SSH Secure Shell
by
Thomas König
What is SSH Secure Shell? From
SSH (Secure Shell) is a program to log into another computer over a
network, to execute commands in a remote machine, and to move files from
one machine to another. It provides strong authentication and secure
communications over unsecure channels. It is intended as a replacement for
rlogin, rsh, and rcp.
Additionally, SSH provides secure X connections and secure forwarding
of arbitrary TCP connections.
Why should I use it?
The traditional BSD 'r' - commmands (rsh, rlogin, rcp) are vulnerable
to different kinds of attacks. Somebody who has root access to machines on
the network, or physical access to the wire, can gain unauthorized access
to systems in a variety of ways. It is also possible for such a person to
log all the traffic to and from your system, including passwords (which
SSH never sends in the clear).
The X Window System also has a number of severe vulnerabilities. With
SSH, you can create secure remote X sessions which are transparent to the
user. As a side effect, using remote X clients with SSH is more convenient
for users.
Users can continue to use old .rhosts and /etc/hosts.equiv files;
changing over to ssh is mostly transparent for them. If a remote site does
not support SSH, a fallback mechanism to rsh is included.
What kinds of attacks does SSH protect against?
SSH protects against
IP spoofing, where a remote host sends out packets which pretend to
come from another, trusted host. SSH even protects against a spoofer on
the local network, who can pretend he is your router to the outside.
IP source routing, where a host can pretend that an IP packet comes
from another, trusted host.
DNS spoofing, where an attacker forges name server records
Interception of clear text passwords and other data by intermediate
hosts.
Manipulation of data by people in control of intermediate hosts
Attacks based on listening to X authentication data and spoofed
connection to the X11 server.
In other words, ssh never trusts the net; somebody hostile who has
taken over the network can only force SSH to disconnect, but cannot
decrypted or play back the traffic, or hijack the connection.
The above only holds if you actually use encryption. SSH does have an
option to use encryption of type "none" this is only for
debugging purposes, and should not be used.
What kind of attacks does SSH not protect against?
SSH will not help you with anything that compromises your host's
security in some other way. Once an attacker has gained root access to a
machine, he can then subvert SSH, too.
If somebody malevolent has access to your home directory, then security
is nonexistent. This is very much the case if your home directory is
exported via NFS.
How does it work?
For more extensive information, please refer to the README
and RFC
files in the SSH directory. The proposed RFC is also available as an
Internet Draft from ftp//ftp.ietf.org/internet-drafts/draft-ietf-tls-ssh-00.txt
.
All communications are encrypted using IDEA or one of several other
ciphers (three-key triple-DES, DES, RC4-128, TSS, Blowfish). Encryption
keys are exchanged using RSA, and data used in the key exchange is
destroyed every hour (keys are not saved anywhere). Every host has an RSA
key which is used to authenticate the host when RSA host authentication is
used. Encryption is used to protect against IP-spoofing; public key
authentication is used to protect against DNS and routing spoofing.
RSA keys are also used to authenticate hosts.
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