Tribblix: manual page: rshd.8

IN.RSHD(8) Maintenance Commands and Procedures IN.RSHD(8)

NAME

in.rshd, rshd - remote shell server

SYNOPSIS

in.rshd [-k5eciU] [-s tos] [-S keytab] [-M realm]
[-L env_var] host.port

DESCRIPTION

in.rshd is the server for the rsh(1) program. The server provides
remote execution facilities with authentication based on Kerberos V5
or privileged port numbers.

in.rshd is invoked by inetd(8) each time a shell service is
requested.

When Kerberos V5 authentication is required (this can be set with
Kerberos-specific options listed below), the following protocol is
initiated:

1. Check Kerberos V5 authentication.

2. Check authorization according to rules in
krb5_auth_rules(7).

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 PATH variable is set to /usr/bin.) The shell
inherits the network connections established by in.rshd.

In order for Kerberos authentication to work, a host/<FQDN> Kerberos
principal must exist for each Fully Qualified Domain Name associated
with the in.rshd server. Each of these host/<FQDN> principals must
have a keytab entry in the /etc/krb5/krb5.keytab file on the in.rshd
server. An example principal might be:

host/bigmachine.eng.example.com

See kadmin(8) for instructions on adding a principal to a krb5.keytab
file. See for a discussion of Kerberos authentication.

If Kerberos V5 authentication is not enabled, then in.rshd executes
the following protocol:

1. The server checks the client's source port. If the port is
not in the range 512-1023, the server aborts the
connection. The client's host address (in hex) and port
number (in decimal) are the arguments passed to in.rshd.

2. The server reads characters from the socket up to a null
(\0) byte. The resultant string is interpreted as an
ASCII number, base 10.

3. If the number received in step 2 is non-zero, it is
interpreted as the port number of a secondary stream to be
used for the stderr. A second connection is then created
to the specified port on the client's machine. The source
port of this second connection is also in the range
512-1023.

4. A null-terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is
interpreted as the user identity on the client's machine.

5. A null terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is
interpreted as a user identity to use on the server's
machine.

6. A null terminated command to be passed to a shell is
retrieved on the initial socket. The length of the command
is limited by the upper bound on the size of the system's
argument list.

7. in.rshd then validates the user according to the following
steps. The remote user name is looked up in the password
file and a chdir is performed to the user's home
directory. If the lookup fails, the connection is
terminated. If the chdir fails, it does a chdir to /
(root). If the user is not the superuser, (user ID 0),
and if the pam_rhosts_auth PAM module is configured for
authentication, the file /etc/hosts.equiv is consulted for
a list of hosts considered "equivalent". If the client's
host name is present in this file, the authentication is
considered successful. See the SECURITY section below for
a discussion of PAM authentication.

If the lookup fails, or the user is the superuser, then
the file .rhosts in the home directory of the remote user
is checked for the machine name and identity of the user
on the client's machine. If this lookup fails, the
connection is terminated

8. A null byte is returned on the initial connection and the
command line is passed to the normal login shell of the
user. The PATH variable is set to /usr/bin. The shell
inherits the network connections established by in.rshd.

OPTIONS

The following options are supported:

-5
Same as -k, for backwards compatibility

-c
Requires Kerberos V5 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. This
option is mutually exclusive with the -i option.

-e
Requires the client to encrypt the connection.

-i
Ignores authenticator checksums if provided. This
option ignores authenticator checksums presented by
current Kerberos clients to protect initial connection
information. Option -i is the opposite of option -c.

-k
Allows Kerberos V5 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.

-L env_var
List of environment variables that need to be saved and
passed along.

-M realm
Uses the indicated Kerberos V5 realm. By default, the
daemon will determine its realm from the settings in
the krb5.conf(5) file.

-s tos
Sets the IP TOS option.

-S keytab
Sets the KRB5 keytab file to use. The
/etc/krb5/krb5.keytab file is used by default.

-U
Refuses connections that cannot be mapped to a name
through the getnameinfo(3SOCKET) function.

USAGE

rshd and in.rshd are IPv6-enabled. See ip6(4P). IPv6 is not currently
supported with Kerberos V5 authentication.

The Kerberized rshd service runs on port 544 (kshell). The
corresponding FMRI entry is: :

svc:/network/shell:kshell (rshd with kerberos (ipv4 only))

SECURITY

in.rshd uses pam(3PAM) for authentication, account management, and
session management. The PAM configuration policy, listed through
/etc/pam.conf, specifies the modules to be used for in.rshd. Here is
a partial pam.conf file with entries for the rsh command using rhosts
authentication, UNIX account management, and session management
module.

rsh auth required pam_rhosts_auth.so.1

rsh account required pam_unix_roles.so.1
rsh session required pam_unix_projects.so.1
rsh session required pam_unix_account.so.1

rsh session required pam_unix_session.so.1

If there are no entries for the rsh service, then the entries for the
"other" service are used. To maintain the authentication requirement
for in.rshd, the rsh entry must always be configured with the
pam_rhosts_auth.so.1 module.

in.rshd can authenticate using Kerberos V5 authentication or
pam(3PAM). For Kerberized rsh service, the appropriate PAM service
name is krsh.

FILES

/etc/hosts.equiv

$HOME/.k5login
File containing Kerberos principals that are
allowed access.

/etc/krb5/krb5.conf
Kerberos configuration file.

DIAGNOSTICS

The following diagnostic messages are returned on the connection
associated with stderr, after which any network connections are
closed. An error is indicated by a leading byte with a value of 1 in
step 8 above (0 is returned above upon successful completion of all
the steps prior to the command execution).

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).

Hostname for your address unknown.

No entry in the host name database existed for the client's
machine.

Login incorrect.

No password file entry for the user name existed.

Permission denied.

The authentication procedure described above failed.

Can't make pipe.

The pipe needed for the stderr was not created.

Try again.

A fork by the server failed.

NOTES

The authentication procedure used here assumes the integrity of each
client machine and the connecting medium. This is insecure, but it is
useful in an "open" environment.

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

The pam_unix(7) module is no longer supported. Similar functionality
is provided by pam_authtok_check(7), pam_authtok_get(7),
pam_authtok_store(7), pam_dhkeys(7), pam_passwd_auth(7),
pam_unix_account(7), pam_unix_auth(7), and pam_unix_session(7).

The in.rshd service is managed by the service management facility,
smf(7), under the service identifier:

svc:/network/shell:default

Administrative actions on this service, such as enabling, disabling,
or requesting restart, can be performed using svcadm(8).
Responsibility for initiating and restarting this service is
delegated to inetd(8). Use inetadm(8) to make configuration changes
and to view configuration information for this service. The service's
status can be queried using the svcs(1) command.

June 20, 2021 IN.RSHD(8)