[Snort-devel] Problem with custom preprocessor - FLAG_STREAM_INSERT set in all packets

Arun Koshal akoshal04 at ...2499...
Thu Feb 4 10:23:06 EST 2016


Hi,

I am working on a custom preprocessor which works on HTTP application. The
preprocessor extracts HTTP method and URI from the client GET requests.

In order to retrieve the method and URI, I am depending on the information
decoded by the http_inspect preprocessor and I am retrieving the method and
URI using _dpd.getHttpBuffer() function.

The problem I am facing is - the flag FLAG_STREAM_INSERT is set in all the
packets because of which I do not have method and URI updated in HTTP
buffer. I debugged Snort in GDB and could not find any packet having this
flag clear in any packet inside my preprocessor. Due to this reason my
preprocessor is not working. I am skipping the packets having
FLAG_STREAM_INSERT set using following code:

  if (sess->reassembly == 0) {
    _dpd.streamAPI->set_reassembly(p->stream_session,
STREAM_FLPOLICY_FOOTPRINT, SSN_DIR_FROM_CLIENT,
STREAM_FLPOLICY_SET_ABSOLUTE);
    sess->reassembly = 1;
  }

  if (p->flags & FLAG_STREAM_INSERT) {
    PREPROC_PROFILE_END(myproc_perf_stats);
    return;
  }

Even when I remove the check on FLAG_STREAM_INSERT, I do not get the method
and URI populated in HTTP buffer.

I also observed that when I increased the priority of my preprocessor, this
problem was not happening. But I had to reduce it to PRIORITY_APPLICATION
so that http_inspect is called before my preprocessor.

It seams the http_inspect is causing some problem. The configuration of
http_inspect preprocessor is as follows:

# HTTP normalization and anomaly detection.  For more information, see
README.http_inspect
preprocessor http_inspect: global iis_unicode_map unicode.map 1252
compress_depth 65535 decompress_depth 65535
preprocessor http_inspect_server: server default \
    http_methods { GET POST PUT SEARCH MKCOL COPY MOVE LOCK UNLOCK NOTIFY
POLL BCOPY BDELETE BMOVE LINK UNLINK OPTIONS HEAD DELETE TRACE TRACK
CONNECT SOURCE SUBSCRIBE UNSUBSCRIBE PROPFIND PROPPATCH BPROPFIND
BPROPPATCH RPC_CONNECT PROXY_SUCCESS BITS_POST CCM_POST SMS_POST
RPC_IN_DATA RPC_OUT_DATA RPC_ECHO_DATA } \
    chunk_length 500000 \
    server_flow_depth 0 \
    client_flow_depth 0 \
    post_depth 65495 \
    oversize_dir_length 500 \
    max_header_length 750 \
    max_headers 100 \
    max_spaces 200 \
    small_chunk_length { 10 5 } \
    ports { 80 81 311 383 591 593 901 1220 1414 1741 1830 2301 2381 2809
3037 3128 3702 4343 4848 5250 6988 7000 7001 7144 7145 7510 7777 7779 8000
8008 8014 8028 8080 8085 8088 8090 8118 8123 8180 8181 8243 8280 8300 8800
8888 8899 9000 9060 9080 9090 9091 9443 9999 11371 34443 34444 41080 50002
55555 } \
    non_rfc_char { 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 } \
    enable_cookie \
    extended_response_inspection \
    inspect_gzip \
    normalize_utf \
    unlimited_decompress \
    normalize_javascript \
    apache_whitespace no \
    ascii no \
    bare_byte no \
    directory no \
    double_decode no \
    iis_backslash no \
    iis_delimiter no \
    iis_unicode no \
    multi_slash no \
    utf_8 no \
    u_encode yes \
    webroot no

Please let me know if I need to do something to use http-inspect
preprocessor HTTP buffers for different HTTP parameters.

Any suggestion to resolve the issue will really be great help.

Thanks and regards,
Arun
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