[Enhancement] routing response should support asis upstream

[douglarek]

Improvement Suggestion

Consider the following setup：

 routing {
        # According to the request of dns query, decide to use which DNS upstream.
        # Match rules from top to bottom.
        request {
            qname(suffix:corp1.com, suffix:corp2.com, suffix:corp3.com) -> asis
            # fallback is also called default.
            fallback: alidns
        }
        # According to the response of dns query, decide to accept or re-lookup using another DNS upstream.
        # Match rules from top to bottom.
        response {
            # Trusted upstream. Always accept its result.
            upstream(googledns) -> accept
            # Possibly polluted, re-lookup using googledns.
            ip(geoip:private) && !qname(geosite:cn) -> googledns
            # fallback is also called default.
            fallback: accept
        }
    }

These domain names(suffix:corp1.com, suffix:corp2.com, suffix:corp3.com) are internal company domains and will actually follow the response ip(geoip:private) && !qname(geosite:cn) -> googledns, which is normal but unnecessary.

There are two solutions, one is the cumbersome version:

ip(geoip:private) && !qname(geosite:cn, suffix:corp1.com, suffix:corp2.com, suffix:corp3.com) -> googledns

The second one(currently not supported, it's just as described in the issue title):

upstream(asis) -> accept # level=fatal msg="failed to build DNS response routing: failed to parse 'upstream(asis, googledns)': upstream "asis" not found; please define it in "dns.upstream""

Is it possible that asis is an upstream after all since it is considered built-in?

Potential Benefits

More convenient and reasonable.

[dae-prow]

Thanks for opening this issue!

[Mitsuhaxy]

asia不是一个固定的upstream，是根据原始dns请求里的目标dns服务器确定的。

[douglarek]

asia不是一个固定的upstream，是根据原始dns请求里的目标dns服务器确定的。

Besides the customized ones, the rest are probably as-is, which seems to be fixed as well.