Chapter 6. Network applications

Modern mail service are under some limitations in order to minimize exposure to the spam (unwanted and unsolicited email) problems.

This is because:

Some programs on Debian expect to access the /usr/sbin/sendmail command to send emails as their default or customized setting since the mail service on a UNIX system functioned historically as:

The OpenSSH SSH daemon supports SSH protocol 2 only.

Please read "/usr/share/doc/openssh-client/README.Debian.gz", ssh(1), sshd(8), ssh-agent(1), and ssh-keygen(1), ssh-add(1) and ssh-agent(1).

	Warning
	"/etc/ssh/sshd_not_to_be_run" must not be present if one wishes to run the OpenSSH server. Don't enable rhost based authentication (HostbasedAuthentication in /etc/ssh/sshd_config).

The following starts an ssh(1) connection from a client.

If you use the same user name on the local and the remote host, you can eliminate typing "username@".

One can avoid having to remember passwords for remote systems by using "PubkeyAuthentication" (SSH-2 protocol).

On the remote system, set the respective entries, "PubkeyAuthentication yes", in "/etc/ssh/sshd_config".

Generate authentication keys locally and install the public key on the remote system by the following.

$ ssh-keygen -t rsa
$ cat .ssh/id_rsa.pub | ssh user1@remote "cat - >>.ssh/authorized_keys"

You can add options to the entries in "~/.ssh/authorized_keys" to limit hosts and to run specific commands. See sshd(8) "AUTHORIZED_KEYS FILE FORMAT".

It is safer to protect your SSH authentication secret keys with a pass phrase. If a pass phrase was not set, use "ssh-keygen -p" to set it.

Place your public SSH key (e.g. "~/.ssh/id_rsa.pub") into "~/.ssh/authorized_keys" on a remote host using a password-based connection to the remote host as described above.

$ ssh-agent bash
$ ssh-add ~/.ssh/id_rsa
Enter passphrase for /home/username/.ssh/id_rsa:
Identity added: /home/username/.ssh/id_rsa (/home/username/.ssh/id_rsa)

No remote password needed from here on for the next command.

$ scp foo username@remote.host:foo

Press ^D to terminating ssh-agent session.

For the X server, the normal Debian startup script executes ssh-agent as the parent process. So you only need to execute ssh-add once. For more, read ssh-agent(1) and ssh-add(1).

If you have an SSH shell account on a server with proper DNS settings, you can send a mail generated on your workstation as an email genuinely sent from the remote server.

$ ssh username@example.org /usr/sbin/sendmail -bm -ti -f "username@example.org" < mail_data.txt

To establish a pipe to connect to port 25 of remote-server from port 4025 of localhost, and to port 110 of remote-server from port 4110 of localhost through ssh, execute on the local host as the following.

# ssh -q -L 4025:remote-server:25 4110:remote-server:110 username@remote-server

This is a secure way to make connections to SMTP/POP3 servers over the Internet. Set the "AllowTcpForwarding" entry to "yes" in "/etc/ssh/sshd_config" of the remote host.

If you have problems, check the permissions of configuration files and run ssh with the "-v" option.

Use the "-p" option if you are root and have trouble with a firewall; this avoids the use of server ports 1 — 1023.

If ssh connections to a remote site suddenly stop working, it may be the result of tinkering by the sysadmin, most likely a change in "host_key" during system maintenance. After making sure this is the case and nobody is trying to fake the remote host by some clever hack, one can regain a connection by removing the "host_key" entry from "~/.ssh/known_hosts" on the local host.