Linux Network Administration

Transcription

1 Secure Remote Connections with OpenSSH Objective At the conclusion of this module, the student will be able to: Configure the ssh daemon start, stop, and restart sshd 17 January 2005 NETW SSH 2 SSH 1

2 Remote Connections Often we need to make a remote connection to a system in which we have access privileges telnet consists of a client and server that permits this connection Once a connection is established, we are operating at a CLI and can do anything that we are permitted to do if we were connected by a console device Although the connection requires a login and password, these are sent as plain text and could be detected by unauthorized users This lack of secure connection has caused many organizations to use Secure Shell client server environments to be implemented 17 January 2005 NETW SSH 3 Secure Shell Secure shells like OpenSSH, a FREE version of the SSH suite of network connectivity tools, is replacing telnet in environments where security is paramount Networks connected to a public network like the Internet ssh connections encrypt all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks 17 January 2005 NETW SSH 4 SSH 2

3 What is SSH SSH (or Secure SHell) is designed to replace older, less secure terminal applications used to log into remote hosts, such as telnet or rsh allows users to log into host systems remotely Unlike FTP or Telnet, SSH encrypts the login session makes it impossible for intruders to collect unencrypted passwords A related program called scp replaces older programs designed to copy files between hosts, such as rcp Using secure methods to remotely log into remote systems decreases the risks for both the client system and the remote host 17 January 2005 NETW SSH 5 Features of SSH SSH is a protocol which facilitates secure communications between two systems using a client/server architecture. The SSH protocol provides the following safeguards: After an initial connection, the client can verify that it is connecting to the same server it connected to previously The client transmits its authentication information to the server using strong, 128 bit encryption All data sent and received during a session is transferred using 128 bit encryption makes intercepted transmissions extremely difficult to decrypt and read The client can forward X11 applications from the server using X11 forwarding this provides a secure means to use graphical applications over a network Using a technique called port forwarding, an SSH server can become a conduit to make insecure protocols like POP secure 17 January 2005 NETW SSH 6 SSH 3

4 OpenSSH Red Hat Linux includes the general OpenSSH package opensshserver - the OpenSSH server openssh-clients - the OpenSSH client Originally created for BSD and now ported to most other UNIX/Linux distros The OpenSSL package from installs several important cryptographic libraries this enables OpenSSH to provide encrypted communications Windows Based SCP Clients WinSCP is a graphical SCP Client for Windows Putty is a text based SCP client for Windows 17 January 2005 NETW SSH 7 Why Use SSH Secure Shell protects against IP spoofing a remote host sends out packets which pretend to come from another, trusted IP source routing a host can pretend that an IP packet comes from another, trusted host DNS spoofing an attacker forges name server records Interception of cleartext 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 17 January 2005 NETW SSH 8 SSH 4

5 SSH Minimizes Communications Threats Interception of communication between two systems the attacker can be somewhere on the network between the two communicating entities, copying any information passed between them The attacker may intercept and keep the information or alter the information and send it to the intended recipient This attack can be mounted with a packet sniffer, a common network utility. Impersonation of a particular host an attacker's system is configured to pose as the intended recipient of a transmission the user's system will remain unaware it is communicating with the wrong host This attack can be mounted through techniques known as DNS poisoning or IP spoofing DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously duplicated host IP spoofing occurs when an intruder sends network packets that falsely appear to be from a trusted host on the network 17 January 2005 NETW SSH 9 Sequence of Events for SSH Connection A cryptographic handshake is made so that the client can verify that it is communicating with the correct server The transport layer of the connection between client and remote host is encrypted using a symmetric cipher The client authenticates itself to the server The remote client can now interact safely with the remote host over the encrypted connection 17 January 2005 NETW SSH 10 SSH 5

