6.2 The rare Command

6.2 The rare Command

Quick Answer: | rare <field> finds the least common values of a field. It uses the same options as top (default limit=10, plus count and percent). Choose rare when the exam question is about uncommon or outlier values—not averages or most-frequent rankings.

Topic 5.2 The rare command sits beside top in Domain 5.0 (15%). Splunk’s own reference states that rare operates identically to top, except it returns the least frequent values instead of the most frequent. If you already understand top, rare is mostly a direction flip—but the exam still tests whether you pick the right direction.

Why uncommon values matter

Operations and security workflows often care about rarity:

• Status codes that almost never appear may signal a new failure mode
• Usernames that show up once may be misconfigurations or probing
• Hosts that barely send a sourcetype may be offline or mis-forwarding
• Method/URI pairs seen once may be exploratory attacks or broken clients

top answers “what dominates?” rare answers “what barely shows up?” Both are frequency tools; neither replaces stats for sums and averages.

Syntax (same family as top)

... | rare [<options>] <field-list> [BY <field-list>]

Example:

index=web sourcetype=access_combined
| rare status

You get the least common status values (default ten), each with count and percent.

index=security
| rare limit=20 user BY action

This returns, for each action, up to twenty least common user values among events that reached the command.

Comparing top and rare side by side

Exam wording cues:

• most / common / frequent / popular / busiest → lean top
• least / rare / uncommon / infrequent / unusual → lean rare
• average / sum / distinct / min / max → lean stats

Worked troubleshooting scenarios

Scenario A — sparse HTTP statuses

index=web earliest=-7d
| rare limit=15 status

If almost all traffic is 200/304, rare surfaces odd codes (418, 451, custom app codes) that top would bury beneath the popular ones.

Scenario B — uncommon users failing login

index=auth action=failure
| rare limit=10 user

Here “rare” among failures means users who failed infrequently in the filtered set—useful when you already scoped to failures and want sparse usernames, not the noisiest attackers (top user would show those).

Scenario C — rare combinations

index=web
| rare limit=5 method, status

A combination like DELETE + 500 may be rarer (and more interesting) than either field alone.

Result shape and UI expectations

Like top, rare is a transforming command:

• Primary readable output is a Statistics table
• Default columns include the field(s) you named plus count and percent
• Percent is still relative to events that entered rare, after prior filters

Briefly: charting those rare-value tables comes later when you save reports/visualizations. Domain 5.2 tests that you can produce the rare-value table, not design a dashboard.

Limits and “all rare values”

limit=0 on this command family asks Splunk to return all qualifying tuples up to the configured maximum (documented ceiling tied to maxresultrows / limits settings). On the User exam, you rarely configure limits.conf; just know:

• Default presentation is 10
• You can raise or lower with limit=
• Extremely large limits are a memory/performance concern in real life—even if the exam only asks what the option does

Common traps

1. Picking rare when the stem says “most common.” Mirror-image mistakes are frequent on multiple-choice exams.
2. Thinking rare means “events with rare=true.” There is no such default field; rare is a command.
3. Using rare instead of a filter. If you only want status=500, filter first (status=500). rare ranks uncommon values; it does not mean “only error events.”
4. Forgetting BY scope. | rare user BY host is not the same as a global | rare user.
5. Assuming rare values are always security incidents. Rarity is a signal, not a verdict—exam answers stay at the SPL behavior level.

Decision tree for Domain 5.1–5.2

1. Need frequency ranking of field values? → top or rare
2. Most vs least? → choose direction
3. Need a number of rows other than 10? → set limit
4. Need per-category rankings? → add BY
5. Need math beyond frequency (sum/avg/dc)? → leave this pair and use stats

Internalize that tree and 5.2 questions reduce to reading the adjective in the stem (least vs most) and matching options.