kshd - kerberized remote shell server

kshd [ -kr45ec ] [-D port] [-L variable]

Krshd is the server for the rcmd(3) routine and, consequently, for the rsh(1) program. The server provides remote execution facilities with authentication based on privileged port numbers from trusted hosts or the Kerberos authentication system.

The kshd server is invoked by inetd(8) when it receives a connection on the port indicated in /etc/inetd.conf. A typical /etc/inetd.conf configuration line for krshd might be:

kshell stream tcp nowait root /usr/sbin/kshd kshd -5c

When a service request is received, the following protocol is initiated:

1)

Authentication is checked

2)

Check authorization via the access-control files .k5login and .klogin in the user's home directory.

3)

A null byte is returned on the initial socket and the command line is passed to the normal login shell of the user. The shell inherits the network connections established by krshd.

Krshd can be configured by command-line arguments passed by inetd(8). The options are:

-5

Allow Kerberos5 authentication with the .k5login access control file to be trusted. If this authentication system is used by the client and the authorization check is passed, then the user is allowed to log in. If the user has no .k5login file, the login will be authorized if the results of krb5_aname_to_localname conversion matches the account name. Unless special rules are configured, this will be true if and only if the Kerberos principal of the connecting user is in the default local realm and the principal portion matches the account name.

-4

Allow Kerberos4 authentication with the .klogin access control file to be trusted. If this authentication system is used by the client and the authorization check is passed, then the user is allowed to log in.

-k

Allow Kerberos5 and Kerberos4 as acceptable authentication mechanisms. This is the same as including -4 and -5.

-e

Require the client to encrypt the connection. Only Kerberos5 clients support encryption.

-L variable

Carry through the current value of the specified variable into the environment of the child. This option can be used to preserve up to four variables.

-c

Require Kerberos5 clients to present a cryptographic checksum of initial connection information like the name of the user that the client is trying to access in the initial authenticator. This checksum provides additionl security by preventing an attacker from changing the initial connection information. To benefit from this security, only Kerberos5 should be trusted; Kerberos4 and rhosts authentication do not include this checksum. If this option is specified, older Kerberos5 clients that do not send a checksum in the authenticator will not be able to authenticate to this server. This option is mutually exclusive with the -i option.

If neither the -c or -i options are specified,then checksums are validated if presented. Since it is difficult to remove a checksum from an authenticator without making the authenticator invalid, this default mode is almost as significant of a security improvement as -c if new clients are used. It has the additional advantage of backwards compatability with some clients. Unfortunately, clients before Kerberos V5, Beta5, generate invalid checksums; if these clients are used, the -i option must be used.

-i

Ignore authenticator checksums if provided. This option ignore authenticator checksusm presented by current Kerberos clients to protect initial connection information; it is the opposite of -c. This option is provided because some older clients--particularly clients predating the release of Kerberos V5 Beta5 (May 1995)--present bogus checksums that prevent Kerberos authentication from succeeding in the default mode.

Krshd supports six options which may be used for testing:

-S keytab

Set the keytab file to use.

-M realm

Set the Kerberos realm to use.

-A

Don't allocate a reserved port for the stderr connection.

-P path

Use the argument to find the Kerberos binaries. Normally a compiled in argument is used.

-D port

Run in standalone mode, listening on port. The daemon will exit after one connection and will not background itself.

-w [ip|maxhostlen[,[no]striplocal]]

Controls the form of the remote hostname passed to login(1). Specifying ip results in the numeric IP address always being passed to login(1). Specifying a number, maxhostlen, sets the maximum length of the hostname passed to login(1) before it will be passed as a numeric IP address. If maxhostlen is 0, then the system default, as determined by the utmp or utmpx structures, is used. The nostriplocal and striplocal options, which must be preceded by a comma, control whether or not the local host domain is stripped from the remote hostname. By default, the equivalent of striplocal is in effect.

Except for the last one listed below, all diagnostic messages are returned on the initial socket, after which any network connections are closed. An error is indicated by a leading byte with a value of 1 (0 is returned in step 3 above upon successful completion of all the steps prior to the execution of the login shell).

``locuser too long''

The name of the user on the client's machine is longer than 16 characters.

``remuser too long''

The name of the user on the remote machine is longer than 16 characters.

``command too long ''

The command line passed exceeds the size of the argument list (as configured into the system).

``Login incorrect.''

No password file entry for the user name existed.

``No remote directory.''

The chdir command to the home directory failed.

``Permission denied.''

The authentication procedure described above failed.

``Can't make pipe.''

The pipe needed for the stderr, wasn't created.

``Try again.''

A fork by the server failed.

``<shellname>: ...''

The user's login shell could not be started. This message is returned on the connection associated with the stderr, and is not preceded by a flag byte.

rshd(8), rsh(1), rcmd(3)

A facility to allow all data exchanges to be encrypted should be present.

A more extensible protocol should be used.