In this case, looking at your code:
Qt Code: 
  1. arguments << "-c" << "iptables -A "<<ui->comboBox->currentText()<<" -p icmp -i eth0 -j DROP";
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Using QProcess you must know that every argument should be independent. Something like this:

Qt Code: 
  1. arguments << "-c" << "iptables" << "-A" << ui->comboBox->currentText() << "-p" << "icmp" << "-i"  << "eth0"  << "-j" << "DROP";
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... should work. But, two points about this:
1- I never tried commands with so many arguments, but this should not be a problem.
2- Actually, your program to execute should not be "/bin/bash", it should be "iptables" directly, because QProcess at the end executes every command at the bash, so you don't need to specify this.

Maybe this code could be better:
QString prog = "iptables";
QStringList arguments;
arguments "-A" << ui->comboBox->currentText() << "-p" << "icmp" << "-i" << "eth0" << "-j" << "DROP";
QProcess* process = new QProcess(this);
process->start(prog , arguments);
process->waitForFinished();
QString tmp = process->readAll();
qDebug() << tmp;
Hope it helps. :-)