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 29 	<TITLE>Networking Properties</TITLE>
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 32 <H1 style="text-align:center">Networking Properties</H1>
 33 <p>
 34 There are a few standard system properties used to
 35 alter the mechanisms and behavior of the various classes of the
 36 java.net package. Some are checked only once at startup of the VM,
 37 and therefore are best set using the -D option of the java command,
 38 while others have a more dynamic nature and can also be changed using
 39 the <a href="../../lang/System.html#setProperty(java.lang.String,java.lang.String)">System.setProperty()</a> API.
 40 The purpose of this document is to list
 41 and detail all of these properties.</P>
 42 <p>
 43 If there is no special note, a property value is checked every time it is used.</P>
 44 <a id="Ipv4IPv6"></a>
 45 <H2>IPv4 / IPv6</H2>
 46 <UL>
 47 	<LI><P><B>{@systemProperty java.net.preferIPv4Stack}</B> (default: false)<BR>
 48 	If IPv6 is available on the operating system the
 49 	underlying native socket will be, by default, an IPv6 socket which
 50 	lets applications connect to, and accept connections from, both
 51 	IPv4 and IPv6 hosts. However, in the case an application would
 52 	rather use IPv4 only sockets, then this property can be set to <B>true</B>.
 53 	The implication is that it will not be possible for the application
 54 	to communicate with IPv6 only hosts.</P>
 55 	<LI><P><B>{@systemProperty java.net.preferIPv6Addresses}</B> (default: false)<BR>
 56 	When dealing with a host which has both IPv4
 57 	and IPv6 addresses, and if IPv6 is available on the operating
 58 	system, the default behavior is to prefer using IPv4 addresses over
 59 	IPv6 ones. This is to ensure backward compatibility: for example,
 60 	for applications that depend on the representation of an IPv4 address
 61 	(e.g. 192.168.1.1). This property can be set to <B>true</B> to
 62 	change that preference and use IPv6 addresses over IPv4 ones where
 63 	possible, or <B>system</B> to preserve the order of the addresses as
 64     returned by the operating system.</P>
 65 </UL>
 66 <P>Both of these properties are checked only once, at startup.</P>
 67 <a id="Proxies"></a>
 68 <H2>Proxies</H2>
 69 <P>A proxy server allows indirect connection to network services and
 70 is used mainly for security (to get through firewalls) and
 71 performance reasons (proxies often do provide caching mechanisms).
 72 The following properties allow for configuration of the various type
 73 of proxies.</P>
 74 <UL>
 75 	<LI><P>HTTP</P>
 76 	<P>The following proxy settings are used by the HTTP protocol handler.</P>
 77 	<UL>
 78 		<LI><P><B>{@systemProperty http.proxyHost}</B> (default: &lt;none&gt;)<BR>
 79 	        The hostname, or address, of the proxy server.
 80 		</P>
 81 		<LI><P><B>{@systemProperty http.proxyPort}</B> (default: {@code 80})<BR>
 82 	        The port number of the proxy server.</P>
 83 		<LI><P><B>{@systemProperty http.nonProxyHosts}</B> (default: {@code localhost|127.*|[::1]})<BR>
 84 	        Indicates the hosts that should be accessed without going
 85 	        through the proxy. Typically this defines internal hosts.
 86 	        The value of this property is a list of hosts,
 87 		separated by the '|' character. In addition, the wildcard
 88 	        character '*' can be used for pattern matching. For example,
 89 		{@code -Dhttp.nonProxyHosts="*.foo.com|localhost"}
 90 		will indicate that every host in the foo.com domain and the
 91 		localhost should be accessed directly even if a proxy server is
 92 		specified.</P>
 93                 <P>The default value excludes all common variations of the loopback address.</P>
