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ntp: drop support for long NTPv4 MACs

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Don't accept NTPv4 packets which have a MAC longer than 24 octets to
strictly follow RFC 7822, which specifies the maximum length of a MAC
and the minimum length of the last extension field to avoid an ambiguity
in parsing of the packet.

This removes an ugly hack that was needed to accept packets that
contained one or more extension fields without a MAC, before RFC 7822
was written and NTP implementations started using truncated MACs.

The long MACs were used by chrony in versions 2.x when configured to
authenticate a server or peer with a key using a 256-bit or longer hash
(e.g. SHA256). For compatibility with chrony >= 4.0, these clients/peers
will need to have "version 3" added to the server/peer line in
chrony.conf.
This commit is contained in:
Miroslav Lichvar
2020-09-10 10:22:27 +02:00
parent 2bb88b45c6
commit fdfcabd79b
3 changed files with 20 additions and 30 deletions
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12
ntp_core.c
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@@ -2095,15 +2095,6 @@ NCR_ProcessRxUnknown(NTP_Remote_Address *remote_addr, NTP_Local_Address *local_a
CLG_LogAuthNtpRequest();
}
/* If it is an NTPv4 packet with a long MAC and no extension fields,
respond with an NTPv3 packet to avoid breaking RFC 7822 and keep
the length symmetric. Otherwise, respond with the same version. */
if (info.version == 4 && info.ext_fields == 0 && info.auth.mode == NTP_AUTH_SYMMETRIC &&
info.auth.mac.length > NTP_MAX_V4_MAC_LENGTH)
version = 3;
else
version = info.version;
local_ntp_rx = local_ntp_tx = NULL;
tx_ts = NULL;
interleaved = 0;
@@ -2133,6 +2124,9 @@ NCR_ProcessRxUnknown(NTP_Remote_Address *remote_addr, NTP_Local_Address *local_a
poll = CLG_GetNtpMinPoll();
poll = MAX(poll, message->poll);
/* Respond with the same version */
version = info.version;
/* Send a reply */
transmit_packet(my_mode, interleaved, poll, version, kod, NULL,
&message->receive_ts, &message->transmit_ts,