New Cygwin release 1.7 (which will be released in a few months) will contain IPv6 support, but rsync configure doesn't currently detect it. I'm using the following patch to enable the detection: --- origsrc/rsync-3.0.4/configure.in 2008-09-06 18:32:26.000000000 +0200 +++ src/rsync-3.0.4/configure.in 2009-02-15 20:38:41.532123700 +0100 @@ -198,8 +198,18 @@ [don't even try to use IPv6])) if test x"$enable_ipv6" != x"no"; then AC_MSG_CHECKING([ipv6 stack type]) - for i in inria kame linux-glibc linux-inet6 toshiba v6d zeta; do + for i in cygwin inria kame linux-glibc linux-inet6 toshiba v6d zeta; do case $i in + cygwin) + AC_EGREP_CPP(yes, [ +#include <netinet/in.h> +#ifdef _CYGWIN_IN6_H +yes +#endif], + [ipv6type=$i; + AC_DEFINE(INET6, 1, [true if you have IPv6]) + ]) + ;; inria) # http://www.kame.net/ AC_EGREP_CPP(yes, [ -- Lapo Luchini - http://lapo.it/