 94         </UL>
 95 	<LI><P>HTTPS<BR>This is HTTP over SSL, a secure version of HTTP
 96 	mainly used when confidentiality (like on payment sites) is needed.</P>
 97 	<P>The following proxy settings are used by the HTTPS protocol handler.</P>
 98 	<UL>
 99 		<LI><P><B>{@systemProperty https.proxyHost}</B> (default: &lt;none&gt;)<BR>
100 	        The hostname, or address, of the proxy server.
101 		</P>
102 		<LI><P><B>{@systemProperty https.proxyPort}</B> (default: {@code 443})<BR>
103 	        The port number of the proxy server.</P>
104 		<P>The HTTPS protocol handler will use the same nonProxyHosts
105 		property as the HTTP protocol.</P>
106 	</UL>
107 	<LI><P>FTP</P>
108 	<P>The following proxy settings are used by the FTP protocol handler.</P>
109 	<UL>
110 		<LI><P><B>{@systemProperty ftp.proxyHost}</B> (default: &lt;none&gt;)<BR>
111 	        The hostname, or address, of the proxy server.
112 		</P>
113 		<LI><P><B>{@systemProperty ftp.proxyPort}</B> (default: {@code 80})<BR>
114 	        The port number of the proxy server.</P>
115 		<LI><P><B>{@systemProperty ftp.nonProxyHosts}</B> (default: {@code localhost|127.*|[::1]})<BR>
116 	        Indicates the hosts that should be accessed without going
117 	        through the proxy. Typically this defines internal hosts.
118 	        The value of this property is a list of hosts, separated by
119 	        the '|' character. In addition, the wildcard character
120 		'*' can be used for pattern matching. For example,
121 		{@code -Dhttp.nonProxyHosts="*.foo.com|localhost"}
122 		will indicate that every host in the foo.com domain and the
123 		localhost should be accessed directly even if a proxy server is
124 		specified.</P>
125                 <P>The default value excludes all common variations of the loopback address.</P>
126 	</UL>
127 	<LI><P>SOCKS<BR>This is another type of proxy. It allows for lower-level
128 	type of tunneling since it works at the TCP level. In effect,
129 	in the Java(tm) platform setting a SOCKS proxy server will result in
130 	all TCP connections to go through that proxy, unless other proxies
131 	are specified. If SOCKS is supported by a Java SE implementation, the
132 	following properties will be used:</P>
133 	<UL>
134 		<LI><P><B>{@systemProperty socksProxyHost}</B> (default: &lt;none&gt;)<BR>
135 	        The hostname, or address, of the proxy server.</P>
136 		<LI><P><B>{@systemProperty socksProxyPort}</B> (default: {@code 1080})<BR>
137 	        The port number of the proxy server.</P>
138                 <LI><P><B>{@systemProperty socksProxyVersion}</B> (default: {@code 5})<BR>
139                 The version of the SOCKS protocol supported by the server. The
140                 default is {@code 5} indicating SOCKS V5. Alternatively,
141                 {@code 4} can be specified for SOCKS V4. Setting the property
142                 to values other than these leads to unspecified behavior.</P>
143 		<LI><P><B>{@systemProperty java.net.socks.username}</B> (default: &lt;none&gt;)<BR>
144 	        Username to use if the SOCKSv5 server asks for authentication
145 	        and no {@link java.net.Authenticator java.net.Authenticator} instance was found.</P>
146 		<LI><P><B>{@systemProperty java.net.socks.password}</B> (default: &lt;none&gt;)<BR>
147 	        Password to use if the SOCKSv5 server asks for authentication
148 	        and no {@code java.net.Authenticator} instance was found.</P>
149 		<P>Note that if no authentication is provided with either the above
150 		properties or an Authenticator, and the proxy requires one, then
151 		the <B>user.name</B> property will be used with no password.</P>
152 	</UL>
153 	<LI><P><B>{@systemProperty java.net.useSystemProxies}</B> (default: {@code false})<BR>
154 	On Windows systems, macOS systems, and Gnome systems it is possible to
155 	tell the java.net stack, setting this property to <B>true</B>, to use
156 	the system proxy settings (all these systems let you set proxies
157 	globally through their user interface). Note that this property is
158 	checked only once at startup.</P>
159 </UL>
160 <a id="MiscHTTP"></a>
161 <H2>Misc HTTP URL stream protocol handler properties</H2>
162 <UL>
