2016-12-20 11:01



I have a program in go which accepts URLs from clients and gets them using the net/http package. Before doing further processing, I would like to check if the URL maps to private (non-routable / RFC1918 networks) address space.

The straight-forward way would be to perform an explicit DNS request and check the address for the known private ranges. After that, perform the HTTP GET request for the URL.

Is there a better way to accomplish this? Preferably integrating with http.Client so it can be performed as a part of the GET request.

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  • dtiopy6088 dtiopy6088 3年前

    You might also want to include checks for loopback (IPv4 or IPv6) and/or IPv6 link-local or unique-local addresses. Here is an example with a list of RFC1918 address plus these others and a simple check against them as isPrivateIP(ip net.IP):

    var privateIPBlocks []*net.IPNet
    func init() {
        for _, cidr := range []string{
            "",    // IPv4 loopback
            "",     // RFC1918
            "",  // RFC1918
            "", // RFC1918
            "::1/128",        // IPv6 loopback
            "fe80::/10",      // IPv6 link-local
            "fc00::/7",       // IPv6 unique local addr
        } {
            _, block, err := net.ParseCIDR(cidr)
            if err != nil {
                panic(fmt.Errorf("parse error on %q: %v", cidr, err))
            privateIPBlocks = append(privateIPBlocks, block)
    func isPrivateIP(ip net.IP) bool {
        for _, block := range privateIPBlocks {
            if block.Contains(ip) {
                return true
        return false
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  • dongling2545 dongling2545 5年前

    It seems there's no better way to accomplish than the one I described. Combining code from @MichaelHausenblas with the suggestion from @JimB, my code ended up kind of like this.

    func privateIP(ip string) (bool, error) {
        var err error
        private := false
        IP := net.ParseIP(ip)
        if IP == nil {
            err = errors.New("Invalid IP")
        } else {
            _, private24BitBlock, _ := net.ParseCIDR("")
            _, private20BitBlock, _ := net.ParseCIDR("")
            _, private16BitBlock, _ := net.ParseCIDR("")
            private = private24BitBlock.Contains(IP) || private20BitBlock.Contains(IP) || private16BitBlock.Contains(IP)
        return private, err
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