7
1 changed files with 24 additions and 0 deletions
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24
ntp_core.c
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@ -67,6 +67,8 @@ typedef enum {
AUTH_NONE = 0, /* No authentication */
AUTH_CRYPTO_NAK, /* Empty MAC indicating authentication error */
AUTH_SYMMETRIC, /* MAC using symmetric key (RFC 1305, RFC 5905) */
AUTH_MSSNTP, /* MS-SNTP authenticator field */
AUTH_MSSNTP_EXT, /* MS-SNTP extended authenticator field */
} AuthenticationMode;
/* ================================================== */
@ -1105,6 +1107,19 @@ check_packet_format(NTP_Packet *message, int length)
/* ================================================== */
static int
is_zero_data(unsigned char *data, int length)
{
int i;
for (i = 0; i < length; i++)
if (data[i])
return 0;
return 1;
}
/* ================================================== */
static int
check_packet_auth(NTP_Packet *pkt, int length,
AuthenticationMode *auth_mode, uint32_t *key_id)
@ -1156,6 +1171,15 @@ check_packet_auth(NTP_Packet *pkt, int length,
if (remainder >= NTP_MIN_MAC_LENGTH) {
*auth_mode = AUTH_SYMMETRIC;
*key_id = ntohl(*(uint32_t *)(data + i));
/* Check if it is an MS-SNTP authenticator field or extended authenticator
field with zeroes as digest */
if (version == 3 && *key_id) {
if (remainder == 20 && is_zero_data(data + i + 4, remainder - 4))
*auth_mode = AUTH_MSSNTP;
else if (remainder == 72 && is_zero_data(data + i + 8, remainder - 8))
*auth_mode = AUTH_MSSNTP_EXT;
}
} else {
*auth_mode = AUTH_NONE;
*key_id = 0;