Allow the use of TCP instead of UDP for queries by setting options usevc

in resolv.conf which sets RES_USEVC.

Reviewed by:	ume
MFC after:	17 days
Differential Revision:	https://reviews.freebsd.org/D16607

lib/libc/resolv/res_init.c

@@ -679,6 +679,8 @@ res_setoptions(res_state statp, const char *options, const char *source)
 		       statp->options |= RES_INSECURE2;
 		} else if (!strncmp(cp, "rotate", sizeof("rotate") - 1)) {
 			statp->options |= RES_ROTATE;
+		} else if (!strncmp(cp, "usevc", sizeof("usevc") - 1)) {
+			statp->options |= RES_USEVC;
 		} else if (!strncmp(cp, "no-check-names",
 				    sizeof("no-check-names") - 1)) {
 			statp->options |= RES_NOCHECKNAME;

share/man/man5/resolver.5

@@ -28,7 +28,7 @@
 .\"     @(#)resolver.5	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd October 12, 2015
+.Dd August 16, 2018
 .Dt RESOLVER 5
 .Os
 .Sh NAME
@@ -127,6 +127,10 @@ is one of the following:
 sets
 .Dv RES_DEBUG
 in _res.options.
+.It Sy usevc
+sets
+.Dv RES_USEVC
+to use TCP instead of UDP for queries.
 .It Sy ndots: Ns Ar n
 sets a threshold for the number of dots which must appear in a name given to
 .Fn res_query