native C socket/connect problems

[Fred Templin]

Hello,

I am using NDK with Android Studio 14.1 Beta. I have a tablet that is running lollipop

(Android v. 5.0.1), and I am building for API 21. My code consists of a simple java

front end derived from the ToyVpn sample code that comes with the SDK. The java

code calls a C library that I developed, debugged and tested under Linux.

In the C library, I am using the standard networking socket API. I successfully create

a socket in the main routine as:

  addr_fd = socket(AF_INET, SOCK_DGRAM, 0);

where "addr_fd" is a global integer. Then, in a signal handler routine I connect to

the socket as follows:

  struct sockaddr_in addr;

  addr.sin_family = AF_INET;

  addr.sin_port = port;  /* a valid UDP port number */

  addr.sin_addr = addr; /* a valid IPv4 address */

  if (connect(addr_fd, (struct sockaddr *)&addr, sizeof(addr)) != 0)

    syslog(LOG_ERR, "connect failed: %s", strerror(errno));

  else

    syslog(LOG_ERR, "connect succeeded");

When I run the code, it always reports "connect failed: Network is Unreachable"

even though the network was set up correctly before I ran the app. After

terminating the app, however, I also get "Network Unreachable" when I try

to ping addr with a different app even though the pings succeeded before I

ran my app. So, my app is clearly messing up the network somehow in

addition to not being able to connect. Any ideas on how to fix this?

Thanks - Fred

 

 

[J Decker]

sure you have network permissions in your manifest?

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[Fred Templin]

Thanks for the reply. In my file named ./app/src/main/AndroidManifest.xml, I have:

<uses-permission android:name="android.permission.INTERNET"></uses-permission>

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"></uses-permission>

<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"></uses-permission>

<uses-permission android:name="android.permission.CHANGE_WIFI_STATE"></uses-permission>

Is that the correct Manifest file, or do I need to add these lines somewhere else?

In my app, I am running the following native C code:

    while (1) {

        int fd;

        struct sockaddr_in addr;

        fd = socket (AF_INET, SOCK_DGRAM, 0);

        if (fd < 0) {

                syslog(LOG_ERR, "socket: %s\n", strerror(errno));

                break;

         }

        addr.sin_family = AF_INET;

        addr.sin_port = 0;

        addr.sin_addr.s_addr = pr->ip;

        if (connect (fd, (struct sockaddr*)&addr, sizeof(addr))== 0)

                syslog(LOG_ERR, "socket connect works!\n");

        else

                syslog(LOG_ERR, "socket connect fails: %s\n", strerr(errno));

        close (fd);

        sleep(2);

    }

When the code runs, it prints:

    socket connect works!

    socket connect fails: Network is unreachable

    socket connect fails: Network is unreachable

    socket connect fails: Network is unreachable

    ...

In other works, the connect works the first time then fails all subsequent times. As

a side effect of running this code in my app, the networking system in the entire

tablet seems to get destroyed. In particular, when I try to ping from a terminal

emulator window I get "Network is Unreachable".

This code works under Linux. Is there something I am either forgetting or doing

wrong under Android?

Thanks - Fred

[Fred Templin]

About network permissions in the manifest, I am a little bit confused as to *which*

file to add the permissions to. I see the following files as possible candidates:

(1)  ./app/build.gradle

(2)  ./app/src/main/AndroidManifest.xml

(3)  ./app/build/intermediates/manifest/androidTest/all/debug/AndroidManifest.xml

(4)  ./app/build/intermediates/manifests/full/all/debug/AndroidManifest.xml

I have done nothing to (1). In (2), I have added the following lines:

<uses-permission android:name="android.permission.INTERNET"></uses-permission>

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"></uses-permission>

<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"></uses-permission>

<uses-permission android:name="android.permission.CHANGE_WIFI_STATE"></uses-permission>

After building my project in Android Studio, I note that the "uses-permission" lines

show up in (4) but do not show up in (3). I assume (3) and (4) are auto-generated.

Am I somehow missing something? The existence of file (3) with no permissions

worries me. Could that be the reason the network is failing?

Thanks - Fred

On Tuesday, September 22, 2015 at 8:33:25 PM UTC-7, J Decker wrote:

[J Decker]

Okay I don't think port 0 is a valid port.
and you don't bind your socket to any port so there's no way it can
send or receive (having no src or dest for the other side to send to)

http://www.cs.odu.edu/~cs476/fall03/lectures/sockets.htm

[Fred Templin]

Another update. In the C code I provided in the previous message, I inserted the

line: "sleep (1)" before the first socket/connect invocation. When I did that, I got

"socket connect fails: Network is unreachable" in the very first attempt. So, the

condition appears to be timing related.

So, somehow the way in which my native C code is being invoked is trashing

the network within a short amount of time after startup. So, the condition does

not appear to be related to socket/connect directly, but is somehow related to

the very invocation of the code. Ideas are welcome!

Thanks - Fred 

[Fred Templin]

Many thanks for the idea. I tried changing the port to "addr.sin_port = htons(8060);"

but there was no change in the behavior - still getting success on the first connect

and "network is unreachable" on all subsequent connects. I am not using the socket

to send or receive any packets (only to discover the address of the outgoing

interface to be used to send to the destination) so setting port to 0 should be OK,

and works fine on linux.

It worries me that the automatically generated AndrodManifest.xml file that

shows up in "./app//build/intermediates/manifest/androidTest/all/debug" does

not have any of the "uses-permission" lines that the other AndroidManifest.xml

files have. Could this be the problem? And, what do I need to do to make

the "uses-permission" lines show up there?

Thanks - Fred