IsDomainName checks if a string is a presentation-format domain name (currently restricted to hostname-compatible "preferred name" LDH labels and SRV-like "underscore labels"; see golang.org/issue/12421).

ParseCIDR parses s as a CIDR notation IP address and prefix length, like "192.0.2.0/24" or "2001:db8::/32", as defined in RFC 4632 and RFC 4291.

RandomUserAgent generate random User-Agent.

DefaultHttpClient default http client use http.Client timeout 5 Second ignore certificate warnings.

USER_AGENT http header User-Agent's.