According to struts 1.x digestor it says that if the 3rd parameter i.e. "-//Apache Software Foundation//DTD Struts Configuration 1.3//EN" does not match with the string as specified in struts 1.x dtd file then it will look for the dtd file in 4th parameter url i.e. http://struts.apache.org/dtds/struts-config_1_3.dtd
This is the normal phenomena and often we do not get to know since we are roughly always connected online.It's only when internet is not working we get socket error.
So resolution for the problem should be that please specify the 3rd parameter from your dtd file, which in your case is struts-config_1_3.dtd.
Dig in struts1.x jar file and look for this dtd file and open it, copy the string and paste in your struts-config.xml file.
i hope this should solve the problem.
MCA , SJCP
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