arcserve-KB : Hostname or Domain contains an _ (underscore) character

Last Update: 2017-06-23 10:53:03 UTC



Arcserve Tech Support and Development teams have  investigated issue(s)  that occurs, when a customer environment contains an underscore character in the FQDN (fully qualified domain name).


  • backup_server1 (underscore in the host name)
  • backupserver.sub.customer_domain1 (underscore in the FQDN / domain part)


Attention: Customers who have such an server or domain with an underscore are requested to NOT upgrade from 6.5 to 6.5 update 1 until they remove the underscore from their FQDN names 



From UDP 6.5 to UDP 6.5 update 1 the built-in Apache component was upgraded from version 2.4.10 to 2.4.25, and the latter version no longer allows any underscore character in the FQDN. 


Underscore characters were popular in Windows NT 3.x and NT 4.0 environments, used in NetBIOS computer names and NetBIOS domain names - before DNS was being used for networking.
With the Upgrade to Windows 2000, Microsoft started to warn against underscores - even for pure WINS environments without DNS.


As per , all FQDN should only compose of the following :

1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
   to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
   sign (-), and period (.). Note that periods are only allowed when 
they serve to delimit components of "domain style names".

It is unclear why an underscore was working in previous versions of apache, and if this overall issue should be considered a regression, or a bug that was finally fixed (albeit with a negative impact for those affected).


Microsoft Windows warns customers, when a host name with an underscore is chosen, but sadly it doesn't prevent that choice:

It is unclear why the message above is only a soft warning, which would allow users to continue by clicking on Yes (and with that accepting the problems with applications and network hardware). It seems that any other character, that had been compatible in WINS (non-DNS) environments, is not allowed by Windows - and in the error message that is displayed below, the underscore is also correctly included as not being allowed:


When a period is included in the host name, the following Windows error message shows the allowed characters for host names, which matches RFC952 correctly:


In our research, we found it also noteworthy that the Microsoft DNS server has a switch, which allows its name checking to be changed between "Strict RFC (ANSI)" and "None RFC (ANSI)". The difference between those settings is that None RFC allows for underscores.

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