implement and document DIRECT for auto-detect-proxy

There is a subtile difference between an empty setting and "DIRECT" in
the configuration as the later overrides the generic settings while the
earlier does not. Also, non-zero exitcodes should really be reported as
an error rather than silently discarded.

1 files changed, 12 insertions, 7 deletions

diff --git a/doc/apt.conf.5.xml b/doc/apt.conf.5.xml
index 6aa0c9629..d71f99c0a 100644
--- a/doc/apt.conf.5.xml
+++ b/doc/apt.conf.5.xml
@@ -444,15 +444,20 @@ APT::Compressor::rev {
      only if the client uses a known identifier.</para>
 
      <para><literal>Acquire::http::Proxy-Auto-Detect</literal> can be used to
-     specify an external command to discover the http proxy to use. Apt expects
-     the command to output the proxy on stdout in the style
-     <literal>http://proxy:port/</literal>. This will override the
-     generic <literal>Acquire::http::Proxy</literal> but not any specific
-     host proxy configuration set via 
-     <literal>Acquire::http::Proxy::$HOST</literal>.
+     specify an external command to discover the http proxy to use. The first
+     and only parameter is an URI denoting the host to be contacted to allow
+     for host-specific configuration. APT expects the command to output the
+     proxy on stdout as a single line in the style <literal>http://proxy:port/</literal>
+     or the word <literal>DIRECT</literal> if no proxy should be used. No output
+     indicates that the generic proxy settings should be used.
+
+     Note that auto-detection will not be used for a host if a host-specific proxy
+     configuration is already set via <literal>Acquire::http::Proxy::<replaceable>HOST</replaceable></literal>.
 
      See the &squid-deb-proxy-client; package for an example implementation that
-     uses avahi. This option takes precedence over the legacy option name 
+     uses avahi.
+
+     This option takes precedence over the legacy option name
      <literal>ProxyAutoDetect</literal>.
      </para>