CSCE 815 Network Security CSCE 815 Network Security Lecture 26 Lecture 26CSCE 815 Network Security CSCE 815 Network Security Lecture 26 Lecture 26

SSH and SSH ImplementationSSH and SSH Implementation

April 24, 2003

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Machines to AttackMachines to Attack

129.252.140.3129.252.140.3

129.252.140.7129.252.140.7

NOT!!!NOT!!!

129.252.140.1 - gateway129.252.140.1 - gateway

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SSH (Secure Shell)SSH (Secure Shell)

http://www.openssh.org/ http://www.openssh.org/

SSH 3.6.1 Released April 1, 2003SSH 3.6.1 Released April 1, 2003

OpenSSH encrypts all traffic (including passwords) to OpenSSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection effectively eliminate eavesdropping, connection hijacking, and other network-level attacks hijacking, and other network-level attacks

OpenSSH provides a myriad of secure tunneling OpenSSH provides a myriad of secure tunneling capabilities capabilities E.g. tunneling X connections

OpenSSH provides variety of authentication methods. OpenSSH provides variety of authentication methods.

Port 22 when used over TCP/IP (most common)Port 22 when used over TCP/IP (most common)

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SSH PictureSSH Picture

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SSH SuiteSSH Suite

ssh replaces telnet and rshssh replaces telnet and rsh

scp (secure copy) which replaces rcpscp (secure copy) which replaces rcp

sftp (secure ftp) which replaces ftpsftp (secure ftp) which replaces ftp

sshd (secure shell daemon) which is the serversshd (secure shell daemon) which is the server

Others: Others: sshssh-add-add, , sshssh-agent-agent, , ssh-keysignssh-keysign, , ssh-keyscanssh-keyscan, , ssh-keygenssh-keygen and sftp-server and sftp-server

Protocols: SSH 1.3 thru SSH 2.0 are supported Protocols: SSH 1.3 thru SSH 2.0 are supported

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OpenSSH SSH client configurationOpenSSH SSH client configuration

sshssh__config (5) sshconfig (5) ssh__config SSH client config SSH client configuration files configuration files

$HOME/.ssh/config$HOME/.ssh/config

/etc/ssh/ssh_config/etc/ssh/ssh_config

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SSH Protocol 2.0SSH Protocol 2.0

SSH Protocol Architecture - SSH Protocol Architecture - ArchitectureArchitecture describes the describes the overall design of SSH-2 overall design of SSH-2

SSH Transport Layer Protocol - provides a single, full-SSH Transport Layer Protocol - provides a single, full-duplex, flow-controlled, byte-oriented connection duplex, flow-controlled, byte-oriented connection from client to server, with privacy, integrity, and from client to server, with privacy, integrity, and man-in-the-middle protection man-in-the-middle protection

SSH Authentication Protocol - identifies the client to the SSH Authentication Protocol - identifies the client to the server server

SSH Connection Protocol - provides richer, application-SSH Connection Protocol - provides richer, application-support services such as TCP port and X forwarding support services such as TCP port and X forwarding

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SSH ArchitectureSSH Architecture

Host Keys - Each server host SHOULD have a host keyHost Keys - Each server host SHOULD have a host key

Two different trust models can be used:Two different trust models can be used:

1.1. client has a local database that associates each host client has a local database that associates each host name with the corresponding public host key name with the corresponding public host key

2.2. host name-to-key association is certified by some host name-to-key association is certified by some trusted certification authority trusted certification authority

All implementations SHOULD provide an option to not All implementations SHOULD provide an option to not accept host keys that cannot be verified. accept host keys that cannot be verified.

