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See this page as a slide show

CT320 Secure Shell

Overview

1. The Before Times
2. Compatibility
3. ssh & scp
4. The ~/.ssh Directory
5. System Configuration
6. Asymmetric Encryption
7. Generating a Key Pair
8. Port Forwarding

The Before Times

These protocols were in common use, back in the day:

ftp, rcp

Copy files between hosts

telnet, rlogin

Log in remotely

rsh

Execute a remote command

They transmit information (including passwords!) in plain text over the internet. Ack! Don’t use them. Disable (chmod a-x) ftpd, rlogind, and rshd, or uninstall the packages.

Compatibility

Some systems, such as Ubuntu 18.10, still provide symlinks for backward compatibility:

    % type rlogin
    rlogin is /usr/bin/rlogin

    % ls -log /usr/bin/rlogin
    lrwxrwxrwx 1 24 Nov 21 21:09 /usr/bin/rlogin -> /etc/alternatives/rlogin

    % ls -log /etc/alternatives/rlogin
    lrwxrwxrwx 1 15 Nov 21 21:09 /etc/alternatives/rlogin -> /usr/bin/slogin

    % ls -log /usr/bin/slogin
    lrwxrwxrwx 1  3 Nov 21 21:09 /usr/bin/slogin -> ssh

That’s adorable. Just avoid them.

Clients

• Distinguish between ssh protocol, an ssh client, and an sshd server.
• There are many ssh clients:
  • ssh is a Linux command that uses the ssh protocol for remote login.
  • scp is a Linux command that uses the ssh protocol to copy files.
  • PuTTY (alias putty) is a Windows terminal emulator that uses the ssh protocol to connect to a remote system.
  • WinSCP is a Windows program that uses the ssh protocol to copy files.
• An ssh server doesn’t care what sort of computer the packets are coming from, and an ssh client doesn’t care what sort of computer it’s talking to.
  • As long as they both talk proper ssh protocol, it works.

ssh & scp

• ssh: Execute a command on a remote system, or remotely log in to a remote system
• scp: Copy a file to/from a remote system
• ssh and scp use the same encryption, configuration, etc.
• There is no scp port or scp daemon, just an ssh port (22) and an ssh daemon (sshd).

Use of ssh

Execute a remote command:

    ssh applin@denver.cs.colostate.edu id

Start a interactive remote session:

    ssh applin@denver.cs.colostate.edu

• If username@ is omitted, then the current username is used.
• A full hostname may not be required, if the domain is the same.

Quoting

What’s the difference between these two commands?

    ssh applin@denver cat a*b

    ssh applin@denver "cat a*b"

How about these?

    ssh applin@denver date; pwd

    ssh applin@denver "date; pwd"

Use of scp

As does cp, scp supports using a directory as a destination, or renaming the file.

    scp alpha denver:
    scp beta applin@denver:
    scp gamma applin@denver:delta
    scp epsilon applin@denver:/tmp
    scp zeta applin@denver:/tmp/iota

Either the source, the destination, both, or neither can be a remote system:

    scp denver:kappa lambda
    scp denver:omicron lansing:pi
    scp sigma tau

scp also supports the -p and -r options from cp.

The ~/.ssh Directory

• The ~/.ssh directory contains a number of files, including:
  • authorized_keys
  • known_hosts
  • config
  • id_ed25519
  • id_ed25519.pub
  • id_rsa
  • id_rsa.pub
• My ~/.ssh directory is unreadable/unwritable/unexecutable by anybody but me.
  • Extremism in the defense of security is no vice.

~/.ssh/authorized_keys

• ~/.ssh/authorized_keys contains a list of authorized public keys.
• It’s a list of people given permission to log in without a password, or to copy files to/from this account without a password.
  • Really, it’s a list of computers.
    • Actually, it’s a list of public keys.
• To enable login to that computer, add your public key to the ~/.ssh/authorized_keys file.

~/.ssh/known_hosts

~/.ssh/known_hosts contains the host keys for the hosts that we’ve connected to previously. If anybody tries to impersonate a host, they won’t have the same host key.

