1<html> 2<title>Ganymed SSH-2 for Java FAQ</title> 3<body> 4 5<a name="oben"></a> 6<h1>Ganymed SSH-2 for Java FAQ</h1> 7 8<p> 9This FAQ includes information regarding topics that were discussed in e-mails between developers and users 10of the Ganymed SSH-2 for Java library. 11</p> 12<p> 13Ganymed SSH-2 for Java homepage: <a href="http://www.cleondris.ch/opensource/ssh2/">http://www.cleondris.ch/opensource/ssh2/</a><br> 14Last update of FAQ: apr-16-2010. 15</p> 16<p> 17Please report bugs, typos and any kind of suggestions to Christian Plattner (christian.plattner at cleondris.ch). 18</p> 19 20<hr> 21 22<h2>Sections:</h2> 23 24<p> 25<ul> 26<li><a href="#env">When I start program XYZ with putty (or openssh, ..., whatever) then everything works. 27However, if I use "Session.execCommand", then XYZ behaves differently or does not work at all!</a></li> 28 29<li><a href="#blocking">My program sometimes hangs when I only read output from stdout! 30Or: can you explain me the story about the shared stdout/stderr window in the SSH-2 protocol? 31Or: what is this "StreamGobbler" thing all about?</a></li> 32 33<li><a href="#buffered">Why are the session's Input- and OutputStreams not buffered?</a></li> 34 35<li><a href="#sessioncommands">Why can't I execute several commands in one single session?</a></li> 36 37<li><a href="#sessionlimit">I cannot open more than 10 concurrent sessions (or SCP clients).</a></li> 38 39<li><a href="#passwordauth">Password authentication fails, I get "Authentication method password not 40supported by the server at this stage".</a></li> 41 42<li><a href="#puttygen">Why does public key authentication fail with my putty key?</a></li> 43 44<li><a href="#catmethod">I am sending data to a remote file using the "cat" method, but not all data is being written.</a></li> 45 46<li><a href="#pumptoremote">I want to pump data into a remote file, but the amount of data to be sent 47is not known at the time the transfer starts.</a></li> 48 49<li><a href="#swingshell">Do you have an example for the usage of feature XYZ?</a></li> 50 51<li><a href="#maven">Where is the official Maven repository?</a></li> 52 53</ul> 54</p> 55 56<hr><a name="env"></a><h2>When I start program XYZ with putty (or openssh, ..., whatever) then everything 57works. However, if I use "Session.execCommand", then XYZ behaves differently or does not work at all!</h2> 58 59<h3>Short answer:</h3> 60 61<p> 62The most often source of problems when executing a command with <tt>Session.execCommand()</tt> 63are missing/wrong set environment variables on the remote machine. Make sure that the minimum needed 64environment for XYZ is the same, independentely on how the shell is being invoked. 65</p> 66 67<p> 68Example quickfix for bash users: 69</p> 70 71<p> 72<ol> 73<li>Define all your settings in the file <tt><b>~/.bashrc</b></tt></li> 74<li>Make sure that the file <tt><b>~/.bash_profile</b></tt> only contains the line <tt><b>source 75~/.bashrc</b></tt>.</li> 76<li>Before executing <tt>Session.execCommand()</tt>, do NOT aquire any type of pseudo terminal in the 77session. Be prepared to consume stdout and stderr data.</li> 78</ol> 79</p> 80 81<p> 82<b>Note:</b> If you really want to mimic the behavior of putty, then don't use Session.execCommand(), 83instead aquire a pty (pseudo terminal) and then start a shell (use <tt>Session.requestPTY()</tt> and 84<tt>Session.startShell()</tt>). You then have to communicate with the shell process at the other end 85through stdin and stdout. However, you also have to implement terminal logic (e.g., escape sequence 86handling (unless you use a "dumb" pty), "expect-send" logic (output parsing, shell prompt detection), etc.). 87</p> 88 89<h3>Long answer:</h3> 90 91<p> 92If you login by using putty, then putty will normally request a "xterm" pty and your assigned shell 93(e.g., bash) will be started (a so called "interactive login shell"). In contrast, if you use 94<tt>Session.execCommand()</tt> to start a command then (unless you ask for it) no pty will be aquired 95and the command will be given to the shell as an argument (with the shell's "-c" option). 96</p> 97 98<p> 99The way a shell is being invoked has an effect on the set of initialization files which will be read be the shell. 100</p> 101 102<p> 103To demonstrate the difference, try the following (from the command line, e.g., with an OpenSSH client): 104</p> 105 106<p> 107<ol> 108<li>Login interactively and print the environment with the "env" command:<br> <br> 109<tt><b>[user@host ~] ssh 127.0.0.1<br> 110[user@host ~] env</b></tt><br> <br> 111</li> 112<li>Let the ssh server execute the "env" command (equivalent to using <tt>Session.executeCommand()</tt>):<br> <br> 113<tt><b>[user@host ~] ssh 127.0.0.1 "env"</b></tt> 114</li> 115</ol> 116</p> 117 118<p> 119If you compare the two outputs, then you will (unless you have adjusted your shell's settings) 120observe different environments. 