BUILD INSTRUCTIONS: To build this, you'll need to remove my key.* settings in ant.properties. You'll have to build a "debug" apk instead of a "release" one. Android will not let you install a debug apk on top of a release one, so you have to remove stock SimpleSSHD first before installing the debug build. Then follow these steps (roughly the "doit" script): ndk-build -j8 && mv libs/armeabi/scp libs/armeabi/libscp.so && mv libs/armeabi/sftp-server libs/armeabi/libsftp-server.so && mv libs/armeabi/rsync libs/armeabi/librsync.so && mv libs/armeabi/buffersu libs/armeabi/libbuffersu.so && ant debug The mv steps are very important, because ant will only package the necessary binaries if they have a .so extension (even though they are stand alone executables). DEVELOPMENT JOURNAL: December 6, 2014. The idea is to make a proper ssh implementation for Android. Important features: * it should run happily without root (on a non-root port) * it should be a regular android app requiring no special permissions, and not requiring any 'magic' executable files * should not rely on busybox * preferably support sftp * open source The existing apps are either expensive, don't work, need root, or too complicated, or a mix of all of the above. And none of them are open source. I figure I'll start with dropbear, which I will run through JNI instead of putting it in its own binary (because making such a binary executable is a bit of a hack). So that's the plan........ December 14, 2014. I got dropbear to compile under the Android NDK, so now it's time to work on the Android side of it. I need: * a Service that can be started, stopped, and queried for whether it's running or not * a Thread to implement the Service's work (by calling into dropbear's main()), which can also be stopped. * a config UI with at least these choices: - bool: start on boot (def: false) - number: port number (def: 2222) - string: path to authorized_keys file (def: /sdcard/ssh) - string: name of default shell (def: /system/bin/sh -l) - string: default path for HOME (def: /sdcard/ssh) - button: start or (if it's running) stop December 15, 2014. Getting to the fun part. Process management... To start sshd, it seems like I can startService(). Then in the Service's onStartCommand(), call startForeground() so it won't be killed (return START_STICKY too?). The question is if dropbear's main() should run under a separate Thread, or a separate Process. The trouble with a Thread is that it might be hard to kill. The trouble with a process is that there is no way to report back status (such as a failure to start sshd). Connectbot starts a new process for its shell -- it really doesn't have a choice because the shell binary isn't linked with Connectbot, and exec() in a thread stinks. To stop it, it just closes stdin/stdout!!! So zombies can (and do) linger. I suppose dropbear could be in its own process if it had something like stdin/stdout to communicate failure? Or it could just write error messages to (i.e.) /sdcard/ssh/log. To stop the service, it would just use kill(). I am curious how the main waiting-for-connections loop looks, but even if it uses select(), I'm not sure how I would honor Thread.interrupt() or whatever. It's not guaranteed to interrupt select(), and I'm not keen on adding an arbitrary timeout/polling feature to it. December 20, 2014. So, I added a builtin scp endpoint. It was pretty straight forward, except dropbear defaults to vfork(), which blocks the parent until the child runs execve()!! Anyways, I noticed that scp doesn't quote its arguments to the remote scp. That means you can't conveniently copy a remote file with a space in its name (it becomes two files). But the upside is that this is where wildcards are handled -- by the shell! So I need to either run it as a separate executable launched through the shell, or make my own implementation of wildcards. It is easy, using a $(BUILD_EXECUTABLE) script, to get ndk to build an executable. But it is only packaged up if it is named "gdbserver" (and debug apk), or "libfoo.so". The good news is that libfoo.so can be executed in /data/data/org.galexander.sshd/lib/libfoo.so, so that is a viable option. Doing the expansion myself is not necessarily hard either, though. I need a library function called glob(), which is apparently not part of bionic. But I have the idea some cut and paste would resolve that with very little extra work on my part. December 21, 2014. Well, bionic libc *does* provide fnmatch(), and even scandir() (a shortcut for readdir). In the best case, though, that still leaves me with a bit of a path parsing conundrum (I have to tell scandir which directory to operate on). And also a bit of an escape character conundrum -- \* and "*" should not act like wildcards. Those are not insurmountable but I think I've talked myself out of it. So then the question is, do I figure out how to ship an executable, or do I do some hack like open a pipe to "/system/bin/sh echo filespec" and use the shell solely for expansion? I'm developing the idea that it's actually pretty easy to ship an executable, I just need to find some -pre-package step where I can do "mv scp libscp.so" and then it will ship. ndk-build will not let me make