Reading Zeek Log Formats

Zed is capable of reading both common Zeek log formats. This document provides guidance for what to expect when reading logs of these formats using the Zed command line tools.

Zeek TSV

Zeek TSV is Zeek's default output format for logs. This format can be read automatically (i.e., no -i command line flag is necessary to indicate the input format) with the Zed tools such as zq.

The following example shows a TSV conn.log being read via zq and output as ZSON.

conn.log

#separator \x09
#set_separator  ,
#empty_field    (empty)
#unset_field    -
#path   conn
#open   2019-11-08-11-44-16
#fields ts  uid id.orig_h   id.orig_p   id.resp_h   id.resp_p   proto   service duration    orig_bytes  resp_bytes  conn_state  local_orig  local_resp  missed_bytes    history orig_pkts   orig_ip_bytes   resp_pkts   resp_ip_bytes   tunnel_parents
#types  time    string  addr    port    addr    port    enum    string  interval    count   count   string  bool    bool    count   string  count   count   count   count   set[string]
1521911721.255387   C8Tful1TvM3Zf5x8fl  10.164.94.120   39681   10.47.3.155 3389    tcp -   0.004266    97  19  RSTR    -   -   0   ShADTdtr    10  730 6   342 -

Example

zq -Z 'head 1' conn.log

Output

{
    _path: "conn",
    ts: 2018-03-24T17:15:21.255387Z,
    uid: "C8Tful1TvM3Zf5x8fl",
    id: {
        orig_h: 10.164.94.120,
        orig_p: 39681 (port=uint16),
        resp_h: 10.47.3.155,
        resp_p: 3389 (port)
    },
    proto: "tcp" (=zenum),
    service: null (string),
    duration: 4.266ms,
    orig_bytes: 97 (uint64),
    resp_bytes: 19 (uint64),
    conn_state: "RSTR",
    local_orig: null (bool),
    local_resp: null (bool),
    missed_bytes: 0 (uint64),
    history: "ShADTdtr",
    orig_pkts: 10 (uint64),
    orig_ip_bytes: 730 (uint64),
    resp_pkts: 6 (uint64),
    resp_ip_bytes: 342 (uint64),
    tunnel_parents: null (|[string]|)
}

Other than Zed, Zeek provides one of the richest data typing systems available and therefore such records typically need no adjustment to their data types once they've been read in as is. The Zed/Zeek Data Type Compatibility document provides further detail on how the rich data types in Zeek TSV map to the equivalent rich types in Zed.

Zeek NDJSON

As an alternative to the default TSV format, there are two common ways that Zeek may instead generate logs in NDJSON format.

1. Using the JSON Streaming Logs package (recommended for use with Zed)
2. Using the built-in ASCII logger configured with redef LogAscii::use_json = T;

In both cases, Zed tools such as zq can read these NDJSON logs automatically as is, but with caveats.

Let's revisit the same conn record we just examined from the Zeek TSV log, but now as NDJSON generated using the JSON Streaming Logs package.

conn.ndjson

{"_path":"conn","_write_ts":"2018-03-24T17:15:21.400275Z","ts":"2018-03-24T17:15:21.255387Z","uid":"C8Tful1TvM3Zf5x8fl","id.orig_h":"10.164.94.120","id.orig_p":39681,"id.resp_h":"10.47.3.155","id.resp_p":3389,"proto":"tcp","duration":0.004266023635864258,"orig_bytes":97,"resp_bytes":19,"conn_state":"RSTR","missed_bytes":0,"history":"ShADTdtr","orig_pkts":10,"orig_ip_bytes":730,"resp_pkts":6,"resp_ip_bytes":342}

Example

zq -Z 'head 1' conn.ndjson

Output

{
    _path: "conn",
    _write_ts: "2018-03-24T17:15:21.400275Z",
    ts: "2018-03-24T17:15:21.255387Z",
    uid: "C8Tful1TvM3Zf5x8fl",
    "id.orig_h": "10.164.94.120",
    "id.orig_p": 39681,
    "id.resp_h": "10.47.3.155",
    "id.resp_p": 3389,
    proto: "tcp",
    duration: 0.004266023635864258,
    orig_bytes: 97,
    resp_bytes: 19,
    conn_state: "RSTR",
    missed_bytes: 0,
    history: "ShADTdtr",
    orig_pkts: 10,
    orig_ip_bytes: 730,
    resp_pkts: 6,
    resp_ip_bytes: 342
}

When we compare this to the TSV example, we notice a few things right away that all follow from the records having been previously output as JSON.

1. The timestamps like _write_ts and ts are printed as strings rather than the Zed time type.
2. The IP addresses such as id.orig_h and id.resp_h are printed as strings rather than the Zed ip type.
3. The connection duration is printed as a floating point number rather than the Zed duration type.
4. The keys for the null-valued fields in the record read from TSV are not present in the record read from NDJSON.

If you're familiar with the limitations of the JSON data types, it makes sense that Zeek chose to output these values in NDJSON as it did. Furthermore, if you were just seeking to do quick searches on the string values or simple math on the numbers, these limitations may be acceptable. However, if you intended to perform operations like aggregations with time-based grouping or CIDR matches on IP addresses, you would likely want to restore the rich Zed data types as the records are being read. The document on shaping Zeek NDJSON provides details on how this can be done.

The Role of _path

Zeek's _path field plays an important role in differentiating between its different log types (conn, dns, etc.) For instance, shaping Zeek NDJSON relies on the value of the _path field to know which Zed type to apply to an input NDJSON record.

If reading Zeek TSV logs or logs generated by the JSON Streaming Logs package, this _path value is provided within the Zeek logs. However, if the log was generated by Zeek's built-in ASCII logger when using the redef LogAscii::use_json = T; configuration, the value that would be used for _path is present in the log file name but is not in the NDJSON log records. In this case you could adjust your Zeek configuration by following the Log Extension Fields example from the Zeek docs. If you enter path in the locations where the example shows stream, you will see the field named _path populated just like was shown for the JSON Streaming Logs output.