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Full Discussion: Controlling a child's stdin/stdout (not working with scp)
Top Forums Programming Controlling a child's stdin/stdout (not working with scp) Post 302402713 by DreamWarrior on Wednesday 10th of March 2010 12:09:42 PM
Old 03-10-2010
Controlling a child's stdin/stdout (not working with scp)
All,

Ok...so I know I *should* be able to control a process's stdin and stdout from the parent by creating pipes and then dup'ing them in the child. And, this works with all "normal" programs that I've tried. Unfortunately, I want to intercept the stdin/out of the scp application and it seems scp is actively refusing to allow me to. The code is:

Code:
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int main()
{
	char	usr[32];
	char	host[32];
	char	remote_path[64];
	char	local_path[64];
	
	int	outfd[2];
	int	infd[2];
	
	/* set usr/host/remote_path/local_path to something meaningful */
	
	/* create pipes for stdin/out to dup in the child */	
	
	pipe(outfd); /* Where the parent is going to write to */
	pipe(infd); /* From where parent is going to read */
	
	if(!fork())
	{
		char	cmd[1024];
		
		/* child, close stdin/out and use parent pipes instead */
			
		close(STDOUT_FILENO);
		close(STDIN_FILENO);
		
		dup2(outfd[0], STDIN_FILENO);
		dup2(infd[1], STDOUT_FILENO);
		
		/* close all pipe ends, needed end has been dup'd other end
		     is not needed */
		     
		close(outfd[0]);
		close(outfd[1]);
		close(infd[0]);
		close(infd[1]);
		
		/* execute scp */
		
		sprintf(cmd, "scp %s@%s:%s %s",
			usr, host, remote_path, local_path);
		
		return system(cmd);
	}

	/* parent */
	
	char input[100];
		
	close(outfd[0]); /* These are being used by the child */
	close(infd[1]);
	
	input[read(infd[0],input,100)] = 0; /* Read from child's stdout */
	
	printf("%s",input);

	/* once I can actually read/write to scp here, I'll have to determine if this is
	     the password prompt or the "add to host key file" prompt and respond
	     accordingly. */
	
	close(outfd[1]);
	close(infd[0]);
}

I know the code works if I use something other than scp, I can write and read to the pipes all day long and control the process as desired. But, it seems that scp is specifically figuring out what the controlling terminal of the process is and actively refusing to co-operate with my pipes.

I then thought maybe I could try to "disown" the terminal by calling "setsid()" right before invoking scp (and tried it right after the fork). Well, that "works," but then the stupid thing connects to my X server and pops up a box to ask for the password. Of course, preventing scp from asking me anything is the exact reason I want to control the standard in of the process so I can funnel the password in through there.

So...does any one have another idea how I can redirect stdin/stdout for this most difficult process? It seems I'll actually need to "hook into" the terminal directly since scp is "so smart." Indeed, I believe scp may be openning up its own connection to the terminal to get input and send output....

Thanks....

P.S. I know there is a better way to do this, like creating the ssh keys so scp never asks for passwords to begin with. Unfortunately, the environment where I'm trying to use this disallows this. They want us to enter passwords every time and this "stupid" rule is circumventing my ability to automate tasks through scp; of course ftp is also taboo. So...I want this "scp wrapper" to read my local password file and just pass it through so I can get back to my regularly scheduled automation. Otherwise, I'll have to enter the passwords again and again while the scripts run...and that's just...well...pointless.
 

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LEARN ABOUT SUSE

scp

SCP(1)							    BSD General Commands Manual 						    SCP(1)

NAME

     scp -- secure copy (remote file copy program)

SYNOPSIS

     scp [-1246BCpqrv] [-c cipher] [-F ssh_config] [-i identity_file] [-l limit] [-o ssh_option] [-P port] [-S program] [[user@]host1:]file1 ...
	 [[user@]host2:]file2

DESCRIPTION

     scp copies files between hosts on a network.  It uses ssh(1) for data transfer, and uses the same authentication and provides the same secu-
     rity as ssh(1).  Unlike rcp(1), scp will ask for passwords or passphrases if they are needed for authentication.