121</p> 122 123<p> 124<b>If you are interested in the details, then please read the <tt>INVOCATION</tt> section in man page 125for the bash shell. You may notice that the definitions of "interactive" and "non-interactive" 126(and combinations with "login") are little bit tricky.</b> 127</p> 128 129[<a href="#oben">TOP</a>] 130 131<hr><a name="blocking"></a><h2>My program sometimes hangs when I only read output from stdout! 132Or: can you explain me the story about the shared stdout/stderr window in the SSH-2 protocol? 133Or: what is this "StreamGobbler" thing all about?</h2> 134 135<p> 136In the SSH-2 low level protocol, each channel (e.g., session) has a receive window. When the remote 137SSH daemon has filled up our receive window, it must wait until we have consumed the input and are ready to accept new data. 138</p> 139 140<p> 141Unfortunately, the SSH-2 protocol defines a shared window for stderr and stdout. As a consequence, 142if, for example, the remote process produces a lot of stderr data and you never consume it, then after 143some time the local receive window will be full and the sender is blocked. If you then try to read() 144from stdout, your call will be blocked: there is no stdout data (locally) available and the SSH daemon 145cannot send you any, since the receive window is full (you would have to read some stderr data first 146to "free" up space in the receive window). 147</p> 148 149<p> 150Fortunately, Ganymed SSH-2 uses a 30KB window - the above described scenario should be very rare. 151</p> 152 153<p> 154Many other SSH-2 client implementations just blindly consume any remotely produced data into a buffer 155which gets automatically extended - however, this can lead to another problem: in the extreme case 156the remote side can overflow you with data (e.g., leading to out of memory errors). 157</p> 158 159<p> 160What can you do about this? 161</p> 162 163<p> 164<ol> 165<li><b>Bad: Do nothing</b> - just work with stderr and stdout Inputstreams and hope that the 30KB 166window is enough for your application.</li> 167 168<li><b>Better, recommended for most users:</b> use two worker threads that consume remote stdout 169and stderr in parallel. Since you probably are not in the mood to program such a thing, you can use 170the StreamGobbler class supplied with Ganymed SSH-2. The Streamgobbler is a special InputStream that 171uses an internal worker thread to read and buffer internally all data produced by another InputStream. 172It is very simple to use:<br> <tt><b><pre>InputStream stdout = new StreamGobbler(mysession.getStdout()); 173 174InputStream stderr = new StreamGobbler(mysession.getStderr());</pre></b></tt> 175You then can access stdout and stderr in any order, in the background the StreamGobblers will 176automatically consume all data from the remote side and store in an internal buffer.</li> 177 178<li><b>Advanced:</b> you are paranoid and don't like programs that automatically extend buffers 179without asking you. You then have to implement a state machine. The condition wait facility offered by 180<tt>Session.waitForCondition()</tt> is exactly what you need: you can use it to wait until either stdout 181or stderr data has arrived and can be consumed with the two InputStreams. You can either use the return value 182of <tt>Session.waitForCondition()</tt> or check with <tt>InputStream.available()</tt> 183(for stdout and stderr) which InputStream has data available (i.e., a <tt>read()</tt> call will not block). 184Be careful when wrapping the InputStreams, also do not concurrently call read() on the InputStreams while calling 185<tt>Session.waitForCondition()</tt> (unless you know what you are doing).<br>Please have a look a the 186<tt>SingleThreadStdoutStderr.java</tt> example.</li> 187 188<li><b>The lazy way:</b> you don't mind if stdout and stderr data is being mixed into the same 189stream. Just allocate a "dumb" pty and the server will hopefully not send you any data on the stderr 190stream anymore. <b>Note:</b> by allocating a pty, the shell used to execute the command will probably 191behave differently in terms of initialization (see also <a href="#env">this question</a>).</li> 192</ol> 193</p> 194 195 196[<a href="#oben">TOP</a>] 197 198<hr><a name="buffered"></a><h2>Why are the session's Input- and OutputStreams not buffered?</h2> 199 200<p> 201If you need it, then this library offers quite a raw type of access to the SSH-2 protocol stack. 