Recent versions of glibc will issue both ipv4 (A) and ipv6 (AAAA) address queries at the same time, from the same port. Sometimes upstream routers will systemically drop the ipv4 queries. The symptom of this problem is that 'getent hosts example.com' only returns ipv6 (or perhaps only ipv4) addresses. The workaround for this is to specify the option 'single-request' in /etc/resolv.conf. This option enables that.
option