frameworks\base\core\java\android\net\EthernetManager.java

public boolean setEthernetEnabled(boolean enabled) {
        Log.d(TAG,enabled ? "turn on Ethernet" : "turn off Ethernet");
        try {
            return mService.setEthernetEnabled(enabled);
        } catch (RemoteException e) {
            return false;
        }
    }

frameworks\opt\net\ethernet\java\com\android\server\ethernet\EthernetService.java

EthernetServiceImpl.java

public boolean setEthernetEnabled(boolean enable) {
        //enforceChangePermission();
        Log.i(TAG,"setEthernetEnabled() : enable="+enable);
        if ( enable ) {
           return mTracker.setInterfaceUp();
        } else {
           return mTracker.setInterfaceDown(); 
        }
    }

EthernetNetworkFactory.java

public boolean setInterfaceUp() {
       try {
           if(!TextUtils.isEmpty(mIface)) {
               mNMService.setInterfaceUp(mIface);
               sendEthIfaceStateChangedBroadcast(EthernetManager.ETHER_IFACE_STATE_UP);
               return true;
           }
           else
              Log.e(TAG,"mIface is null");
        }catch (Exception e) {
            Log.e(TAG, "Error downing interface " + mIface + ": " + e);
        }
      return false;
    }

\frameworks\base\services\core\java\com\android\server\NetworkManagementService.java

    public void setInterfaceUp(String iface) {
        mContext.enforceCallingOrSelfPermission(CONNECTIVITY_INTERNAL, TAG);
        final InterfaceConfiguration ifcg = getInterfaceConfig(iface);
        ifcg.setInterfaceUp();
        setInterfaceConfig(iface, ifcg);
    }
    
    
    public void setInterfaceConfig(String iface, InterfaceConfiguration cfg) {
        mContext.enforceCallingOrSelfPermission(CONNECTIVITY_INTERNAL, TAG);
        LinkAddress linkAddr = cfg.getLinkAddress();
        if (linkAddr == null || linkAddr.getAddress() == null) {
            throw new IllegalStateException("Null LinkAddress given");
        }

        final Command cmd = new Command("interface", "setcfg", iface,
                linkAddr.getAddress().getHostAddress(),
                linkAddr.getPrefixLength());
        for (String flag : cfg.getFlags()) {
            cmd.appendArg(flag);
        }

        try {
            mConnector.execute(cmd);
        } catch (NativeDaemonConnectorException e) {
            throw e.rethrowAsParcelableException();
        }
    }    

system\netd\server\CommandListener.cpp

CommandListener::InterfaceCmd::InterfaceCmd() :
                 NetdCommand("interface") {
}

int CommandListener::InterfaceCmd::runCommand(SocketClient *cli,
                                                      int argc, char **argv) {
   
        else if (!strcmp(argv[1], "setcfg")) {
            // arglist: iface [addr prefixLength] flags
            if (argc < 4) {
                cli->sendMsg(ResponseCode::CommandSyntaxError, "Missing argument", false);
                return 0;
            }
            ALOGD("Setting iface cfg");

            struct in_addr addr;
            int index = 5;

            ifc_init();

            if (!inet_aton(argv[3], &addr)) {
                // Handle flags only case
                index = 3;
            } else {
                if (ifc_set_addr(argv[2], addr.s_addr)) {
                    cli->sendMsg(ResponseCode::OperationFailed, "Failed to set address", true);
                    ifc_close();
                    return 0;
                }

                // Set prefix length on a non zero address
                if (addr.s_addr != 0 && ifc_set_prefixLength(argv[2], atoi(argv[4]))) {
                   cli->sendMsg(ResponseCode::OperationFailed, "Failed to set prefixLength", true);
                   ifc_close();
                   return 0;
               }
            }

            /* Process flags */
            for (int i = index; i < argc; i++) {
                char *flag = argv[i];
                if (!strcmp(flag, "up")) {
                    ALOGD("Trying to bring up %s", argv[2]);
                    if (ifc_up(argv[2])) {
                        ALOGE("Error upping interface");
                        cli->sendMsg(ResponseCode::OperationFailed, "Failed to up interface", true);
                        ifc_close();
                        return 0;
                    }
                } else if (!strcmp(flag, "down")) {
                    ALOGD("Trying to bring down %s", argv[2]);
                    if (ifc_down(argv[2])) {
                        ALOGE("Error downing interface");
                        cli->sendMsg(ResponseCode::OperationFailed, "Failed to down interface", true);
                        ifc_close();
                        return 0;
                    }
                } else if (!strcmp(flag, "broadcast")) {
                    // currently ignored
                } else if (!strcmp(flag, "multicast")) {
                    // currently ignored
                } else if (!strcmp(flag, "running")) {
                    // currently ignored
                } else if (!strcmp(flag, "loopback")) {
                    // currently ignored
                } else if (!strcmp(flag, "point-to-point")) {
                    // currently ignored
                } else {
                    cli->sendMsg(ResponseCode::CommandParameterError, "Flag unsupported", false);
                    ifc_close();
                    return 0;
                }
            }

            cli->sendMsg(ResponseCode::CommandOkay, "Interface configuration set", false);
            ifc_close();
            return 0;

    }
    return 0;
}

system\core\libnetutils\ifc_utils.c
发送命令到内核；

int ifc_up(const char *name)
{
    int ret = ifc_set_flags(name, IFF_UP, 0);
    if (DBG) printerr("ifc_up(%s) = %d", name, ret);
    return ret;
}

static int ifc_set_flags(const char *name, unsigned set, unsigned clr)
{
    struct ifreq ifr;
    ifc_init_ifr(name, &ifr);

    if(ioctl(ifc_ctl_sock, SIOCGIFFLAGS, &ifr) < 0) return -1;
    ifr.ifr_flags = (ifr.ifr_flags & (~clr)) | set;
    return ioctl(ifc_ctl_sock, SIOCSIFFLAGS, &ifr);
}