Exercise: Understanding and Examining Bro Logs

During the course of its normal operation, Bro produces a large volume of log files. This series of exercises examines the Bro log output format, and highlights a few extremely useful utilities that can be used to extract data from and/or process this information.

Part 1: Generating Logs

Exercise

Run bro with the -r option, and provide the http.pcap file. For more information on bro options, please run bro with the -h option.

Note

Logs will be generated in the current working directory!

Solution

mkdir /tmp/bro-logs
cd /tmp/bro-logs
bro -r traces/http.pcap

Part 2: Matching Records to Log Fields

Exercise

For this, you’ll need misc.pcap.

Run this command:

bro -r misc.pcap

and then interpret the fields in each of the resulting logs. Examine relevant records in the associated script files; be sure to look for the &log directive when examining those files.

Note

Record definitions can normally be found in $PREFIX/share/bro/base/protocols/<PROTO>/main.bro in the in the installation directory (or scripts/base/protocols/... in the Bro source tree).

Part 3: Connection Statistics

Bro summarizes each TCP and UDP connection as a single line in the conn.log. Because these connection summaries are quite detailed, you can extract plenty useful statistics from it. For the following two parts, use the log files generated from the trace 2009-M57-day11-18.trace.gz via bro -r 2009-M57-day11-18.trace.pcap.

Exercise

List the connections by in increasing order of duration, i.e., the longest connections at the end.

Solution

awk 'NR > 4' < conn.log | sort -t$'\t' -k 9 -n

The duration field records the number of seconds per connection. Its location is by default field number 9. Because we want the whole line without the comments, we skip the first four lines. Then we sort by the same field (-k 9) numerically (-n).

138     udp     -       -       -       -       S0      -       0       D       1       229     0       0
1258532683.876479       6XCmETILKQj     192.168.1.103   138     192.168.1.255   138     udp     -       -       -       -       S0      -       0       D       1       240     0       0
1258532824.338291       Up71DFBWvn9     192.168.1.104   138     192.168.1.255   138     udp     -       -       -       -       S0      -       0       D       1       229     0       0
1258533406.310783       t3XZuevetC4     192.168.1.103   138     192.168.1.255   138     udp     -       -       -       -       S0      -       0       D       1       240     0       0
1258533546.501981       l26lscUQs05     192.168.1.104   138     192.168.1.255   138     udp     -       -       -       -       S0      -       0       D       1       229     0       0
1258533745.340248       7mcJ1A7fK8f     192.168.1.1     5353    224.0.0.251     5353    udp     -       -       -       -       S0      -       0       D       1       105     0       0
1258533766.050097       Ah3IckDD2p4     192.168.1.102   138     192.168.1.255   138     udp     -       -       -       -       S0      -       0       D       1       229     0       0
1258534161.354320       0tze0IPUea6     192.168.1.102   1180    68.216.79.113   37      tcp     -       -       -       -       S0      -       0       S       1       48      0       0
...

Exercise

Find all connections that are longer than one minute.

Solution

awk 'NR > 4 && $9 > 60' conn.log

We look again at field number 9, but this time add another filter to display only those lines whose duration is greater than 60 seconds.

1258535660.158200       qho9DFjZlob     192.168.1.104   1196    65.55.184.16    443     tcp     ssl     67.887666       57041   8510    RSTR    -       0       ShADdar 54      59209   26      9558
1258543996.442969       Y5tkDNnwDci     192.168.1.103   138     192.168.1.255   138     udp     -       60.629434       560     0       S0      -       0       D       3       644     0       0
1258551306.134546       enTlgt1EdPg     192.168.1.104   138     192.168.1.255   138     udp     -       61.005932       549     0       S0      -       0       D       3       633     0       0
1258561885.476082       7PMLf3OCwgh     192.168.1.105   49210   65.55.184.155   443     tcp     ssl     66.419106       55531   7475    RSTR    -       0       ShADdar 52      57623   21      8323
1258562522.926514       RLxZxpgN0X4     192.168.1.104   1386    74.125.164.85   80      tcp     http    63.735504       683     30772   SF      -       0       ShADadfF        13      1211    28      31900
1258562636.223671       dYR9bmgCfqf     192.168.1.104   1387    74.125.164.85   80      tcp     http    65.450666       694     11708   SF      -       0       ShADadfF        9       1062    14      12276
1258562701.674828       jnI3OrUbadg     192.168.1.104   1423    74.125.164.85   80      tcp     http    65.169595       3467    60310   SF      -       0       ShADadfF        21      4315    54      62478
1258562522.748378       y0HGSpY912g     192.168.1.104   1385    74.125.19.102   80      tcp     http    244.158006      950     1800    SF      -       0       ShADadfF        6       1198    6       2048
1258562766.844923       tJHQEO0r3I4     192.168.1.104   1424    74.125.164.85   80      tcp     http    75.058910       3384    83613   SF      -       0       ShADadfF        23      4312    72      86501
1258562679.607431       rtqNEtM6mu2     192.168.1.104   1413    74.125.19.148   80      tcp     http    252.293422      427     347     SF      -       0       ShADadfF        6       675     6       595
...

