根据http://golang.org/pkg/net/#Dialer：

type Dialer struct {
        // Timeout is the maximum amount of time a dial will wait for
        // a connect to complete. If Deadline is also set, it may fail
        // earlier.
        //
        // The default is no timeout.
        //
        // With or without a timeout, the operating system may impose
        // its own earlier timeout. For instance, TCP timeouts are
        // often around 3 minutes.

因此，默认超时（不考虑操作系统施加的限制）为无。

可以使用设置超时SetDeadline。

我认为默认的OSX超时可以通过进行检查sysctl net.inet.tcp。