a target with a "." in it directly. ... Now scp, sftp-server, and rsync work as separate executables... rsync does fail at -z because it needs it's own custom zlib...The stock "external" one seems to lack the "old-style compress" method. There is another commandline option for the new (deflate?) technique, but I think the normal -z ought to work too. But that is literally the last feature! Then just release details. December 29, 2014. First problem report from a user. Lollipop (Android 5.0) requires "PIE" executables -- position independent code. I think that is a modern equivalent to -fpic that Android is now requiring so that it can randomize addresses to try to obscure stack smashing attacks that rely on fixed addresses. It is epicly lame. Anyways, the big fuck-you from Google is that Ice Cream Sandwich (Android 4.1) and earlier require fixed-position code. So one binary will not generally work on both. Here is a good summary: https://code.google.com/p/android-developer-preview/issues/detail?id=888 There is something called "run_pie" which you can wrap your executables in that lets older Android run PIE executables. It would require a relatively small change to the exec() call to prepend it with "run_pie". That seems like a hack. The suggested remedy is to build two different apks! Yuck! Anyways, it is only executables (not libraries -- they are position independent already) that are affected. And apparently static executables don't care one way or the other. So that is my remedy -- static executables for the moment. I tested them and it is only a little bit bigger -- 904kB of binaries instead of 668kB. January 18, 2015. Markus Ethen suggested it display the current IP address so you know where to ssh to, in case it isn't convenient to use static dhcp or to remember the address. That seems to be easier said than done. You can use WifiManager, but that won't give your IP address unless you're on wifi. That is probably "good enough", but it is certainly not ideal. There is also java.net.NetworkInterface, which seems to return a random ipv6 address. Ah-hah! It is fe80::macaddr, which is a bogus "local-connection only" ipv6 address, like 192.168, but automatically-generated without dhcp. So if I skip that, it finds the proper ipv4 address! Pfew! I was thinking I'd have to directly use /proc/net/dev and SIOCGIFCONF, just like ifconfig does, but it works fine with java.net.NetworkInterface. June 20, 2015. At some points, rsync is only write()ing, and assumes that the other end will receive it all. The other end does a little bit of write()ing of its own, and then is happy to read() all it wants. So this written stuff may sit in a buffer somewhere indefinitely. If that happens, an infelicity in the design of SuperSU causes everything to wedge. Of course, this is only if you set the shell to /system/xbin/su as a way of having root access for rsync. Anyways, I made a new program, "buffersu", which is just a stdin/stdout-buffering wrapper for rsync that is guaranteed to always perform any read() that is possible at any time, no matter how many write()s are outstanding. That seems to do the trick. June 21, 2016. Chris Moore reports that rsync and sftp do not like files larger than 2GB. rsync was easy - it just needed an additional #define in rsync/config.h to enable its builtin support for using stat64/lseek64/off64_t/etc. Now that I'm investigating sftp, I find this surprising fact about bionic (though the glibc man page for stat(2) tried to tell me this) - stat64 and stat are the same thing! But off64_t and lseek64 are significant. That should make converting sftp pretty convenient. Especially since sftp already uses "u_int64_t" instead of off_t. p.s. Chris Moore gave me this command to test sftp, which turned out to be useful: curl -v --pubkey .ssh/id_rsa.pub -r 2147482624-2147484672 -k sftp://mushroom:2222/sdcard/ssh/buh -o buh-new As for scp, it's not as clear what needs to be done. It doesn't use lseek. But it does use off_t a bit, including on an index in a for loop that is compared against st_size (which is 64-bit). So I'll just change all of the off_t to off64_t and hope for the best. sftp and rsync work! Not gonna bother testing scp on big files... October 1, 2016. Jared Stafford told me startForeground() improves responsiveness on Nougat. There had been a comment suggesting startForeground(), but I never got around to trying it because it has worked "well enough". With Nougat, though, there is a definite tendency for SimpleSSHD to be non-responsive. I'm not sure exactly what its cause is, but the symptom I notice most frequently is that the first ssh connection after a while will be delayed "a long time" - on the order of 10-30 seconds, or maybe indefinitely sometimes. Oddly, a second connection can sometimes get through undelayed, even before the first connection does. It is as if the fork() of process for the new connection is where the delay is, not in the listen() call. That's not overall too surprising, Nougat is a lot harsher about background processes as part of a Google push to reduce power consumption on idle devices. Another concern is related to a change back in July - sometimes the system will kill the sshd process for no