问获取客户端IP地址: REMOTE_ADDR、HTTP_X_FORWARDED_FOR，还有什么有用的？

EN

public static class ClientIP
{
    // based on http://www.grantburton.com/2008/11/30/fix-for-incorrect-ip-addresses-in-wordpress-comments/
    public static string ClientIPFromRequest(this HttpRequestBase request, bool skipPrivate)
    {
        foreach (var item in s_HeaderItems)
        {
            var ipString = request.Headers[item.Key];

        if (String.IsNullOrEmpty(ipString))
            continue;

        if (item.Split)
        {
            foreach (var ip in ipString.Split(','))
                if (ValidIP(ip, skipPrivate))
                    return ip;
        }
        else
        {
            if (ValidIP(ipString, skipPrivate))
                return ipString;
        }
    }

    return request.UserHostAddress;
}

private static bool ValidIP(string ip, bool skipPrivate)
{
    IPAddress ipAddr;

    ip = ip == null ? String.Empty : ip.Trim();

    if (0 == ip.Length
        || false == IPAddress.TryParse(ip, out ipAddr)
        || (ipAddr.AddressFamily != AddressFamily.InterNetwork
            && ipAddr.AddressFamily != AddressFamily.InterNetworkV6))
        return false;

    if (skipPrivate && ipAddr.AddressFamily == AddressFamily.InterNetwork)
    {
        var addr = IpRange.AddrToUInt64(ipAddr);
        foreach (var range in s_PrivateRanges)
        {
            if (range.Encompasses(addr))
                return false;
        }
    }

    return true;
}

/// <summary>
/// Provides a simple class that understands how to parse and
/// compare IP addresses (IPV4) ranges.
/// </summary>
private sealed class IpRange
{
    private readonly UInt64 _start;
    private readonly UInt64 _end;

    public IpRange(string startStr, string endStr)
    {
        _start = ParseToUInt64(startStr);
        _end = ParseToUInt64(endStr);
    }

    public static UInt64 AddrToUInt64(IPAddress ip)
    {
        var ipBytes = ip.GetAddressBytes();
        UInt64 value = 0;

        foreach (var abyte in ipBytes)
        {
            value <<= 8;    // shift
            value += abyte;
        }

        return value;
    }

    public static UInt64 ParseToUInt64(string ipStr)
    {
        var ip = IPAddress.Parse(ipStr);
        return AddrToUInt64(ip);
    }

    public bool Encompasses(UInt64 addrValue)
    {
        return _start <= addrValue && addrValue <= _end;
    }

    public bool Encompasses(IPAddress addr)
    {
        var value = AddrToUInt64(addr);
        return Encompasses(value);
    }
};

private static readonly IpRange[] s_PrivateRanges =
    new IpRange[] { 
            new IpRange("0.0.0.0","2.255.255.255"),
            new IpRange("10.0.0.0","10.255.255.255"),
            new IpRange("127.0.0.0","127.255.255.255"),
            new IpRange("169.254.0.0","169.254.255.255"),
            new IpRange("172.16.0.0","172.31.255.255"),
            new IpRange("192.0.2.0","192.0.2.255"),
            new IpRange("192.168.0.0","192.168.255.255"),
            new IpRange("255.255.255.0","255.255.255.255")
    };


/// <summary>
/// Describes a header item (key) and if it is expected to be 
/// a comma-delimited string
/// </summary>
private sealed class HeaderItem
{
    public readonly string Key;
    public readonly bool Split;

    public HeaderItem(string key, bool split)
    {
        Key = key;
        Split = split;
    }
}

// order is in trust/use order top to bottom
private static readonly HeaderItem[] s_HeaderItems =
    new HeaderItem[] { 
            new HeaderItem("HTTP_CLIENT_IP",false),
            new HeaderItem("HTTP_X_FORWARDED_FOR",true),
            new HeaderItem("HTTP_X_FORWARDED",false),
            new HeaderItem("HTTP_X_CLUSTER_CLIENT_IP",false),
            new HeaderItem("HTTP_FORWARDED_FOR",false),
            new HeaderItem("HTTP_FORWARDED",false),
            new HeaderItem("HTTP_VIA",false),
            new HeaderItem("REMOTE_ADDR",false)
    };
}