NAME¶

amanda-auth - Communication/Authentication methods between Amanda server and client

DESCRIPTION¶

Amanda offers 8 methods of communication between Amanda server (sometimes also called the tape server) and clients, each with its own authentication method. The desired communication method is specified by the auth parameter in the amanda.conf file (amanda.conf(5)) commonly as a dumptype. Valid values to the auth parameter are bsd, bsdudp, bsdtcp, krb5, local, rsh, and ssh. The authentication and communication method is used during the backup process amdump (amdump(8)) as well as the recovery process amrecover (amrecover(8)).

COMPILATION AND GENERAL INFORMATION¶

The communication method and thus type of authentication that will be used by the Amanda server is specified by the auth parameter in the dumptype for each disklist entry (DLE). The auth parameter thus may be easily and globally specified in the "global" dumptype. If auth is not specified, the bsdtcp communication method is used. See amanda.conf(5) for more information on Amanda configuration and dumptypes, and disklist(5) for more information on disklists.

On the client side, the Amanda daemon amandad validates the connection depending on the value of the auth argument passed to it (see Amanda(8)). Also, when it comes to recovery, the auth parameter can be specified in the amanda-client.conf(5) file to specify the communication method to be used by the client to the server.

When Amanda is being built from source code, desired communication and thus authentication methods (shown as "Authentication") must be specified as configure options at compilation time.

Authentication	  Configure option(s)


 bsd			--with-bsd-security      --with-amandahosts (pre-2.6)


 bsdtcp		--with-bsdtcp-security   --with-amandahosts (pre-2.6)


 bsdudp		--with-bsdudp-security   --with-amandahosts (pre-2.6)


 krb5		--with-krb5-security


 local		 (always included)


 rsh			--with-rsh-security


 ssh			--with-ssh-security


 ssl			--with-ssl-security

There are additional configure options for bsd, bsdudp, and bsdtcp to allow for specifying explicit UDP and TCP port ranges.

   --with-udpportrange


   --with-tcpportrange


   --with-low-tcpportrange

See PORT USAGE below for more information.

There are additional configure options for Kerberos 5 if you so desire.

   --with-krb5-security=DIR       where libkrb.a lives   [see below]

If configuring with --with-krb5-security, the configure script will search under /usr/kerberos/lib, /usr/cygnus/lib, /usr/lib, and /opt/kerberos/lib for the kerberos bits, libkrb.a, in this order. Kerberos support will not be added if it does not find them. If the kerberos bits are found under some other hierarchy, you can specify this via --with-krb5-security=DIR where DIR is where the kerberos bits live. The configure script will then look in the 'lib' directory under this hierarchy for libkrb.a.

The auth parameter selects a communication/authentication method to use between the client and the backup server. These methods are described each in their own section below.

BSD, BSDUDP, AND BSDTCP COMMUNICATION AND AUTHENTICATION¶

For additional information including example configurations, see http://wiki.zmanda.com/index.php/Configuring_bsd/bsdudp/bsdtcp_authentication.

The bsd, bsdudp, and bsdtcp communication methods use either UDP, TCP, or both protocols operating as a network service to authenticate and exchange data between server and clients.

The authentication proceeds as follows: for a new, incoming connection, Amanda verifies that the source port is in the reserved range (less than 1024), which for UNIX hosts suggests that the remote user has root privileges. Amanda then verifies that the reverse DNS for the remote address matches the forward DNS; that is, that the address maps to a hostname which maps back to the same address. Finally, the remote system must provide a username that matches the username in .amandahosts.

In addition to compilation and general configuration (see COMPILATION AND GENERAL INFORMATION above), the authentication method that the client is configured to receive is specified by the auth parameter in the network service configuration for Amanda. The authentication method used by an Amanda client to reach the server during recovery is the authentication method specified by the auth parameter in the client's Amanda network service configuration or in its amanda-client.conf file (see amanda-client.conf(5)).