Exercise

Find all IP addresses of web servers that send more than 1 MB back to a client.

Solution

bro-cut service resp_bytes id.resp_h < conn.log \
    | awk '$1 == "http" && $2 > 1000000 { print $3 }' \
    | sort -u

First, we extract the relevant fields from the conn.log, which are id.resp_h, service, and resp_bytes. The idea is to filter all connections labeled as HTTP where the responder (i.e., the server) sent more than 1,000,000 bytes.

Recall awk‘s pattern-action statement, wich looks like pattern { action }. The filter conditions appear in the pattern, whereas the print directives in the action. Here, we print only the third field that we extracted with bro-cut, namely id.resp_h. Finally, we weed out duplicates via sort -u.

130.59.10.36
137.226.34.227
151.207.243.129
193.1.193.64
198.189.255.73
198.189.255.74
198.189.255.82
208.111.128.122
208.111.129.48
208.111.129.62
65.54.95.201
65.54.95.209
65.54.95.7
68.142.123.21
68.142.123.31

Exercise

Are there any web servers on non-standard ports (i.e., 80 and 8080)?

Solution

bro-cut service id.resp_p id.resp_h < conn.log \
    | awk '$1 == "http" && ! ($2 == 80 || $2 == 8080) { print $3 }' \
    | sort -u

This awk exercise is similar to the above in terms of complexity, with the only difference being a different filter expression. The output is empty, meaning that Bro did not find any web servers on non-standard ports in this trace.

Exercise

Show a breakdown of the number of connections by service.

Solution

bro-cut service < conn.log | sort | uniq -c | sort -n

This is a typical aggregation question. The standard procedure almost always contains a combination of sort and uniq. The main idea is to massage the lines such that sorting and counting them yields a reasonable output. The advantage of this approach is that it does not accumulate any in-memory state and can rely on external sorting, which is imperative for large sets of logs.

One can also think about these aggregation tasks as a MapReduce job, where the first part of the pipeline is the map phase, sort the shuffle phase, and uniq a primitive reducer.

   2 ftp
   2 ftp-data
  21 smtp
 113 ssl
1786 -
2386 http
3992 dns

Exercise

Show the top 10 destination ports in descending order.

Solution

bro-cut id.resp_p < conn.log | sort | uniq -c | sort -rn | head -n 10

In the spirit as above, we aggregate the destination ports and sort the final output again to emit only the top 10 values.

3455 53
2730 80
 776 138
 553 137
 196 67
 189 139
  87 5353
  73 443
  62 37
  53 995

Exercise

What are the top 10 hosts (originators) that send the most traffic?

Solution

bro-cut id.orig_h orig_bytes < conn.log             \
    | sort                                          \
    | awk '{ if (host != $1) {                      \
                 if (size != 0)                     \
                     print $1, size;                \
                  host=$1;                          \
                  size=0                            \
              } else                                \
                  size += $2                        \
            }                                       \
            END {                                   \
                if (size != 0)                      \
                     print $1, size                 \
                }'                                  \
    | sort -k 2                                     \
    | head -n 10

This is a more involved example with a more complicated "reducer" function. The main idea is to order the output such that the traffic of one host is grouped together. Each group can then processed with constant space in awk by only maintaining two variables host and size. Finally, once we have the per-host aggregate of the sent volume, we sort the second field (-k 2) and display the top 10 entries.

192.168.1.103 1079461
192.168.1.104 1332571
192.168.1.105 2050085
192.168.1.102 207289
192.168.1.1 2172
169.254.173.77 6116
192.168.1.105 800067

Part 4: HTTP Statistics

Exercise

What are the distinct browsers in this trace? What are the distinct MIME types of the downloaded URLS?

Solution

bro-cut user_agent < http.log | sort -u
bro-cut resp_mime_types < http.log | sort -u

First, we extract the relevant field with bro-cut and then restrict the output to the distinct values. The query is not very complicated, yet can still be quite insightful.