202Of course, many people don't need that kind of low level access. If you need buffered streams, 203then you should the do the same thing as you would probably do with the streams of a TCP socket: 204wrap them with instances of BufferedInputStream and BufferedOutputStream. In case you use 205StreamGobblers for the InputStreams, then you don't need any additional wrappers, since the 206StreamGobblers implement buffering already. 207</p> 208<p> 209This code snippet will probably work well for most people: 210</p> 211<p> 212<tt> 213<pre> 214InputStream stdout = new StreamGobbler(mysession.getStdout()); 215InputStream stderr = new StreamGobbler(mysession.getStderr()); 216OutputStream stdin = new BufferedOutputStream(mysession.getStdin(), 8192); 217</pre> 218</tt> 219</p> 220 221[<a href="#oben">TOP</a>] 222 223<hr><a name="sessioncommands"></a><h2>Why can't I execute several commands in one single session?</h2> 224<p> 225If you use <tt>Session.execCommand()</tt>, then you indeed can only execute only one command per session. 226This is not a restriction of the library, but rather an enforcement by the underlying SSH-2 protocol 227(a <tt>Session</tt> object models the underlying SSH-2 session). 228</p> 229<p> 230There are several solutions: 231</p> 232<p> 233<ul> 234<li><b>Simple: Execute several commands in one batch</b>, e.g., something like <tt>Session.execCommand("echo 235Hello && echo again")</tt>.</li> 236<li><b>Simple: The intended way: simply open a new session for each command</b> - once you have opened a 237connection, you can ask for as many sessions as you want, they are only a "virtual" construct.</li> 238<li><b>Advanced: Don't use <tt>Session.execCommand()</tt>, but rather aquire a shell with 239<tt>Session.startShell()</tt></b>. See also <a href="#env">this question</a>.</li> 240</ul> 241</p> 242 243 244[<a href="#oben">TOP</a>] 245 246<hr><a name="sessionlimit"></a><h2>I cannot open more than 10 concurrent sessions (or SCP clients).</h2> 247<p> 248You are probably using OpenSSH. By looking at their source code you will find out that there 249is a hard-coded constant called MAX_SESSIONS in the session.c file which is set to "10" by default. 250This is a per connection limit. Unfortunately, it is not a run-time tunable parameter. 251However, this limit has no effect on the number of concurrent port forwardings. Please note: this information 252is based on the OpenSSH 4.3 release. 253</p> 254<p> 255Possible solutions: 256<ul> 257<li>(a) Recompile your SSH daemon</li> 258<li>(b) Try to live with this limit and keep the number of concurrent sessions <= 10.</li> 259<li>(c) Distribute your sessions over multiple concurrent SSH connections.</li> 260</ul> 261</p> 262<p> 263Just for completeness: starting from release 210, the thrown exception may look as follows:<br> 264<tt> 265<pre> 266java.io.IOException: Could not open channel (The server refused to open the channel (SSH_OPEN_ADMINISTRATIVELY_PROHIBITED, 'open failed')) 267</pre> 268</tt> 269</p> 270 271[<a href="#oben">TOP</a>] 272 273<hr><a name="passwordauth"></a><h2>Password authentication fails, I get "Authentication method password 274not supported by the server at this stage".</h2> 275 276<p> 277Many default SSH server installations are configured to refuse the authentication type "password". 278Often, they only accept "publickey" and "keyboard-interactive". You have different options: 279</p> 280 281<p> 282<ul> 283<li><b>Enable password authentication.</b> E.g., in case of OpenSSH on Fedora, edit 284<code>/etc/sshd/sshd_config</code> and change the value of "PasswordAuthentication" to "yes", 285then send a HUP signal to the daemon so that it re-reads its configuration.</li> 286<li><b>Switch to public-key authentication.</b> Probably the best choice.</li> 287<li><b>Try to use keyboard-interactive authentication.</b> If you have a GUI that interacts with a user, 288then this is doable (check out the SwingShell.java example).</li> 289</ul> 290</p> 291 292<p> 293In general it is a good idea to call either <code>Connection.getRemainingAuthMethods()</code> 294or <code>Connection.isAuthMethodAvailable()</code> before using a certain authentication method. 295</p> 296 297<p> 298Please note that most servers let you in after one successful authentication step. However, in rare cases 299you may encounter servers that need several steps. I.e., if one of the <code>Connection.authenticateWithXXX()</code> 300methods returns <code>false</code> and <code>Connection.isAuthenticationPartialSuccess()</code> returns 301<code>true</code>, then further authentication is needed. For each step, to find out which authentication methods 302may proceed, you can use either the <code>Connection.getRemainingAuthMethods()</code> 303or the <code>Connection.isAuthMethodAvailable()</code> method. Again, please have a look into the 304SwingShell.java example. 