163 	<LI><P><B>{@systemProperty http.agent}</B> (default: &ldquo;Java/&lt;version&gt;&rdquo;)<BR>
164 	Defines the string sent in the User-Agent request header in http
165 	requests. Note that the string &ldquo;Java/&lt;version&gt;&rdquo; will
166 	be appended to the one provided in the property (e.g. if
167 	{@code -Dhttp.agent="foobar"} is used, the User-Agent header will
168 	contain &ldquo;foobar Java/1.5.0&rdquo; if the version of the VM is
169 	1.5.0). This property is checked only once at startup.</P>
170 	<LI><P><B>{@systemProperty http.keepAlive}</B> (default: {@code true})<BR>
171 	Indicates if persistent connections should be supported. They improve
172 	performance by allowing the underlying socket connection to be reused
173 	for multiple HTTP requests. If this is set to true then persistent
174 	connections will be requested with HTTP 1.1 servers.</P>
175 	<LI><P><B>{@systemProperty http.maxConnections}</B> (default: {@code 5})<BR>
176 	If HTTP keepalive is enabled (see above) this value determines the
177 	maximum number of idle connections that will be simultaneously kept
178 	alive, per destination.</P>
179 	<LI><P><B>{@systemProperty http.maxRedirects}</B> (default: {@code 20})<BR>
180 	This integer value determines the maximum number, for a given request,
181 	of HTTP redirects that will be automatically followed by the
182 	protocol handler.</P>
183 	<LI><P><B>{@systemProperty http.auth.digest.validateServer}</B> (default: {@code false})</P>
184 	<LI><P><B>{@systemProperty http.auth.digest.validateProxy}</B> (default: {@code false})</P>
185 	<LI><P><B>{@systemProperty http.auth.digest.cnonceRepeat}</B> (default: {@code 5})</P>
186 	<P>These 3 properties modify the behavior of the HTTP digest
187 	authentication mechanism. Digest authentication provides a limited
188 	ability for the server  to authenticate itself to the client (i.e.
189 	By proving it knows the user's password). However, not all HTTP
190 	servers support this capability and by default it is turned off. The
191 	first two properties can be set to true to enforce this check for
192 	authentication with either an origin or proxy server, respectively.</P>
193 	<P>It is usually not necessary to change the third property. It
194 	determines how many times a cnonce value is re-used. This can be
195 	useful when the MD5-sess algorithm is being used. Increasing this
196 	value reduces the computational overhead on both client and server
197 	by reducing the amount of material that has to be hashed for each
198 	HTTP request.</P>
199 	<LI><P><B>{@systemProperty http.auth.ntlm.domain}</B> (default: &lt;none&gt;)<BR>
200 	NTLM is another authentication scheme. It uses the
201 	{@code java.net.Authenticator} class to acquire usernames and passwords when
202 	they are needed. However, NTLM also needs the NT domain name. There are
203 	3 options for specifying that domain:</P>
204 	<OL>
205 	  <LI><P>Do not specify it. In some environments the domain is
206 	      actually not required and the application does not have to specify
207 	      it.</P>
208 	  <LI><P>The domain name can be encoded within the username by
209 	      prefixing the domain name, followed by a backslash '\' before the
210 	      username. With this method existing applications that use the
211 	      authenticator class do not need to be modified, as long as users
212 	      are made aware that this notation must be used.</P>
213 	  <LI><P>If a domain name is not specified as in method 2) and this
214 	      property is defined, then its value will be used as the domain
215 	      name.</P>
216 	</OL>
217 </UL>
218 <P>All these properties are checked only once at startup.</P>
219 <a id="AddressCache"></a>
220 <H2>Address Cache</H2>
221 <P>The java.net package, when doing name resolution, uses an address
222 cache for both security and performance reasons. Any address
223 resolution attempt, be it forward (name to IP address) or reverse (IP
224 address to name), will have its result cached, whether it was
225 successful or not, so that subsequent identical requests will not
226 have to access the naming service. These properties allow for some
227 tuning on how the cache is operating.</P>
228 <UL>