     File names may contain a user and host specification to indicate that the file is to be copied to/from that host.	Local file names can be
     made explicit using absolute or relative pathnames to avoid scp treating file names containing ':' as host specifiers.  Copies between two
     remote hosts are also permitted.

     The options are as follows:

     -1      Forces scp to use protocol 1.

     -2      Forces scp to use protocol 2.

     -4      Forces scp to use IPv4 addresses only.

     -6      Forces scp to use IPv6 addresses only.

     -B      Selects batch mode (prevents asking for passwords or passphrases).

     -C      Compression enable.  Passes the -C flag to ssh(1) to enable compression.

     -c cipher
	     Selects the cipher to use for encrypting the data transfer.  This option is directly passed to ssh(1).

     -F ssh_config
	     Specifies an alternative per-user configuration file for ssh.  This option is directly passed to ssh(1).

     -i identity_file
	     Selects the file from which the identity (private key) for public key authentication is read.  This option is directly passed to
	     ssh(1).

     -l limit
	     Limits the used bandwidth, specified in Kbit/s.

     -o ssh_option
	     Can be used to pass options to ssh in the format used in ssh_config(5).  This is useful for specifying options for which there is no
	     separate scp command-line flag.  For full details of the options listed below, and their possible values, see ssh_config(5).

		   AddressFamily
		   BatchMode
		   BindAddress
		   ChallengeResponseAuthentication
		   CheckHostIP
		   Cipher
		   Ciphers
		   Compression
		   CompressionLevel
		   ConnectionAttempts
		   ConnectTimeout
		   ControlMaster
		   ControlPath
		   GlobalKnownHostsFile
		   GSSAPIAuthentication
		   GSSAPIDelegateCredentials
		   HashKnownHosts
		   Host
		   HostbasedAuthentication
		   HostKeyAlgorithms
		   HostKeyAlias
		   HostName
		   IdentityFile
		   IdentitiesOnly
		   KbdInteractiveDevices
		   LogLevel
		   MACs
		   NoHostAuthenticationForLocalhost
		   NumberOfPasswordPrompts
		   PasswordAuthentication
		   PKCS11Provider
		   Port
		   PreferredAuthentications
		   Protocol
		   ProxyCommand
		   PubkeyAuthentication
		   RekeyLimit
		   RhostsRSAAuthentication
		   RSAAuthentication
		   SendEnv
		   ServerAliveInterval
		   ServerAliveCountMax
		   StrictHostKeyChecking
		   TCPKeepAlive
		   UsePrivilegedPort
		   User
		   UserKnownHostsFile
		   VerifyHostKeyDNS

     -P port
	     Specifies the port to connect to on the remote host.  Note that this option is written with a capital 'P', because -p is already
	     reserved for preserving the times and modes of the file in rcp(1).

     -p      Preserves modification times, access times, and modes from the original file.

     -q      Quiet mode: disables the progress meter as well as warning and diagnostic messages from ssh(1).

     -r      Recursively copy entire directories.  Note that scp follows symbolic links encountered in the tree traversal.

     -S program
	     Name of program to use for the encrypted connection.  The program must understand ssh(1) options.

     -v      Verbose mode.  Causes scp and ssh(1) to print debugging messages about their progress.  This is helpful in debugging connection,
	     authentication, and configuration problems.

     The scp utility exits 0 on success, and >0 if an error occurs.

SEE ALSO

     rcp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh_config(5), sshd(8)

HISTORY

     scp is based on the rcp(1) program in BSD source code from the Regents of the University of California.

AUTHORS

     Timo Rinne <tri@iki.fi>
     Tatu Ylonen <ylo@cs.hut.fi>

BSD
								 February 8, 2010							       BSD
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