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import_headers

message:import_headers(SPECS)
Since: Dev Builds Only

The functionality described in this section requires a dev build of KumoMTA. You can obtain a dev build by following the instructions in the Installation section.

Iterates the headers of the message, importing matching header values into the message metadata. SPECS is an array of tables, one per header name or pattern, each describing how that header should be matched, named in the metadata, and optionally removed.

This is a more flexible alternative to message:import_x_headers. The same call can be used to import multiple distinct headers with different rules, and can also be used to strip the matched headers from the message body in a single pass.

Spec fields

Each entry in SPECS accepts the following fields:

  • name (required, string) — the header name to match.

Matching is always case-insensitive. The pattern can be either a precise header name (e.g. "Subject") or a name with a single trailing * wildcard (e.g. "X-*"), which matches any header whose name begins with the literal portion of the pattern. Bare *, leading wildcards, and interior wildcards are not supported.

  • match (optional, string, default "last") — which matching headers to capture:

    • "first" — capture the first matching header value as a string.
    • "last" — capture the last matching header value as a string.
    • "all" — capture every matching header value as an array of strings.

  • transform (optional, string, default "snake_case") — how to derive the metadata key from the matched header name. All transforms produce a deterministic key. Examples below assume a matched header named X-Campaign-Id:

    • "snake_case" — x_campaign_id (matches the transform performed by message:import_x_headers)
    • "kebab_case" — x-campaign-id
    • "camel_case" — xCampaignId
    • "pascal_case" — XCampaignId

  • target (optional, string) — explicitly set the metadata key to use, bypassing transform. Only valid when name is a precise header name; using target with a wildcard pattern produces an error.

  • remove (optional, bool, default false) — when true, the matched header instances are removed from the message body after their values have been captured.

If a spec produces no matches, no metadata is written for it (including for match = "all", which writes nothing rather than an empty array).

When more than one spec could match a header, the first spec in SPECS that matches wins. This lets you place a precise rule for a specific header in front of a broader wildcard catch-all.

Examples

Import all X- headers into metadata, and strip them from the message:

msg:import_headers {
  { name = 'X-*', remove = true },
}

Capture every Received: header as an array, while also lifting the subject:

msg:import_headers {
  { name = 'Received', match = 'all' },
  { name = 'Subject' },
}

Treat one specific header specially, while still importing the rest of the X- headers with the default transform:

msg:import_headers {
  { name = 'X-Campaign-Id', target = 'campaign_id' },
  { name = 'X-*' },
}

Use a different naming style:

msg:import_headers {
  { name = 'X-*', transform = 'camel_case' },
}
-- X-Campaign-Id is captured as `xCampaignId`

See Also