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In a couple of fields in my Windows Forms application, I am asking a user to supply either an IP address or a hostname. Is there a good regular expression (regex) to validate the entered value? Or is there another method that I should consider? Please note I would prefer that the user enters a FQDN or an IP address.

DavidRR
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ThaKidd KG5ORD
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    Don't forget to consider the implications of IPv6 addresses if you want this application to last. – Dave Markle Jun 17 '10 at 12:39
  • possible duplicate of [Regular expression to match hostname or IP Address?](http://stackoverflow.com/questions/106179/regular-expression-to-match-hostname-or-ip-address) – jjnguy Jun 17 '10 at 12:39
  • jjnguy - Did not mean to repost. I will check this out and my apologies if i did. Dave - been looking at that aspect myself and realize that I may be going down the wrong path in my app at the moment since it is current based only on IPv4. Am very worried about that. – ThaKidd KG5ORD Jun 17 '10 at 12:54
  • Added "Or is there another method that I should consider?" since you were receptive to a non-regex approach. That also keeps this question from being flagged as a [duplicate](http://stackoverflow.com/questions/106179). – DavidRR May 03 '16 at 13:47

4 Answers4

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In the interest of future-proofing your application, I'd suggest just using IPAddress.TryParse() to determine if the input is an IP address.

Checking for a 'valid' hostname is more difficult, also because you didn't specify whether the hostname has to exist or not. If it does, the easiest way would be to use Dns.GetHostEntry() to see if that yields a result. You can't get much more accurate validation based on the description you gave.

ErikHeemskerk
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  • Would Dns.GetHostEntry() be useful if the user typed in a host and then clicked on a button to validate the host entry...like the Check Names button on Active Directory related fields? – ThaKidd KG5ORD Jun 17 '10 at 12:52
  • Certainly, I've seen this done in multiple tools, the first that springs to mind is IIS SMTP Server Config (when setting the server's FQDN). – ErikHeemskerk Jun 17 '10 at 12:58
  • See [IPAddress.AddressFamily](https://msdn.microsoft.com/en-us/library/system.net.ipaddress.addressfamily.aspx) to distinguish between an IPv4 and IPv6 address. Also see [this answer](http://stackoverflow.com/a/53500/1497596). – DavidRR May 03 '16 at 14:05
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try to call IPAddress.TryParse , if it fails try to Dns.GetHostByName

Andrey
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If you like to validate if the format is valid, you can use

bool isValid = Uri.CheckHostName(hostname) == UriHostNameType.Dns;

instead of Regex, wich can be error-prone. I use this in a pre-configuration application, where the user can specify a hostname which will be set in the future. For this purpose, Dns.GetHostEntry is not suiteable because it trys to resolve the hostname, which will fail since it's not assigned yet.

So it depends on your needs. If the user should enter a hostname who actually can be resolved, you may want to use Dns.GetHostEntry() or IPAddress.TryParse(). If you just want to do input validation, try Uri.CheckHostName.

Lion
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  • Checking for single word like Uri.CheckHostName("name") is also valid since it's valid for local network, so for internet addresses it's also required to check if there is at least 1 dot and that it's not at the end, since "name." is also valid – Vitaly Mar 15 '23 at 15:21
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Why would you need to ask for the IP address and for the hostname for a Windows Form application? If these are the local PC details, you could get these from:-

1) To get the hostname, you can call Dns.GetHostName() (see MSDN reference)

2) To get the IP address, you can enumerate the IP address via Dns.GetHostByName() (see MSDN reference)

Syd
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  • The data would not be used to test things out on the local system. It would be used to determine whether a SSH server exists and also whether the data a user enters is real (as in blahblahblah == wrong, 66.234.34.23 == correct). This question is dealing completely with remote hosts. – ThaKidd KG5ORD Jun 17 '10 at 13:00
  • @ThaKidd, thank you for the clarification. On the note, I think you can still use Dns.GetHostByName(hostname) to verify if the hostname is valid. Ditto for the the IP address which you can use Dns.GetHostByAddress() to check the address. – Syd Jun 17 '10 at 13:26
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    Dns.GetHostByName and Dns.GetHostByAddress have both been marked obsolete. – ErikHeemskerk Jun 17 '10 at 14:56