1. diff --git a/includes/common.php b/includes/common.php
2. index 4d58f60aa..d98423368 100644
3. --- a/includes/common.php
4. +++ b/includes/common.php
5. @@ -649,7 +649,7 @@ function is_valid_hostname($hostname)
6.      // maximum length is 253 characters, maximum segment size is 63
7.  
8.      return (
9. -        preg_match("/^([a-z\d](-*[a-z\d])*)(\.([a-z\d](-*[a-z\d])*))*\.?$/i", $hostname) //valid chars check
10. +        preg_match("/^([a-z\d](-*[a-z\d_])*)(\.([a-z\d](-*[a-z\d])*))*\.?$/i", $hostname) //valid chars check
11.          && preg_match("/^.{1,253}$/", $hostname) //overall length check
12.          && preg_match("/^[^\.]{1,63}(\.[^\.]{1,63})*\.?$/", $hostname)
13.      );
14.