network: enable EDNS for DNS queries

[stek29]

Enable EDNS0 for all DNS queries to match c-ares defaults and support responses larger than 512 bytes.

Partially addresses #10588

[cosmo0920]

Hi,
The patch looks good but we need to follow DCO workflow.
So, could you add Signed-Off line by git add -s?
The detail is here: https://github.com/fluent/fluent-bit/pull/10589/checks?check_run_id=45884364947

[edsiper]

@stek29 thanks for this PR.

wondering what's the right path here and potential side-effects of this change. The biggest user of c-ares is Curl, however I cannot find a reference to this flag being used in Curl and wondering the reason about it:

• https://github.com/curl/curl/blob/master/lib/asyn-ares.c#L160-L192

looking at c-ares options init functionality, the flag ARES_FLAG_EDNS is not enabled by default:

• https://github.com/c-ares/c-ares/blob/71663adcd95acdb1c198806d626dfc8640d132be/src/lib/ares_init.c#L238

removed.. "why do we really need this flag ? " (edited)

looking at #10588 more in detail now

[stek29]

@edsiper just to clarify:

it is enabled by default in c-ares, but it's part of flags — so it's only enabled if ARES_OPT_FLAGS is unset:
https://github.com/c-ares/c-ares/blob/71663adcd95acdb1c198806d626dfc8640d132be/src/lib/ares_init.c#L132-L135

and most clients, including curl, just don’t set the ARES_OPT_FLAGS.
but fluent-bit does set this option, because it conditionally sets flag ARES_FLAG_USEVC:

fluent-bit/src/flb_network.c

Lines 1091 to 1100 in 9def01d

	/* c-ares options: Set the transport layer to the desired protocol */
	optmask = ARES_OPT_FLAGS;
	if (dns_mode == FLB_DNS_USE_TCP) {
	opts.flags = ARES_FLAG_USEVC;
	}
	*result = ares_init_options((ares_channel *) &lookup_context->ares_channel,
	&opts, optmask);