By default, Amanda use the "amanda" service name and associated port set in /etc/services. It can be changed by setting the dumptype client-port option to a different port number or a different service name. All examples are for the service name "amanda" that uses the default port 10080.

    amandaserver.example.com   backup   amdump

    amandaclient1.example.com   root   amindexd amidxtaped

   service_name socket_type protocol wait/nowait amanda_backup_user absolute_path_to_amandad amandad server_args

   amanda dgram udp wait backup /path/to/amandad amandad -auth=bsd amdump

   amanda stream tcp nowait backup /path/to/amandad amandad -auth=bsdtcp amdump

   amanda stream tcp nowait backup /path/to/amandad amandad -auth=bsdtcp amdump amindexd amidxtaped

   amandaidx stream tcp nowait backup /usr/local/libexec/amanda/current/amindexd   amindexd
   amidxtape stream tcp nowait backup /usr/local/libexec/amanda/current/amidxtaped amidxtaped

service amanda
{
	only_from               = Amanda server
	socket_type             = socket type
	protocol                = protocol
	wait                    = yes/no
	user                    = amanda backup user
	group                   = amanda backup user group id
	groups                  = yes
	server                  = absolute path to amandad
	server_args             = amandad server arguments
	disable                 = no
}

service amanda
{
	only_from       = amandaserver.example.com
	socket_type     = dgram
	protocol        = udp
	wait            = yes
	user            = backup
	group           = disk
	groups          = yes
	server          = /path/to/amandad
	server_args     = -auth=bsd amdump
	disable         = no 
}

service amanda
{
	only_from       = amandaserver.example.com amandaclient.example.com
	socket_type     = stream
	protocol        = tcp
	wait            = no
	user            = backup
	group           = disk
	groups          = yes
	server          = /path/to/amandad
	server_args     = -auth=bsdtcp amdump
	disable         = no 
}

service amandaidx
{
	socket_type		= stream
	protocol		= tcp
	wait			= no
	user			= amanda
	group			= disk
	server			= /usr/local/libexec/amanda/amindexd 
	disable			= no
}
service amidxtape
{
	socket_type		= stream
	protocol		= tcp
	wait			= no
	user			= amanda
	group			= disk
	server			= /usr/local/libexec/amanda/amidxtaped
	disable			= no
}

   Key:


       UP = Unreserved Port


    RPpAP = Reserved Port per Amanda Process


   UPpDLE = Unreserved Port per DLE


     [..] = Configure options that can be used at compile time to define port ranges
Authentication	Protocol	Amanda server					Amanda client
bsd			udp		1 RPpAP [--with-udpportrange]		10080
			tcp		1 UP [--with-tcpportrange]		3 UPpDLE [--with-tcpportrange]
bsdudp		udp		1 RPpAP [--with-udpportrange]		10080
			tcp		1 UP [-with-tcpportrange]		1 UPpDLE [--with-tcpportrange]
bsdtcp		tcp		1 RPpAP [--with-low-tcpportrange]	10080

KERBEROS COMMUNICATION AND AUTHENTICATION¶

For more detail, see http://wiki.zmanda.com/index.php/Kerberos_authentication.

Amanda supports Kerberos 5 communication methods between Amanda server and client.

General information including compilation are given above (see COMPILATION AND GENERAL INFORMATION above).

Kerberos 5 uses TCP and the server uses only one TCP port and data streams are multiplexed to this port.

The krb5 driver script defaults to:

/*


 * The lifetime of our tickets in minutes.


 */
#define AMANDA_TKT_LIFETIME     (12*60)
/*


 * The name of the service in /etc/services.


 */
#define AMANDA_KRB5_SERVICE_NAME        "k5amanda"

You can currently only override these by editing the source code.

The kerberized AMANDA service uses a different port on the client hosts. The /etc/services line is:

   k5amanda      10082/tcp

And the /etc/inetd.conf line is:

   k5amanda stream tcp nowait root /usr/local/libexec/amanda/amandad amandad -auth=krb5

Note that you're running this as root, rather than as your dump user. AMANDA will set its UID down to the dump user at times it doesn't need to read the keytab file, and give up root permissions entirely before it goes off and runs dump. Alternately you can change your keytab files to be readable by user amanda. You should understand the security implications of this before changing the permissions on the keytab.