305</p> 306 307[<a href="#oben">TOP</a>] 308 309<hr><a name="puttygen"></a><h2>Why does public key authentication fail with my putty key?</h2> 310<p> 311When using putty private keys (e.g., .ppk files) with public key authentication, you get a 312"Publickey authentication failed" exception. The reason is that the library currently is not able to 313directly handle private keys in the proprietary format used by putty. However, you can use the 314"puttygen" tool (from the putty website) to convert your key to the desired format: load your key, 315then go to the conversions menu and select "Save OpenSSH key" (which saves the key in openssl PEM format, 316e.g., call it "private.pem"). 317</p> 318 319[<a href="#oben">TOP</a>] 320 321<hr><a name="catmethod"></a><h2>I am sending data to a remote file using the "cat" method, but not all data is being written.</h2> 322<p> 323Please read carefully the answer to the following <a href="#pumptoremote">question</a>. 324</p> 325 326[<a href="#oben">TOP</a>] 327 328 329<hr><a name="pumptoremote"></a><h2>I want to pump data into a remote file, but the amount of data to be sent 330is not known at the time the transfer starts.</h2> 331<p> 332The SCP protocol communicates the amount of data to be sent at the start of the transfer, 333so SCP remains out of consideration. Possible other solutions: 334<ul> 335<li>Use the SFTP client. Recommended.</li> 336<li>Execute "cat > filename.txt" on the remote side and pump the data into stdin. This method is NOT recommended (and won't work on Windows...).</li> 337</ul> 338</p> 339<p> 340Be careful if you use the "cat" approach, as it may happen that not all your data will be 341written. If you close the stdin stream and immediatelly close the session (or the whole connection) then 342some SSH servers do not send the pending data to the process being executed ("cat" in this case). 343You have to wait until "cat" has received the EOF and terminates before closing the session. However, 344waiting for the termination may not always work, since SSH servers sometimes "forget" to send the exit code 345of the remote process. The following code MAY work: 346</p> 347<p> 348<tt> 349<pre> 350Session sess = conn.openSession(); 351sess.execCommand("cat > test.txt"); 352OutputStream stdin = sess.getStdin(); 353 354... out.write(...) ... out.write(...) ... 355 356/* The following flush() is only needed if you wrap the */ 357/* stdin stream (e.g., with a BufferedOutputStream). */ 358out.flush(); 359 360/* Now let's send EOF */ 361out.close(); 362 363/* Let's wait until cat has finished */ 364sess.waitForCondition(ChannelCondition.EXIT_STATUS, 2000); 365/* Better: put the above statement into a while loop! */ 366/* In ANY CASE: read the Javadocs for waitForCondition() */ 367 368/* Show exit status, if available (otherwise "null") */ 369System.out.println("ExitCode: " + sess.getExitStatus()); 370/* Now its hopefully safe to close the session */ 371sess.close(); 372</pre> 373</tt> 374</p> 375<p> 376(Just a thought for another solution: execute <code>cat > test.txt && echo "FINISHED"</code> 377and wait until you get "FINISHED" on stdout... - try it on your own risk =) 378</p> 379 380[<a href="#oben">TOP</a>] 381 382<hr><a name="swingshell"></a><h2>Do you have an example for the usage of feature XYZ?</h2> 383<p> 384Please have at look at the examples section in the distribution, especially at the SwingShell.java example. 385</p> 386 387[<a href="#oben">TOP</a>] 388 389<hr><a name="maven"></a><h2>Where is the official Maven repository?</h2> 390<p> 391We regulary get requests for a Maven repository. Please note that <b>there is no such thing as an official Ganymed SSH-2 for Java Maven repository</b>. 392At the moment, we do not have the resources to support specific build systems (be it Maven or anything else). We know that others 393have setup (and not maintained) such repositories. However, we believe that you should 394download security related software only from a trusted source - in other words, download the precompiled .jar file from our website and add it to your 395project. This generic approach will work with every java development enviroment and build system. 396Last warning: please think twice before you use a foreign "repository" to "auto-update" security related components of your project. 397</p> 398 399[<a href="#oben">TOP</a>] 400 401</body> 402</html> 403 404