229 	<LI><P><B>{@systemProperty networkaddress.cache.ttl}</B> (default: see below)<BR>
230 	Value is an integer corresponding to the number of seconds successful
231 	name lookups will be kept in the cache. A value of -1, or any other
232 	negative value for that matter,	indicates a &ldquo;cache forever&rdquo;
233 	policy, while a value of 0 (zero) means no caching. The default value
234 	is -1 (forever) if a security manager is installed, and implementation-specific
235 	when no security manager is installed.</P>
236 	<LI><P><B>{@systemProperty networkaddress.cache.negative.ttl}</B> (default: {@code 10})<BR>
237 	Value is an integer corresponding to the number of seconds an
238 	unsuccessful name lookup will be kept in the cache. A value of -1,
239 	or any negative value, means &ldquo;cache forever&rdquo;, while a
240 	value of 0 (zero) means no caching.</P>
241 </UL>
242 <P>Since these 2 properties are part of the security policy, they are
243 not set by either the -D option or the {@code System.setProperty()} API,
244 instead they are set as security properties.</P>
245 <a id="Unixdomain"></a>
246 <H2>Unix domain sockets</H2>
247 <p>
248 Calling {@link java.nio.channels.ServerSocketChannel#bind(SocketAddress,int) ServerSocketChannel.bind}
249 with a {@code null} address parameter will bind to an <i>automatically assigned</i> socket address.
250 For Unix domain sockets, this means a unique path in some predefined system temporary directory.
251 There are a number of system (and networking) properties that affect this behavior.
252 <p>
253 Unix domain socket addresses are limited in length to approximately 100 
254 bytes (depending on the platform), it is important to ensure that the temporary directory's name
255 together with the filename used for the socket (currently a name similar to
256 {@code socket_1679697142}) does not exceed this limit. The following properties
257 can be used to control the selection of this directory:
258 <ul>
259 	<li><p><b>{@systemProperty jdk.net.unixdomain.tmpdir}</b> This can be set as 
260 	a networking property in {@code conf/net.properties} If set, this specifies the 
261 	directory to use for automatically bound server socket addresses. On some platforms,
262 	(eg some Unix systems) this will have a predefined default value. On others,
263 	(eg Windows) there is no default value. Either way, it is always possible
264 	to override the networking property with a system property of the same name
265 	set on the command line. If neither of the networking nor system property
266 	are set, then some systems (eg Windows)  may check a commonly used environment 
267 	variable as temporary directory.
268 	<li><p><b>{@systemProperty java.io.tmpdir}</b> If the previous step fails to locate
269 	a directory to use, then the directory identified by the system property
270 	{@code java.io.tmpdir} is used.
271 </ul>
272 More information about the platform specific behavior can be seen in the 
273 {@code conf/net.properties} configuration file.
274 <p>
275 <i>Implicit</i> binding of a {@link java.nio.channels.SocketChannel SocketChannel}
276 <p>
277 If a client socket is connected to a remote destination without calling {@code bind} first,
278 then the socket is <i>implicitly</i> bound. In this case, <i>Unix domain</i> sockets 
279 are <i>unnamed</i> (ie. their path is empty). This behavior is not affected by any
280 system or networking properties.
281 <p>
282 <a id="EnhancedExceptions"></a>
283 <H2>Enhanced exception messages</H2>
284 By default, for security reasons, exception messages do not include potentially sensitive
285 security information such as hostnames or Unix domain socket address paths.
286 The following property can be used to relax this restriction, for debugging and other
287 purposes.
288 <ul>
289 	<li><p><b>{@systemProperty jdk.includeInExceptions}</b> This is typically set to
290 	a comma separated list of keywords that refer to exception types whose messages
291 	may be enhanced with more detailed information. If the value includes the string
292 	{@code hostInfo} then socket addresses will be included in exception message
293 	texts (eg hostnames, Unix domain socket address paths).
294 </ul>
295 
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