I/O error : Attempt to load network entity

[paradiseduo]

➜  shadowsocksr-libev git:(master) make
/Applications/Xcode.app/Contents/Developer/usr/bin/make  all-recursive
Making all in libsodium
Making all in src
Making all in libsodium
Making all in include
make[5]: Nothing to be done for `all'.
make[5]: Nothing to be done for `all-am'.
make[4]: Nothing to be done for `all-am'.
make[3]: Nothing to be done for `all-am'.
Making all in libcork
make[2]: Nothing to be done for `all'.
Making all in libipset
make[2]: Nothing to be done for `all'.
Making all in libudns
make[2]: Nothing to be done for `all'.
Making all in libev
make[2]: Nothing to be done for `all'.
Making all in src
make[2]: Nothing to be done for `all'.
Making all in doc
  GEN      shadowsocks-libev.xml
  GEN      shadowsocks-libev.8
xmlto: /Users/a/Desktop/shadowsocksr-libev/doc/shadowsocks-libev.xml does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/Users/a/Desktop/shadowsocksr-libev/doc/shadowsocks-libev.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
                                                                               ^
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Document /Users/a/Desktop/shadowsocksr-libev/doc/shadowsocks-libev.xml does not validate
make[2]: *** [shadowsocks-libev.8] Error 13
rm shadowsocks-libev.xml
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

但是我直接在浏览器里打开
http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
是可以下载的