6 Role of the Transport Layer The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of and after authentication The transport layer accomplishes this by handling the encryption and decryption of data providing integrity protection of data packets as they are sent and received The transport layer also provides compression, speeding the transfer of information 17 January 2005 NETW SSH 11 OpenSSH Key Exchange Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange: Keys are exchanged The public key encryption algorithm is determined The symmetric encryption algorithm is determined The message authentication algorithm is determined The hash algorithm to be used is determined During the key exchange, the server identifies itself to the client with a unique host key If the client has never communicated with this particular server before, the server's key will be unknown to the client and it will not connect OpenSSH gets around this problem by accepting the server's host key after the user is notified and verifies the acceptance of the new host key In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is communicating with the intended server If the host key no longer matches, the user must remove the client's saved version before a connection can occur 17 January 2005 NETW SSH 12 SSH 6

7 OpenSSH Security After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs This generates another set of hash values and a new shared secret value If an attacker were able to determine the hash and shared secret value this information would be useful for a limited period 17 January 2005 NETW SSH 13 Authentication Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported May be a private keyencoded signature or a password The client then tries to authenticate itself to the server using one of these supported methods SSH servers and clients can be configured to allow different types of authentication The server can decide which encryption methods it will support based on its security model, and the client can choose the order of authentication methods to attempt from among the available options This gives each side the optimal amount of control 17 January 2005 NETW SSH 14 SSH 7

8 OpenSSH Configuration Files moduli Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange, which is critical for constructing a secure transport layer ssh_config The system-wide default SSH client configuration file overridden if one is also present in the user's home directory (~/.ssh/config). sshd_config The configuration file for the sshd daemon. ssh_host_dsa_key The DSA private key used by the sshd daemon. ssh_host_dsa_key.pub The DSA public key used by the sshd daemon. 17 January 2005 NETW SSH 15 OpenSSH Configuration Files Cont d sshfihostfikey The RSA private key used by the sshd daemon for version 1 of the SSH protocol. ssh_host_key.pub The RSA public key used by the sshd daemon for version 1 of the SSH protocol. ssh_host_rsa_key The RSA private key used by the sshd daemon for version 2 of the SSH protocol. ssh_host_rsa_key.pub The RSA public key used by the sshd for version 2 of the SSH protocol. 17 January 2005 NETW SSH 16 SSH 8

9 Contents of Users ~/.ssh/ directory authorized_keys holds a list of authorized public keys for servers When the client connects to a server, the server authenticates the client by checking its stored signed public key. id_dsa Contains the DSA private key of the user. id_dsa.pub The DSA public key of the user. id_rsa The RSA private key used by ssh for version 2 of the SSH protocol. id_rsa.pub The RSA public key used by ssh for version 2 of the SSH protocol identity The RSA private key used by ssh for version 1 of the SSH protocol. identity.pub The RSA public key used by ssh for version 1 of the SSH protocol. known_hosts This file contains DSA host keys of SSH servers accessed by the user Very important for ensuring that the SSH client is connecting the correct SSH server 17 January 2005 NETW SSH 17 Starting OpenSSH OpenSSH is installed by default during Linux installations both SSH and SCP share the same configuration file and are governed by the same /etc/init.d/sshd startup script Configure SSH to start at boot using the chkconfig chkconfig sshd on You can also start/stop/restart SSH after script changes service sshd start service sshd stop service sshd restart You must restart the SSH process every time you make a change to the configuration files 17 January 2005 NETW SSH 18 SSH 9

10 /etc/ssh/sshd_config /etc/ssh/ssh_config is SSH client configuration file /etc/ssh/sshd_config is SSH server configuration file By default SSH listens on all your NICs and uses TCP port January 2005 NETW SSH 19 Warning Received When Key Changes WARNING: REMOTE HOST IDENTIFICATION IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that the RSA host key has just been changed. The fingerprint for the RSA key sent by the remote host is 5d:d2:f5:21:fa:07:64:0d:63:1b:3b:ee:a6:58:58:bb. Please contact your system administrator. Add correct host key in /root/.ssh/known_hosts to get rid of this message. Offending key in /root/.ssh/known_hosts:2 RSA host key for has changed and you have requested strict checking. Host key verification failed. 17 January 2005 NETW SSH 20 SSH 10