Dns.GetHostEntryAsync Method (String)
Resolves a host name or IP address to an IPHostEntry instance as an asynchronous operation.
Assembly: System (in System.dll)
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>] static member GetHostEntryAsync : hostNameOrAddress:string -> Task<IPHostEntry>
- Type: System.String
The host name or IP address to resolve.
The hostNameOrAddress parameter is a null reference (Nothing in Visual Basic).
The length of hostNameOrAddress parameter is greater than 255 characters.
An error was encountered when resolving the hostNameOrAddress parameter.
The hostNameOrAddress parameter is an invalid IP address.
This operation will not block. The returned Task(TResult) object will complete after the hostNameOrAddress has been resolved.
This method queries a DNS server for the IP address that is associated with a host name or IP address.
When an empty string is passed as the host name, this method returns the IPv4 addresses of the local host.
If the host name could not be found, the SocketException exception is returned with a value of 11001 (Windows Sockets error WSAHOST_NOT_FOUND). This exception can be returned if the DNS server does not respond. This exception can also be returned if the name is not an official host name or alias, or it cannot be found in the database(s) being queried.
This method assumes that if an IP literal string is passed in the hostNameOrAddress parameter that the application wants an IPHostEntry instance returned with all of the properties set. These properties include the AddressList, Aliases, and HostName. As a result, the implementation of this method exhibits the following behavior when an IP string literal is passed:
The method tries to parse the address. If the hostNameOrAddress parameter contains a legal IP string literal, then the first phase succeeds.
A reverse lookup using the IP address of the IP string literal is attempted to obtain the host name. This result is set as the HostName property.
The host name from this reverse lookup is used again to obtain all the possible IP addresses associated with the name and set as the AddressList property.
For an IPv4 string literal, all three steps above may succeed. But it is possible for a stale DNS record for an IPv4 address that actually belongs to a different host to be returned. This may cause step #3 to fail and throw an exception (there is a DNS PTR record for the IPv4 address, but no DNS A record for the IPv4 address).
For IPv6, step #2 above may fail, since most IPv6 deployments do not register the reverse (PTR) record for an IPv6 address. So this method may return the string IPv6 literal as the fully-qualified domain (FQDN) host name in the HostName property.
The GetHostAddresses method has different behavior with respect to IP literals. If step #1 above succeeds (it successfully parses as an IP address), that address is immediately returned as the result. There is no attempt at a reverse lookup.
IPv6 addresses are filtered from the results of this method if the local computer does not have IPv6 installed. As a result, it is possible to get back an empty IPHostEntry instance if only IPv6 results where available for the hostNameOrAddress.parameter.
This member emits trace information when you enable network tracing in your application. For more information, see Network Tracing in the .NET Framework.
The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.