The following dumptype options apply to krb5:

   auth "krb5"    # use krb5 auth for this host


                  # (you can mingle krb hosts and bsd .rhosts in one conf)

The principal and keytab files that Amanda uses must be set in the amanda.conf file for kerberos 5 dumps to work. You can hardcode this in the source code if you really want to (common-src/krb5-security.c)

   krb5keytab


   krb5principal

For example:

   krb5keytab	  "/etc/krb5.keytab-amanda"


   krb5principal  "amanda/saidin.omniscient.com"

The principal in the second option must be contained in the first. The keytab should be readable by the amanda user (and definitely not world readable!) and is (obviously) on the server. In MIT's kadmin, the following:

   addprinc -randkey amanda/saidin.omniscient.com


   ktadd -k /etc/krb5.keytab-amanda amanda/saidin.omniscient.com

will do the trick. You will obviously want to change the principal name to reflect something appropriate for the conventions at your site.

You must also configure each client to allow the amanda principal in for dumps.

There are several ways to go about authorizing a server to connect to a client.

The normal way is via a .k5amandausers file or a .k5login file in the client user's home directory. The determination of which file to use is based on the way you ran configure on AMANDA. By default, AMANDA will use .k5amandahosts, but if you configured with --without-amandahosts, AMANDA will use .k5login. (similar to the default for .rhosts/.amandahosts-style security). The .k5login file syntax is a superset of the default krb5 .k5login. The routines to check it are implemented in amanda rather than using krb5_kuserok because the connections are actually gssapi based.

This .k5amandahosts/.k5login is a hybrid of the .amandahosts and a .k5login file. You can just list principal names, as in a .k5login file and the principal will be permitted in from any host. If you do NOT specify a realm, then there is no attempt to validate the realm (this is only really a concern if you have cross-realm authentication set up with another realm or something else that allows you multiple realms in your kdc. If you do specify a realm, only that principal@realm will be permitted to connect.

You may prepend this with a hostname and whitespace, and only that principal (with optional realm as above) will be permitted to access from that hostname.

Here are examples of valid entries in the .k5amandahosts:

   service/amanda


   service/amanda@TEST.COM


   dumpmaster.test.com service/amanda


   dumpmaster.test.com service/amanda@TEST.COM

Rather than using a .k5amandahosts or .k5login file, the easiest way is to use a principal named after the destination user, (such as amanda@TEST.COM in our example) and not have either a .k5amandahosts or .k5login file in the destination user's home directory.

There is no attempt to verify the realm in this case (only a concern if you have cross-realm authentication setup).

LOCAL COMMUNICATION¶

The Amanda server communicates with the client internally versus over the network, ie. the client is also the server.

This is the only method that requires no authentication as it is clearly not needed.

The authenticated peer hostname for this authentication will be as reported by gethostname if that succeeds, or the fixed string localhost otherwise.

This method requires the host name in the DLE to be one that refers to this host. Any name for which gataddrinfo is able to return at least one address usable as the local end of a socket will work.

RSH COMMUNICATION AND AUTHENTICATION¶

For more detail, see http://wiki.zmanda.com/index.php/Configuring_rsh_authentication.

The Amanda server communicates with its client as the Amanda user via the RSH protocol.

Please note that RSH protocol itself is insecure and should be used with caution especially on any servers and clients with public IPs.

Each Amanda client communicates with the server using one TCP port and all data streams from the client are multiplexed over one port. The number of Amanda clients is limited by the number of reserved ports available on the Amanda server. Some versions of RSH do not use reserved ports and, thus, this restriction is not valid.

General information including compilation is given above (see COMPILATION AND GENERAL INFORMATION above).

In addition to specifying the auth field in dumptype definition, it might be required to specify client-username and amandad fields. If the backup user name is different on the Amanda client, the user name is specified as client-username. If the location of the Amanda daemon amandad is different on the Amanda client, the location is specified as amandad-path field value.

