Threads in C

We have seen how fork() works in C.  They say, threads are light-weight processes.  Will see how it is done.  Sample program with 3 threads is given here:

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#include <stdio.h> #include <pthread.h> #include <unistd.h> void * threadrun (void * poin) { int k=*(int *)poin; for (int i=1;i<5;i++) { sleep(2); printf("iteration %d of thread %d\n",i,k); } return NULL; } int main() { pthread_t tid1,tid2,tid3; pthread_attr_t attr; pthread_attr_init(&attr); int p=1; pthread_create(&tid1, &attr,threadrun, &p); int q=2; pthread_create(&tid2, &attr,threadrun, &q); int r=3; pthread_create(&tid3, &attr,threadrun, &r); pthread_join(tid1,NULL); pthread_join(tid2,NULL); pthread_join(tid3,NULL); return 0; }

The thread function should take in a void pointer, return a void pointer to a pointer.  The function threadrun will print 4 iterations each while sleeping 2 seconds for each iteration.  We are returning NULL.

Thread Creation
Line 19 - Create an identifier
Line 20/21 - Create an attribute structure and initialize
Line 23 - Create a value to pass it to thread, so that we can identify the thread
Line 24 - Start the Thread

After thread is created, they will start executing independent of each other.  You can see the stdout the sequence in which the prints are made.

Thread Join
Line 30 - We wait till thread1 is returned.
Line 31 - We wait till thread2 is returned
Line 32 - We wait till thread3 is returned

As Thread1 is first in line,  even though thread 2 and 3 returns earlier, our program will wait till thread1 is completed.

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# ./thread iteration 1 of thread 1 iteration 1 of thread 3 iteration 1 of thread 2 iteration 2 of thread 1 iteration 2 of thread 2 iteration 2 of thread 3 iteration 3 of thread 3 iteration 3 of thread 2 iteration 3 of thread 1 iteration 4 of thread 3 iteration 4 of thread 2 iteration 4 of thread 1

Ssh using Openssh

SSH Secure Shell, whenever we want to execute a shell command on a remote machine, it is used.  The whole idea here is to make sure that eaves drop will not be able to decrypt it, as it is encrypted.

Now, we already have a linux machine(172.16.0.1) that has openssh server running.  I have a client linux machine(172.16.0.20).  For the first time, whenever we issue a command.

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# ssh 172.16.0.1 -l root ls Host '172.16.0.1' is not in the trusted hosts file. (ssh-rsa fingerprint sha1!! 3f:61:51:b5:00:02:64:f9:c4:c3:dd:a1:e9:eb:ac:9e:1b:aa:ef:68) Do you want to continue connecting? (y/n) y root@172.16.0.1's password: abc.txt def.txt #

Line 5: It asks whether to include public key in the file ~/.ssh/known_hosts
Line 6: Password is needed

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# cat .ssh/known_hosts 172.16.0.1 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC97bEOOLravOBmbx/P+5BXxEEH49N3tdS3PgynAwdma/OY6fGzFrXhGnKZgYKbkKSTldKpXs3xUdRhjR6jU9VN4GcjzmcFfNgniGdq6SntV/U9lrdCGt2V5dLdTAE8I3q2vQL9i/OQ83fcu9CVKN6/O0jToJ65n9lrgrsFfSjkFWlPJLh2l9Syvgl1OReat6iBetcpetUnC0CTG9ORwpDsl+GbQAXc2VSWirafQRDNFNTC1aOwgUa9YMFY+XUqetU/76nYr4MC8JNoWkqvH0O+6UTR5wKhYDMaDIr9PGtTJF+2Hqt97XYAudieIwBogiGNl5/fF2GKW3j7ykibDAxH #

You can see the entry added in the file.  Now, delete the file.  We can avoid the banner by explicitly adding it by ourselves.  It will be available by two methods:

Using ssh-keyscan command remotely:

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# ssh-keyscan -H 172.16.0.1 # 172.16.0.1 SSH-2.0-OpenSSH_6.2 no hostkey alg # 172.16.0.1 SSH-2.0-OpenSSH_6.2 |1|Dg0BPDSZVlQ397prZu+fKSB31zM=|Vab1XH6An8aw4QQoR1rln8mvwo0= ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC97bEOOLravOBmbx/P+5BXxEEH49N3tdS3PgynAwdma/OY6fGzFrXhGnKZgYKbkKSTldKpXs3xUdRhjR6jU9VN4GcjzmcFfNgniGdq6SntV/U9lrdCGt2V5cLdSAD8I3q2vQL9i/OQ83fcu9CVKN6/O0jToJ65n9lrgrsFfSjkFWlPJLh2l9Syvgl1OReat6iBetcpetUnC0CTG9ORwpDsl+GbQAXc2VSWirafQRDNFNTC1aOwgUa9YMFY+XUqetU/76nYr4MC8JNoWkqvH0O+6UTR5wKhYDMaDIr9PGtTJF+2Hqt97XYAudieIwBogiGNl5/fF2GKW3j7ykibDAxH

On the server machine (172.16.0.1), in the following location

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#cat /etc/ssh/ssh_host_rsa_key.pub ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC97bEOOLravOBmbx/P+5BXxEEH49N3tdS3PgynAwdma/OY6fGzFrXhGnKZgYKbkKSTldKpXs3xUdRhjR6jU9VN4GcjzmcFfNgniGdq6SntV/U9lrdCGt2V5cLdSAD8I3q2vQL9i/OQ83fcu9CVKN6/O0jToJ65n9lrgrsFfSjkFWlPJLh2l9Syvgl1OReat6iBetcpetUnC0CTG9ORwpDsl+GbQAXc2VSWirafQRDNFNTC1aOwgUa9YMFY+XUqetU/76nYr4MC8JNoWkqvH0O+6UTR5wKhYDMaDIr9PGtTJF+2Hqt97XYAudieIwBogiGNl5/fF2GKW3j7ykibDAxH

Copy and paste the corresponding public key in the known_hosts file of client(172.16.0.20)

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# vi .ssh/known_hosts # ssh 172.16.0.1 -l root ls root@172.16.0.1's password: abc.txt def.txt #

Passwordless Login for the server

Whenever we issue ssh to the server, it always asks for password.  To avoid this, we have to keep that user's public key in the 'authorized_key' list of server.  Remember, it has to be public key of user@hostname(~/.ssh/id_rsa.pub) and NOT public key of the hostname(/etc/ssh/ssh_host_rsa_key.pub)

Initially 'id_rsa.pub' would not be present.  There will be only 2 files 'known_hosts' and 'authorized hosts'

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# ls ~/.ssh/ authorized_keys known_hosts # ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/root/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /root/.ssh/id_rsa. Your public key has been saved in /root/.ssh/id_rsa.pub. The key fingerprint is: d6:1c:43:59:bc:fe:fa:36:e4:44:0c:12:96:41:2f:f9 root@svtap01end1.bec.broadcom.net The key's randomart image is: +--[ RSA 2048]----+ | .*B. | | o+oo | | =..+ | | o =. o | | S o.E. | | . . o | | = | | = | | .+.. | +-----------------+ # ls ~/.ssh/ authorized_keys id_rsa id_rsa.pub known_hosts # cat ~/.ssh/id_rsa.pub ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC6gtqYxfBe/dcBtMPh/j1x6eAVOLbn9ooY12ztDs15q4kkjw0YNkHVcw9vX3Ys5eIx2N+5WaFc8uCuZFqK+F3/qybCNkIditLwoC8OOH3zXZeBB+hzkACl1ThEIIHRlqUX79xMokPwOM99OOt6CwKZCl81aRF75+QW6ek8PxMvEm4O9zb22pGU36Qb5PS6IOKfBUbIUi4v7dR5ElBiziPCZqXgFDf5iZAS83hyvSVxl5VG7dhO0GfWGa+KrQ1kwdK5oyIL81PBgkQFrxVzejApSxwBgNs+1B9e8Tq5/FsxcLfUB0w12VQlIedZOeUDhh18+MoUrStdNYrEZqNwt5Ox root@svtap01end1.bec.broadcom.net #

Copy the output of id_rsa.pub to authorized hosts in the server.  After doing lot of trails of copy paste the contents to the file, in-build command 'ssh-copy-id' worked smoothly.

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# ssh-copy-id root@172.16.0.1 /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed /usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys root@172.16.0.1's password: Number of key(s) added: 1 Now try logging into the machine, with: "ssh 'root@172.16.0.1'" and check to make sure that only the key(s) you wanted were added. # ssh 172.16.0.1 ls abc.txt def.txt #

Thank you.