good reason (maybe because they removed it from the recent apps list). The remedy I settled on was to monitor for the sshd process dying from within the regular Android-managed process, and restart it. I guess I didn't write down where in the documentation I found it, but Android seems expressly antagonistic to non-system-managed processes. If they ever get more hard-assed about that, this whole idea goes out the window. Anyways, I implemented startForeground(), and I am unhappy that it requires a Notification. At API 7 (Android 2.1) those are really primitive. For example, the PRIORITY_MIN behavior which will sometimes hide the notification is added in API 16 (Android 4.1). So, I've got it with this stupid old-style notification, and it really doesn't look good, and it is always present. That is not awesome. On the other hand, it's easy to block the notifications, and a few people have expressed an interest in a notification. It doesn't seem worth it to me to upgrade to a newer API just for the better notifications... On the other hand, Google Play shows that I have 862 users: Android 7+ : 3.13% Android 6+ : 37.47% Android 5+ : 67.86% Android 4.1+: 96.62% Android 4.0+: 98.01% The oldest reported version is Android 2.3. So, there are a few people on very old versions, but actually SimpleSSHD is used on newer devices than the average app, which is the reverse of my typical trend. So a few people would be negatively impacted, but not a very large number. I could switch to multi-APK mode so that legacy users are just stuck with an unsupported back-version, which is probably what they truly want anyways.. Anyways, I'm gonna use it with startForeground() and the notifications disabled for a while, and if I find it to be an improvement then I'll just make it so clicking on the notification goes to the app, update the doc, and then publish it. ... Looking up doze mode details for improving a different app, I stumbled across this: https://www.bignerdranch.com/blog/diving-into-doze-mode-for-developers/ One last case which is not mentioned in the Android documentation is an app using a foreground service. Any process using a foreground service is exempt from Doze Mode effects, which is key for long-running applications that you may want to run even if the user has put their phone down for a long period of time -- like a music app, for example. There is, however, a bug in Android Marshmallow which requires a foreground service to be in a separate process from all other activity code, documented here. It is planned to be fixed in Nougat. Not exactly confidence-inspiring. And I haven't come across a description of how exactly doze mode screws up SimpleSSHD, either. It might be worthwhile to do the work to find out where things actually get wedged, it might even be a flaw in Dropbear that makes it so unreliable with Nougat. October 16, 2016. I've been using a foreground service for a while, and it has not caused me any troubles. The thing I have been most anxious about is that it might disable doze mode entirely...but there has not been any noteworthy overnight drain. So, in case someone does have problems with it, I am making it an option that defaults to enabled. Looking through email, I haven't seen any, but I have the idea several users have asked for a notification icon in the past. And now that that is finally implemented, I am curious about other things people have requested that I have not been keen on. And Jan Ondrej's requests come to mind: o) setting to start the service automatically when the application is launched o) QUIT button that stops the service and the activity at once o) not allow wifi to power down when the activity is open I'm really not crazy about integrating any kind of wakelock. And having two buttons still seems silly to me. But with the notification there, the idea that someone will micromanage whether the service is running or not does not seem so far fetched. So I'll go ahead and add "Start on Open" setting. ... Looking at reviews on Google Play Store, I finally found a couple reviews in the last couple months who would have enjoyed password login. I'm still pretty opposed to it because it seems like the usage model would be to enable a default password, login with it to transfer an authorized_keys file, and then disable the default password. That is fine, and would even be kind of convenient (I might use it), but it seems more often than not the default password would be left enabled indefinitely, which is not awesome. And I can't imagine more than 1% of users ever typing a strong password into their phones. But I've thought of a compromise. What if, if there is no authorized_keys file, it accepts passwordless logins but with some sort of obnoxious alert dialog sort of thing to interrupt the user? That way, you would be able to login once to copy the authorized keys file, and the nuissance alert would be no big deal. Then once the authorized keys exists, no further action is necessary. It's a pity there is no convenient way to interact between the Android GUI thread and the sshd thread. Ah-hah! I've got the least effort idea in my head! Under situations that I'm not sure what they should be, it should generate a random