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protodetect: improve DCERPC UDP probing parser and simplify app-layer-detect-proto.c code v3 #11679

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protodetect: improve DCERPC UDP probing parser and simplify app-layer-detect-proto.c code v3 #11679

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protodetect/dcerpc: improve DCERPC UDP probing parser
ilya-bakhtin Jul 21, 2024
fad7b6d
protodetect: simplify app-layer-detect-proto.c code since DCERPC UDP …
ilya-bakhtin Jul 21, 2024
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4 changes: 3 additions & 1 deletion rust/src/dcerpc/dcerpc_udp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -294,9 +294,11 @@ pub unsafe extern "C" fn rs_dcerpc_udp_get_tx_cnt(vtx: *mut std::os::raw::c_void
/// Probe input to see if it looks like DCERPC.
fn probe(input: &[u8]) -> (bool, bool) {
match parser::parse_dcerpc_udp_header(input) {
Ok((_, hdr)) => {
Ok((leftover_bytes, hdr)) => {
let is_request = hdr.pkt_type == 0x00;
let is_dcerpc = hdr.rpc_vers == 0x04 &&
hdr.fragnum == 0 &&
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leftover_bytes.len() >= hdr.fraglen as usize &&
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This looks good. This alone seems good enough to detect your test pcap correctly.
In addition to this, following wireshark's detection logic, possible header field to make probing stronger that isn't already covered:
pkt_type must be <= CANCEL_ACK (10) [this update alone fails the detection test on your s-v pcap though]

lmk wdyt?

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There seem to be more cf https://github.com/secdev/scapy/blob/master/scapy/layers/dcerpc.py#L138

    10: "cancel_ack",
    11: "bind",
    12: "bind_ack",
    13: "bind_nak",
    14: "alter_context",
    15: "alter_context_resp",
    16: "auth3",
    17: "shutdown",
    18: "co_cancel",
    19: "orphaned",

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Indeed there are. That condition check was used by wireshark for the detection so only the first pkt I think (probing). All the other types you mentioned are after a connection is established and some of these are even handled by our dcerpc parser like bind + bindack. This is why I thought of suggesting it.

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I would go without it in case we probe midstream...

(hdr.flags2 & 0xfc == 0) &&
(hdr.drep[0] & 0xee == 0) &&
(hdr.drep[1] <= 3);
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25 changes: 5 additions & 20 deletions src/app-layer-detect-proto.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1401,7 +1401,6 @@ AppProto AppLayerProtoDetectGetProto(AppLayerProtoDetectThreadCtx *tctx, Flow *f
(flags & STREAM_TOSERVER) ? "toserver" : "toclient");

AppProto alproto = ALPROTO_UNKNOWN;
AppProto pm_alproto = ALPROTO_UNKNOWN;

if (!FLOW_IS_PM_DONE(f, flags)) {
AppProto pm_results[ALPROTO_MAX];
Expand All @@ -1419,38 +1418,24 @@ AppProto AppLayerProtoDetectGetProto(AppLayerProtoDetectThreadCtx *tctx, Flow *f
FLOW_RESET_PP_DONE(f, reverse_dir);
}
}

/* HACK: if detected protocol is dcerpc/udp, we run PP as well
* to avoid misdetecting DNS as DCERPC. */
if (!(ipproto == IPPROTO_UDP && alproto == ALPROTO_DCERPC))
goto end;

pm_alproto = alproto;

/* fall through */
SCReturnUInt(alproto);
}
}

if (!FLOW_IS_PP_DONE(f, flags)) {
bool rflow = false;
alproto = AppLayerProtoDetectPPGetProto(f, buf, buflen, ipproto, flags, &rflow);
DEBUG_VALIDATE_BUG_ON(*reverse_flow);
alproto = AppLayerProtoDetectPPGetProto(f, buf, buflen, ipproto, flags, reverse_flow);
if (AppProtoIsValid(alproto)) {
if (rflow) {
*reverse_flow = true;
}
goto end;
SCReturnUInt(alproto);
}
}

/* Look if flow can be found in expectation list */
if (!FLOW_IS_PE_DONE(f, flags)) {
DEBUG_VALIDATE_BUG_ON(*reverse_flow);
alproto = AppLayerProtoDetectPEGetProto(f, flags);
}

end:
if (!AppProtoIsValid(alproto))
alproto = pm_alproto;

SCReturnUInt(alproto);
}

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