Question

I have an FTP server where it is returning an invalid IP Address for a PASV command. Filezilla automatically detects this. How do I validate an IP Address in C# to make sure it is routable?

Here is what filezilla has in the log:
Command: PASV
Response: 227 Entering Passive Mode (10,46,169,44,21,124).
Status: Server sent passive reply with unroutable address. Using server address instead.

Answer 1

For checking private addresses(biggest subset of non-routable addresses) you could something like:

public static bool IsPrivateAddress(this IPAddress addr)
{
    if(addr.AddressFamily == System.Net.Sockets.AddressFamily.InterNetworkV6)
    {
        return addr.IsIPv6LinkLocal || addr.IsIPv6SiteLocal;
    }
    var bytes = addr.GetAddressBytes();
    return
        ((bytes[0] == 10) ||
        ((bytes[0] == 192) && (bytes[1] == 168)) ||
        ((bytes[0] == 172) && ((bytes[1] & 0xf0)==16)));
}

there are more non routable addresses (e.g. loopback, multicast, and experimental blocks) but it seems unlikely you would see those.