Configuring password less access to remote systems using ssh-keygen utility

When we say password less authentication, we mean that the client authentication is carried out using public and private keys.

Generate public/private key pair
The ssh-keygen utility is used to generate the public/private key pair. The keys generated are stored under .ssh directory in the user home directory. The private key is never shared and stored in the local machine whereas the public key is distributed to the machines you want to login to.

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Installing And Configuring SSH server on Windows

We will be making use of Cygwin utilities to configure and run ssh as service on windows machine. Installing Cygwin on a windows machine is pretty straight forward. Download the latest Cygwin installation setup.exe from the Cygwin site and follow the below instructions.

1. Installing Cygwin
Step 1. Double click setup.exe

Step 2. Choose the download source

Step 3. Select the root directory of the Cygwin. This directory is synonymous to / in linux

Step 4. Select the directory where you want to keep the installation files. You can save this directory and use at a later point to install Cygwin on any windows machine using this directory.

Click 'OK' if prompted to create the directory if it does not exists

Step 5. Select the type of connection you are using to connect to internet.

Step 6. Choose a download site.

Step 7. Clicking next opens up the 'Select Packages' screen.

Step 8. Select the Open SSH server and client programs from the 'Select Packages' screen.

Step 9. Click next to start the installation.

This will install the following utilities in your Cygwin's /usr/bin directory

ssh-add.exe
ssh-agent.exe
ssh-host-config
ssh-keygen.exe
ssh-keyscan.exe
ssh-user-config
ssh.exe

2. Configuring ssh as Windows service

Run ssh-host-config utility to configure sshd server on windows. Select 'no' when prompted for 'Should privilege separation be used? (yes/no)'. Select 'yes' when prompted for 'Do you want to install sshd as service?'. Choose default options for other options.

The above will install CYGWIN sshd service on Windows. To start the service execute

net start sshd

3. Connecting using Cygwin ssh client (ssh.exe)

ssh.exe user@ssh-server

4. Connecting through putty

Add the server's host key to registry. This will add an entry into the ~/.ssh/known_hosts file

Login using windows user and password

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POSIX Threads - A simple Example

In our earlier post, we have written multithreaded program using win32 apis. This post deals with writing the multithreaded programs using POSIX thread apis. The important POSIX thread apis are namely pthread_create, pthread_exit and pthread_join

Create a thread

int pthread_create
(
    pthread_t *thread,
    const pthread_attr_t *attr,
    void *( *start_routine)(void *), 
    void *arg
);

Terminate a thread

void pthread_exit (void *retval);

Wait on a thread

int pthread_join (pthread_t thread, void **status);

The Complete Example

#include <pthread.h>
#include <iostream>

//File: SimpleThreadExample.cpp

#define MAX_THREADS 3
using namespace std;

void* func(void *args)
{
    int tp = *(int *)args * 10;
    while(tp--)
    {
        cout << *(int *) args;
        fflush(stdout);
        sleep(1);
    }

    cout << endl;
    pthread_exit((void *)10);
    return (void *)10;
}

int main(void)
{
    pthread_t tid[MAX_THREADS] = {0};
    int param[MAX_THREADS] = { 1, 2, 3 };

    int rc;
    for(int i = 0; i < MAX_THREADS; ++i)
    {
        rc = pthread_create(&tid[i], NULL, func, ¶m[i]);
        if(0 != rc)
        {
            cout << "ERROR: Thread Creation failed" << endl;
        }
        cout << "Thread[" << tid[i] << "] Created" << endl;
    }

    void *status[3];
    for(int i = 0; i < MAX_THREADS; ++i)
    {
        rc = pthread_join(tid[i], &status[i]);
    }

    for(int i = 0; i < MAX_THREADS; ++i)
    {
        cout << "Thread[" << tid[i] << "] returned with exit code=" << (long) status[i] << endl;
    }

    cout << "Main thread terminating" << endl;
    return 0;
}

The makefile for the project

#-----------------------------------------------------------------------
#                   A simple thread example
#-----------------------------------------------------------------------
PROJECT := SimpleThreadExample

SRCEXT := .h .cpp
OBJEXT := .o
EXEEXT := .exe

#compilers
CC := gcc
CXX := g++

#flags
CFLAGS := -o
CXXFLAGS := -o
LDFLAGS := -lpthread -o

#disable implicit suffix rules
.SUFFIXES:
.SUFFIXES: $(SRCEXT) $(OBJEXT) $(EXEEXT)

SRC := \
    SimpleThreadExample.cpp

OBJ := \
    $(SRC:.cpp=.o)

EXE := \
    $(PROJECT)$(EXEEXT)

#define dummy targets
.PHONY: all clean compile link

all : compile link

clean :
    @echo

    @echo "Cleaning Temporary Files..."
    $(RM) $(OBJ) $(EXE)

compile : $(OBJ)

link : $(EXE)

$(OBJ) :
    @echo
    @echo "Compiling Source Files..."
    $(CXX) $(CXXFLAGS) $(OBJ) $(SRC)

$(EXE) :
    @echo
    @echo "Linking..."
    $(LD) $(OBJ) $(LDFLAGS) $(EXE)

run :
    @echo
    @$(EXE)

Building the project. (The output is from Cygwin Environment. You may need to modify the makefile to compile it on a linux environment)

$ gmake clean all

Cleaning Temporary Files...
rm -f SimpleThreadExample.o SimpleThreadExample.exe

Compiling Source Files...
g++ -o SimpleThreadExample.o SimpleThreadExample.cpp

Linking...
ld SimpleThreadExample.o -lpthread -o SimpleThreadExample.exe

Output:
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