[Bug network/19856] New: resolv: send_vc should perform unusable server checks like send_dg

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19856

            Bug ID: 19856
           Summary: resolv: send_vc should perform unusable server checks
                    like send_dg
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

In TCP mode, we do not currently detect servers which do not provide recursion.
 This causes problems if the UDP response signals availability of recursion
(RA=1) and truncation (TC=1), and the TCP response has RA=0.  This can happen
with a reverse DNS proxy.

Specifically, this check appears to be missing in the TCP client code:

                if (anhp->rcode == NOERROR && anhp->ancount == 0
                    && anhp->aa == 0 && anhp->ra == 0 && anhp->arcount == 0) {
                        DprintQ(statp->options & RES_DEBUG,
                                (stdout, "referred query:\n"),
                                *thisansp,
                                (*thisresplenp > *thisanssizp)
                                ? *thisanssizp : *thisresplenp);
                        goto next_ns;
                }

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