For example:
define dumptype rsh_example {


         ...


         auth "rsh"


         client-username "backup"


         amandad-path "/usr/lib/exec/amandad"


         ...
}

SSH COMMUNICATION AND AUTHENTICATION¶

For more detail, see http://wiki.zmanda.com/index.php/How_To:Set_up_transport_encryption_with_SSH.

Amanda client sends data to the server using SSH. SSH keys have to be set up so that Amanda server can communicate with its clients using SSH.

General information including compilation is given above (see COMPILATION AND GENERAL INFORMATION above).

SSH provides transport encryption and authentication. To set up an SSH authentication session, Amanda will run the equivalent of the following to start the backup process. /path/to/ssh -l user_name client.zmanda.com $libexecdir/amandad

To use SSH, you need to set up SSH keys either by storing the passphrase in cleartext, using ssh-agent, or using no passphrase at all. All of these options have security implications which should be carefully considered before adoption.

When you use a public key on the client to do data encryption (see http://wiki.zmanda.com/index.php/How_To:Set_up_data_encryption), you can lock away the private key in a secure place. Both, transport and storage will be encrypted with such a setup. See http://wiki.zmanda.com/index.php/Encryption for an overview of encryption options.

Enable SSH authentication and set the ssh-keys option in all DLEs for that host by adding the following to the DLE itself or to the corresponding dumptype in amanda.conf:

  auth "ssh"


  ssh-keys "/home/backup/.ssh/id_rsa_amdump"

ssh-keys is the path to the private key on the client. If the username to which Amanda should connect is different from the default, then you should also add

  client-username "otherusername"

If your server amandad path and client amandad path are different, you should also add

  amandad-path "/client/amandad/path"

Include the keys used for AMANDA in the clients' authorized_keys file, prepended with the following options:

  from="amanda_server.your.domain.com",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,command="/absolute/path/to/amandad -auth=ssh amdump" ssh-rsa AAAB3..

This will limit that key to connect only from Amanda server and only be able to execute amandad(8). This avoids several attacks that are possible if the no options are specified in the authorized_keys file.

In the same way, prepend the key used for AMANDA in the server's authorized_keys file with:

  from="amanda_client.your.domain.com",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,command="/absolute/path/to/amandad -auth=ssh amindexd amidxtaped"

You can omit the from=.. option if you have too many clients to list, although this has obvious security implications.

Set ssh-keys and any other necessary options in /etc/amanda/amanda_client.conf:

  auth "ssh"


  ssh-keys "/root/.ssh/id_rsa_amrecover"


  client-username "amanda"


  amandad-path "/server/amandad/path"

Besides user keys, SSH uses host keys to uniquely identify each host, to prevent one host from impersonating another. Unfortunately, the only easy way to set up these host keys is to make a connection and tell SSH that you trust the identity:

  $ ssh client1.zmanda.com


  The authenticity of host 'client1.zmanda.com (192.168.10.1)' can't be established.


  RSA key fingerprint is 26:4e:df:a2:be:c8:cb:20:1c:68:8b:cc:c0:3b:8e:9d.


  Are you sure you want to continue connecting (yes/no)?yes

As Amanda will not answer this question itself, you must manually make every connection (server to client and client to server) that you expect Amanda to make. Note that you must use the same username that Amanda will use (that is, ssh client and ssh client.domain.com are distinct).

SSL COMMUNICATION AND AUTHENTICATION¶

See amanda-auth-ssl(7)

SEE ALSO¶

amanda(8), amanda-auth-ssl(7), amanda.conf(5), amanda-client.conf(5), disklist(5), amdump(8), amrecover(8)

The Amanda Wiki: : http://wiki.zmanda.com/

AUTHORS¶

Jean-Louis Martineau <martineau@zmanda.com>

Dustin J. Mitchell <dustin@zmanda.com>

Paul Yeatman <pyeatman@zmanda.com>