password and display it on the phone's screen. Probably it should generate it iff there is no authorized_keys file at all. Since that detection happens for each client connection, then probably all of this logic should go in dropbear itself, and the display should come through dropbear.err. Then we might even want a UI way to delete authorized_keys, perhaps even as a replacement for the current awkward UI. November 19, 2016. I got a user request to update for security. I looked at dropbear and didn't see any relevant security issues, so I'm gonna hold off for a while, but it should be on my radar. I also got a user request for writing to external SD card. He gave this link: http://stackoverflow.com/questions/33162152/storage-permission-error-in-marshmallow It gives me a really strong deja vu, I think I tried that a few months ago when a nearly identical request came in, and it didn't work. I wish I had kept notes for that experiment! I think there are two separate issues. I think Android 6+ push you to use requestPermission() to explicitly enable the WRITE_EXTERNAL_STORAGE permission, but if targetSdkVersion is low enough then you are grandfathered in, so we don't care. The second issue is that Android 5+ require use of a special API to access a removable SD card (which is different from /sdcard, which is typically internal storage protected by WRITE_EXTERNAL_STORAGE on new phones?). That second issue is what is biting people. I don't know any way around that. If Google doesn't back down, then I guess the only plausible way around it would be something like an LD_PRELOAD that intercepts open/lseek/read/write/close to external SD, and replaces them with connection to a local daemon process that is running in a typical Android context and is able to use the awful API. Seems like a lot of work and complication. I might go through with it if I happened to use external SD myself, but I'm of the personal opinion that removable storage is obsolete now that even relatively cheap phones like the base Moto G4 come with 16GB... 640kB ought to be enough for anybody. I told the most recent guy to try SuperSU. I don't have any idea if that will really work, to be honest. October 28, 2017. At the beginning of October, a user notified me that rsync doesn't work on Android 8.0 (Oreo, API 26). In the past week, Google has upgraded my Nexus 5x to Oreo and I can confirm it. It seems that SE Linux or something causes certain system calls to perform the equivalent of: fprintf(stderr, "Bad system call\n"); exit(-1); It should return ENOSYS instead, but someone at Google is not cool enough to be working with Unix. The first one I discovered like this is sigprocmask(), which we can probably do without. The next one is chmod(), which is somewhat more useful. The troubling thing is that I can find a file where the commandline chmod works, but the same chmod() doesn't work in rsync. I figured maybe my NDK was just too old ("r10d"), but I think I can't conveniently upgrade it because Google only distributes the NDK for Linux x86-64 now. I was using SDK 7, so I tried SDK 11, 17, 19, and got the same result. The NDK I have also has a directory for SDK 21, but it has typos in the header files, or they aren't compatible with its version of gcc. But anyways, if SDK 19 doesn't work then maybe it's more than just linking against a bad version of Bionic. Well, I think I've got it - fchmod() works, but chmod() does not. *shrug* March 4, 2018. Looked in the google play console for crashes, and I find a few. 10 people have run into UnsatisfiedLinkError, and they seem to all be using x86. According to https://stackoverflow.com/questions/32598756/android-ndkhow-to-exclude-x86-device-in-google-player It's not plausible to fix this problem by blacklisting x86 devices! Google sucks! And the justification is that there is an ARM emulation layer, but obviously it doesn't work and why should I have to debug it?? Anyways, someday may want to try making a jni/Application.mk and setting APP_ABI in there to support x86 too? Do I really think that will work for scp/rsync/etc? Guh. And this bug has only one report... The startActivity() call to open the documentation is getting a android.content.ActivityNotFoundException. I guess it must mean there is no browser installed or something? I seem to be using the normal way to visit an http programatically.. Only one report, so I guess I just don't care. March 24, 2018. Win Bent emailed me to let me know his x86_64 Asus ZenPad is able to run SimpleSSHD but is unable to run rsync. So, the ARM-on-x86 emulation layer is catching up and now works sometimes, but isn't compatible with the hack for scp/sftp/rsync. Win was kind enough to test a build that supports just armeabi and x86, and reported it works. So I don't need to build support for both x86 and x86_64, apparently. Nice to have a concrete answer to the question! XXX - make the default path for authorized_keys be in the app-private, too, for security ? XXX - show /data/data/org.galexander.sshd/files somewhere as a suggested place to put files XXX - find the /data/data/org.galexander.sshd/lib/ directory programatically, so it will work with whatever bizarre secure android with virtualized apps