I recently worked on a rWhois project that required writing an ASP.NET MVC 4 based admin portal. One of the last things I tackled was adding validation to inputs. Adding validation to a MVC model is super-simple with Data Annotations. By adding these annotations to your model, both client and server-side validation comes to life. There’s a great tutorial describing how to accomplish this on the asp.net site:

http://www.asp.net/mvc/tutorials/mvc-4/getting-started-with-aspnet-mvc4/adding-validation-to-the-model

The in-box DataAnnotations class provides numerous built-in validation attributes that can be applied to any property. Things like strings of specific length, or integers within a range, or even regex matching is supported using the built-in validation attributes – and they are automatically wired up when you use Visual Studio to add views with scaffolding. But what about when the built-in attributes don’t quite do what you need?

I needed to create validation that confirms the input is both in proper CIDR format (ie. 10.0.0.0/8 and not “some text”) and that the supplied CIDR subnet is valid (ie. 192.168.100.0/24 is a proper network/bitmask, but 192.168.100.50/24 is not). Validating that the input is in the proper format can easily be done with regex. Just add the following to annotation to the appropriate property on your model:

[RegularExpression(@"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])(\/(\d|[1-2]\d|3[0-2]))$", ErrorMessage="Not valid CIDR format")]

To check that the supplied input is a valid network/bitmask CIDR combination, I needed custom validation. You can easily extend the ValidationAttribute and implement IClientValidatable to add your custom validation. First, you’ll want to create a new class and reference the DataAnnotations and Mvc namespaces:

using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;

namespace MvcApplication1.Validation
{
    public class CIDRSubnet : ValidationAttribute, IClientValidatable
    {

    }
}

Next, we’ll want to create the appropriate server-side validation by overriding the ValidationResult class and adding the CIDR subnet check:

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        //validate that valus is valid cdir
        string[] cidr = ((string)value).Split(new char[] { '/' });
        string[] ip = cidr[0].Split(new char[] { '.' });
        int i = (Convert.ToInt32(ip[0]) << 24) | (Convert.ToInt32(ip[1]) << 16) | (Convert.ToInt32(ip[2]) << 8) | (Convert.ToInt32(ip[3]));
        int mask = Convert.ToInt32(cidr[1]) == 0 ? 0 : ~((1 << (32-29)) - 1);
        int network = i & mask;

        if (i != network) return new ValidationResult("Not a valid CIDR Subnet");
        return ValidationResult.Success;
    }

Now we want to create our client side validation function. Create a new javascript file and make sure it’s referenced in your View:

function ipv4checker(input) {
    //confirm cidr is valid network
    var cidr = input.split('/');
    var base = cidr[0];
    var bits = cidr[1];
    var ip = base.split('.');
    var i = (ip[0] << 24 | ip[1] << 16 | ip[2] << 8 | ip[3]);
    var mask = bits == 0 ? 0 : ((1 << (32 - bits)) - 1) ^ 0xFFFFFFFF;
    var network = i & mask;

    if (i == network) { return true; }
    else { return false; }
}

Assuming jquery and jquery unobtrusive validation libraries have already been added to your project, you can then add a method to the validator, and wire it up. I’m not passing any parameters here, but you could also include parameters:

//custom vlidation rule - check IPv4
$.validator.addMethod("checkcidr",
    function (value, element, param) {
    return ipv4checker(value);
    });

//wire up the unobtrusive validation
$.validator.unobtrusive.adapters.add
    ("checkcidr", function (options) {
        options.rules["checkcidr"] = true;
        options.messages["checkcidr"] = options.message;
    });

Then, you’ll need to implement the GetClientValidationRules method in your custom validation class:

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        ModelClientValidationRule rule = new ModelClientValidationRule();
        rule.ValidationType = "checkcidr";
        rule.ErrorMessage = "Not a valid CIDR Subnet";
        return new List<ModelClientValidationRule> { rule };
    }

Lastly, add a reference to your custom validation class in the model and add the custom validation attribute to your property:

using System.ComponentModel.DataAnnotations;
using MvcApplication1.Validation;

namespace MvcApplication1.Models
{
    public class Allocation
    {
        [Required]
        [RegularExpression(@"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])(\/(\d|[1-2]\d|3[0-2]))$", ErrorMessage="Not valid CIDR format")]
        [CIDRSubnet(ErrorMessage="Not a valid CIDR Subnet")]
        public string IPNetwork { get; set; }
    }
}