~/.ssh/config

• /etc/ssh/ssh_config and ~/.ssh/config contain configuration information, including:
  • ControlMaster
  • ControlPath
  • ControlPersist
  • Compression
  • ForwardX11
  • Protocol
  • Host aliases

See the ssh_config man page for tons more information.

Sample ~/.ssh/config

    # Share the control circuit for multiple connections:
    ControlMaster auto

    # Put the control circuit socket in MY directory, for safety:
    ControlPath ~/tmp/ssh·mux·%h·%p·%r

    # Keep the control circuit for a while after we stop using it:
    ControlPersist 10m

    # Global options:
    Host *
        Compression yes
        ForwardX11 yes
        Protocol 2

    # Jack’s office at CSU:
    Host applin Applin csu CSU
        User applin
        HostName Greybull.CS.ColoState.Edu

System Configuration

/etc/ssh/sshd_config can make things more difficult for miscreants:

    # Permit only certain users:
    AllowUsers cindy belle aurora snow

    # Don’t allow root login via ssh.  One can use sudo, however:
    PermitRootLogin no

    # Empty password?  You can’t use ssh, then:
    PermitEmptyPasswords no

    # Don’t allow password authentication at all:
    PasswordAuthentication no

    # Don’t permit obsolete ssh protocol 1:
    Protocol 2

    # Everybody expects port 22, therefore:
    Port 13579

~/.ssh/id_*

• ~/.ssh/id_ed25519: private key
• ~/.ssh/id_ed25519.pub: public key
• ~/.ssh/id_rsa: private key
• ~/.ssh/id_rsa.pub: public key

Asymmetric Encryption

• ssh and scp use a pair of keys: one public, one private.
• Your public key is public. You can publish it in the newspaper.
• Your private key is private. If it gets out, abandon it.
• ~/.ssh/authorized_keys contains the public keys for the users authorized to log into this account (or copy files) without giving a password.
• Anybody can encrypt a message with your public key, but only you have the private key to decrypt it. However, you don’t do explicit encryption/decryption with ssh & scp.
• Make one public/private key pair for each device (e.g., laptop). If you lose the laptop, invalidate its keys.

Generating a Key Pair

To generate a public/private key pair:

    ssh-keygen -t ed25519 -C "Jack’s CSU Macbook"

This will create an ed25519 keypair in ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519.pub.

If you’re using older software, you may have to resort to RSA keys. Make sure to use a sufficiently long key.

    ssh-keygen -b 4096 -t RSA -C "Jack’s home desktop"

Ed25519 is recommended as having better security, but requires up-to-date servers & clients. I use ed25519 for all my keys.

Passphrase

• An ssh private key is generally protected by a passphrase.
• It’s like a password, but longer.
  • Why they call it a passphrase is beyond me.
• ssh-agent will remember your passphrase for you.
• If you don’t have a passphrase, who can get to your private key?

Port Forwarding

Consider the following problem:

• You want to access a server on a remote system.
• The computer that you’re on doesn’t have permission to access “outside” systems, for security reasons.
• However, a gateway system does have permission.
• You want to run your command here, and not on some stupid gateway system.

Port forwarding is your solution!

Port Forwarding

    ssh -f -N -Llocal-port:host:host-port user@remote-host

• -f: run in the background
• -N: don’t run a command or start a shell on remote-host
• -L:
  1. Connect to remote-host as user (really, it always does that)
  2. From remote-host, connect to host on port host-port
  3. Forward that connection to local-port on the local machine

Port Forwarding Example

    % wget -q -O- http://icanhazip.com/
    203.0.113.123

    % wget -q -O- http://localhost:12345/

    % wget -O- http://localhost:12345/
    --2026-02-15 08:21:11 --  http://localhost:12345/
    Resolving localhost (localhost)... 127.0.0.1
    Connecting to localhost (localhost)|127.0.0.1|:12345... failed: Connection refused.

    % ssh -f -N -L12345:icanhazip.com:80 applin@denver.cs.colostate.edu

    % wget -q -O- http://localhost:12345/
    129.82.44.141

    % host 129.82.44.141
    205.46.82.129.in-addr.arpa domain name pointer denver.cs.colostate.edu.

Cryptography ain’t Everything

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