Question

I am using two linux machines to simulate some firewall tests… I execute the tests by running nc through ssh on a remote machine… if I spawn the ssh like this, it works…

ssh -i id_dsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no \
    -p 2224 root@a2-idf-lab nc -s 10.26.216.82 10.195.18.132 \
    21 < /var/log/messages

However, if I try to control how much of /var/log/messages with head -c 20 /var/log/messages, the command hangs but I don’t understand why…

ssh -i id_dsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no \
    -p 2224 root@a2-idf-lab nc -s 10.26.216.82 10.195.18.132 \
    21 < head -c 20 /var/log/messages

I also tried this with no better success…

ssh -i id_dsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no \
    -p 2224 root@a2-idf-lab nc -s 10.26.216.82 10.195.18.132 \
    21 < (head -c 20 /var/log/messages)

Question: Why does the second command hang, and how can I accomplish what I need?

FYI, these experiments were really in preparation for sending cat /dev/urandom | base64 | head -c 20 - into nc… bonus points if you can give me cli that would work with nc through an ssh session…

Answer 1

< is shell redirection, it redirects the input stream to read from a file, not to execute a command. try:

head -c 20 /var/log/messages | ssh -i id_dsa -o UserKnownHostsFile=/dev/null \
  -o StrictHostKeyChecking=no \
  -p 2224 root@a2-idf-lab nc -s 10.26.216.82 10.195.18.132 21 

this pipes /var/log/messages from the local machine into nc on the remote machine.
if you want to use the /var/log/messages file on the remote machine, use quotes around the command:

ssh -i id_dsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no \
  -p 2224 root@a2-idf-lab "head -c 20 /var/log/messages |\
     nc -s 10.26.216.82 10.195.18.132 21"