AVGDM-
AVGDM-WVSXX86 85 BUILD=39 LOC=1033 BRD=cnet-0-0
AVGDM-WVSXX86 85 BUILD=40 LOC=1033 PRD=US-F-AVF
AVGINET9-WVSXX86 90 AVI=270.14.71/2510 BUILD=707 LOC=1033 LIC=9I-ASXNN-X4WGW-M0XFR-T84VX-3VX02 DIAG=51E OPF=0 PCA=
AVGINET9-WVSXX86 90 AVI=270.14.72/2511 BUILD=707 LOC=1033 LIC=9I-ASXNN-X4WGW-M0XFR-T84VX-3VX02 DIAG=51E OPF=0 PCA=
AVGINET9-WVSXX86 90FREE AVI=270.14.73/2512 BUILD=707 LOC=1033 LIC=9AVFREE-VKPCB-6BWFM-TRLQR-BRUHP-CP86G DIAG=310 OPF=0 PCA=
AVGINET9-WXPPX86 90 AVI=270.14.71/2510 BUILD=707 LOC=1033 LIC=9I-ASXNN-X4WGW-M0XFR-T84VX-3VX02 DIAG=51E OPF=0 PCA=
AVGINET9-WXPPX86 90 AVI=270.14.72/2511 BUILD=707 LOC=1033 LIC=9I-ASXNN-X4WGW-M0XFR-T84VX-3VX02 DIAG=51E OPF=0 PCA=
AVGINET9-WXPPX86 90 AVI=270.14.73/2512 BUILD=707 LOC=1033 LIC=9I-ASXNN-X4WGW-M0XFR-T84VX-3VX02 DIAG=51E OPF=0 PCA=
Google Update/1.2.183.13;winhttp
Google Update/1.2.183.13;winhttp;cup
JNLP/6.0 javaws/1.6.0_16 (b01) Java/1.6.0_16
MSDW
Microsoft BITS/7.0
Microsoft NCSI
Microsoft-CryptoAPI/5.131.2600.5512
Microsoft-CryptoAPI/6.0
Microsoft-WebDAV-MiniRedir/6.0.6002
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.0.04506)
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0)
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.23) Gecko/20090812 Lightning/0.9 Thunderbird/2.0.0.23
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.23) Gecko/20090812 Thunderbird/2.0.0.23
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/532.0 (KHTML, like Gecko) Chrome/3.0.195.33 Safari/532.0
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
SCSDK-6.0.0
Shockwave Flash
Windows-Update-Agent
clamav/0.92.1
jupdate
live-client/2.0

-
application/octet-stream
application/octet-stream,application/octet-stream
application/pdf
application/vnd.ms-cab-compressed
application/vnd.ms-cab-compressed,application/octet-stream
application/x-123
application/x-dosexec
application/x-elc
application/xml
application/x-shockwave-flash
image/gif
image/jpeg
image/png
image/x-icon
image/x-ms-bmp
text/html
text/plain
text/troff
text/x-c
video/x-flv

Exercise

What are the three most commonly accessed web sites?

Solution

bro-cut host < http.log | sort | uniq -c | sort -n | tail -n 3

In this case, we are interested in the Host header of the HTTP request, which the http.log provides in the host field. We interpret the "most commonly accessed" phrase as number of requests, i.e., number of lines in the log file. The aggregation is similar to what we have seen in the previous part.

231 safebrowsing-cache.google.com
259 scores.espn.go.com
421 download.windowsupdate.com

Exercise

What are the top 10 referred hosts?

Solution

bro-cut referrer < http.log                     \
    | awk 'sub(/[[:alpha:]]+:\/\//, "", $1)     \
           {                                    \
               split($1, s, /\//);              \
               print s[1]                       \
           }'                                   \
    | sort                                      \
    | uniq -c                                   \
    | sort -rn                                  \
    | head -n 10

Although the value of the Referer (sic) header is readily available via the referrer field in the http.log, it may not be in the appropriate format. For example, sometimes we observe values containing a full URL path, and sometimes just the host. Therefore, we perform an extra sanitization step that strips an optional protocol part (sub) and then extracts only the value of the host name of the referring URL.

275 adsatt.espn.go.com
234 espn.go.com
230 www.google.com
217 co108w.col108.mail.live.com
165 www.carmax.com
160 www.toysrus.com
139 support.dell.com
122 www.engadget.com
120 sports.espn.go.com
117 www.msn.com

Part 5: Tweaking Log Output

Exercise

Tell Bro to include the new_separator.bro script, and then re-process http.pcap. After verifying that the separator character has, in fact, changed, modify the separator character defined in new_separator.bro to be something slightly more interesting. Next, re-run Bro and verify that the separator character worked as expected and that the #separator field at the top of the file was updated appropriately. Now, add a line to new_separator.bro that will change the comment character used in the log file; consult base/frameworks/logging/writers/ascii.bro to determine the appropriate incantation.

Solution

Your new_separator.bro should look something like:

redef LogAscii::separator = ",";
redef LogAscii::meta_prefix = "//";

Note

While bro may accept a two-character separator, keep in mind that some parsers may not understand how to correctly parse a CSV file that uses a string of characters to separate individual fields.


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