Internal Domains now support three modes. "os" is unchanged.
"resolvers" becomes explicit resolver with network fallback: the
configured resolvers are tried first, and a timeout, unreachable
resolver, SERVFAIL or NXDOMAIN hands the query to matching VPN DNS
servers, then domain-less VPN DNS servers, then the OS resolver's LAN
nameservers through the new LanOnlyQueryCtx. "resolvers_only" keeps
the previous strict behavior. Down fallback-mode resolvers are skipped
and re-checked in the background at most every 30s. Generated upstreams
are identified by the UpstreamConfig.InternalDomain marker instead of
the internal_ key prefix.
Master adaptations: the fallback state lives on proxyRequest and is
applied in tryUpstreams, shouldContinueWithNextUpstream and
handleAllUpstreamsFailure instead of v1.0's monolithic proxy loop.
Resolver construction uses master's context-aware NewResolver, and the
background re-check marks its context private so transport errors that
name the resolver stay at debug. Tests use master's zap JSON capture
and VPN manager logger.
Port the final merged change from !1009 to master, retaining master's
context-aware logger/resolver APIs, proxy decomposition and firewall
refresh handling. Adapt source regression tests and cover master's
refresh handler and transport-level private-address diagnostics.
Dev Scribe