This option was introduced here: http://sourceware.org/ml/libc-alpha/2009-10/msg00063.html
However, there appears to be no documentation for it. Folks are going to be looking for this as they deploy systems behind firewalls and end up with delays due to DNS, see here: http://www.linuxquestions.org/questions/showthread.php?p=4399340#post4399340
So if you could please document the option, I at least would greatly appreciate it.
Patch from Petr Baudis applied for man-pages-3.40:
@@ -231,6 +231,19 @@ sets
.IR _res.options .
This enables support for the DNS extensions described in RFC\ 2671.
+.BR single-request " (since glibc 2.10)"
+.IR _res.options .
+By default, glibc performs IPv4 and IPv6 lookups in parallel since
+Some appliance DNS servers
+cannot handle these queries properly and make the requests time out.
+This option disables the behavior and makes glibc perform the IPv6
+and IPv4 requests sequentially (at the cost of some slowdown of the
The \fIdomain\fP and \fIsearch\fP keywords are mutually exclusive.