[Bro-Dev] [JIRA] (BIT-1251) content_gap not being raised

Jon Siwek (JIRA) jira at bro-tracker.atlassian.net
Mon Sep 15 10:45:07 PDT 2014

    [ https://bro-tracker.atlassian.net/browse/BIT-1251?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18100#comment-18100 ] 

Jon Siwek commented on BIT-1251:

"content_gap" is intentionally (or maybe just historically... hard to tell) not raised in some situations.  Particularly if Bro doesn't consider a connection fully established, it may not raise it unless you "redef report_gaps_for_partial=T;".  In this case, the default behavior has a detrimental interaction with TCP reassembly -- we miss a segment from the server and start buffering further segments (because we don't know for sure if it will come out of order later), the server sends a FIN and so Bro considers the server endpoint closed, finally the client ACKs enough that we can tell there's a gap, but we don't raise "content_gap" because the connection is no longer "fully established".

Addressing some root problems here might take significant changes/rewrite of the TCP analysis, so you might check in to the workaround of redef'ing "report_gaps_for_partial" (source code comments claim a downside of that is raising "content_gap" falsely in more situations, but not sure how common/relevant that still is).

> content_gap not being raised
> ----------------------------
>                 Key: BIT-1251
>                 URL: https://bro-tracker.atlassian.net/browse/BIT-1251
>             Project: Bro Issue Tracker
>          Issue Type: Problem
>          Components: Bro
>    Affects Versions: git/master
>         Environment: Fedora 20
>            Reporter: Jimmy Jones
>         Attachments: analyser.bro, test5-10mb.pcap.gz
> Using the attached bro script, I extract out the http response, which contains some null padding for the missing packets. However the content_gap event never gets raised, despite http_entity_data not receiving the entire download.

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