Allow repeated IPFIX IEs and safe zero-length NetFlow v9 fields

[ktsaou]

Summary

• Accept repeated Information Elements in IPFIX templates, as required by RFC 7011 Section 8.
• Allow individual zero-length fields in NetFlow v9 templates and options templates when the complete record still has a nonzero size.
• Continue rejecting all-zero records and the IPFIX variable-length sentinel in v9 templates.
• Apply the same v9 validation rules to v9 templates carried through the IPFIX parser.

Motivation

The duplicate-field check rejects valid IPFIX templates that repeat the same Information Element. RFC 7011 explicitly requires collectors to support this pattern.

The v9 zero-length check rejects complete records that are safe to parse because another member consumes input. Rejecting only templates whose total record size is zero preserves the infinite-loop guard without discarding those records.

Tests

• Added end-to-end template and data parsing tests for both cases.
• Added validation tests for safe zero-length members and all-zero rejection.
• cargo test
• cargo clippy --all -- -D warnings
• cargo test --doc
• ./scripts/check-readme-sync.sh

RFC reference: https://www.rfc-editor.org/rfc/rfc7011.html#section-8

[mikemiles-dev]

Thanks for the contribution!