Extensibility - should evolve over timeExtensibility - should evolve over time

protocol allows full negotiation of encryption, integrity, protocol allows full negotiation of encryption, integrity, key exchange, compression, and public key key exchange, compression, and public key algorithms and formats algorithms and formats

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SSH PacketsSSH Packets

Minimum Packets Size is 28Minimum Packets Size is 28

Negligible for large packets, but for character by Negligible for large packets, but for character by character ala telnet this is significant character ala telnet this is significant

28 + 32(TCP/IP) + ethernet28 + 32(TCP/IP) + ethernet

But minimum ethernet packet payload size is 46But minimum ethernet packet payload size is 46

So the increase over the minimum is 4/46 = ~10%So the increase over the minimum is 4/46 = ~10%

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SSH Message Numbers SSH Message Numbers

SSH packets have message numbers in the range 1 to 255. SSH packets have message numbers in the range 1 to 255.

Transport layer protocol: Transport layer protocol: 1 to 19 Transport layer generic (e.g. disconnect, ignore, debug, etc.) 20 to 29 Algorithm negotiation 30 to 49 Key exchange method specific (numbers can be reused for

different authentication methods)

User authentication protocol: User authentication protocol: 50 to 59 User authentication generic 60 to 79 User authentication method specific (numbers can be

reused for different authentication methods)

Connection protocol: Connection protocol: 80 to 89 Connection protocol generic 90 to 127 Channel related messages

Reserved for client protocols: 128 to 191 Reserved for client protocols: 128 to 191

Reserved Local extensions: 192 to 255Reserved Local extensions: 192 to 255

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Authentication requestsAuthentication requests

RequestsRequests byte - SSH_MSG_USERAUTH_REQUEST string - user name (in ISO-10646 UTF-8 encoding [RFC2279]) string - service name (in US-ASCII) string - method name (US-ASCII) The rest of the packet is method-specific

ResponseResponse byte SSH_MSG_USERAUTH_FAILURE string “authentications that can continue” boolean partial success

““Authentications that can continue" is a comma-Authentications that can continue" is a comma-separated list of authentication method names that separated list of authentication method names that may productively continue the authentication dialog. may productively continue the authentication dialog.

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XWindows Normal ConnectionXWindows Normal Connection

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SSH X ConnectionSSH X Connection1.1. SSH attempts to connect to port 22 on remote hostSSH attempts to connect to port 22 on remote host

2.2. SSHD on the machine Remote forks off a child SSHD SSHD on the machine Remote forks off a child SSHD process. If X11 forwarding is enabled, the process process. If X11 forwarding is enabled, the process listens on port 6000 + x (first open one)listens on port 6000 + x (first open one)

3.3. child SSHD child SSHD now forks off the command received from the original SSH

client, usually xterm. SSHD sets the DISPLAY environment xterm to "Remote

4.4. xterm sends all X information to the fake server on xterm sends all X information to the fake server on it's own host it's own host

5.5. fake SSHD-X server encrypts the X information, then fake SSHD-X server encrypts the X information, then sends it to the SSH client on the Local machine. sends it to the SSH client on the Local machine.

6.6. SSH client decrypts the information and sends it to SSH client decrypts the information and sends it to the real X server the real X server

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SSH and Proxy Servers in General SSH and Proxy Servers in General

Two methods “-L” and “-R” optionsTwo methods “-L” and “-R” options

ssh -L ssh -L local-portlocal-port::remote-machineremote-machine::remote-portremote-port \ \

remote-machineremote-machine

This forwards a port (local-port) on the local machine This forwards a port (local-port) on the local machine across an encrypted channel to a server port across an encrypted channel to a server port (remote-port) on the remote machine(remote-port) on the remote machine

ssh -R ssh -R remote-portremote-port::remote-machineremote-machine::local-portlocal-port \ \

remote-machineremote-machine

command to have a port on a remote host act as a command to have a port on a remote host act as a proxy for a local portproxy for a local port

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SSH ReferencesSSH References

Implementation SSH 1 Implementation SSH 1

http://www.cise.ufl.edu/help-system/sshhttp://www.cise.ufl.edu/help-system/ssh//

SSH 3.2 SSH 3.2 http://www.ssh.com/http://www.ssh.com/

Open SSH http://www.openssh.org/Open SSH http://www.openssh.org/

Protocols http://www.snailbook.com/protocols.html Protocols http://www.snailbook.com/protocols.html