fix: echo query id in legacy unicast responses (#496) - #498
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RFC 6762 §6.7 requires a legacy unicast response to echo the querier's message id. The id-echo machinery (`set_id`/`wire_id`) already existed, but `wire_id()` returns 0 whenever `multicast` is true, and nothing ever cleared that flag, so responses always went out with id 0. Standard DNS tools reject this, e.g. `dig` warns "ID mismatch: expected ID N, got 0". Add `DnsOutgoing::set_multicast()` and call it on the legacy-unicast path (`unicast_dest.is_some()`) so `wire_id()` emits the id set via `set_id()`. Multicast responses and the delayed-PTR path keep `multicast = true` and so correctly keep id 0. Extend `test_legacy_unicast_response` to send a query with a distinctive id and assert the response echoes it. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Fixes issue #496 part 1:
Problem:
RFC 6762 §6.7 requires a legacy unicast response to echo the querier's message id. The id-echo machinery already existed, but responses always went out with id 0.
Fix:
Add
DnsOutgoing::set_multicast()and call it on the legacy-unicast path sowire_id()emits the id set viaset_id().