R-Services [512, 513, 514]
info
Basically it was before SSH, it was used to communicate between Unix devices, but the transmissions occured in an unencypted format
R-Command Suite Programs
The R-commands suite consists of the following programs:
rcp (
remote copy)rexec (
remote execution)rlogin (
remote login)rsh (
remote shell)rstat
ruptime
rwho (
remote who)
Command
Service Daemon
Port
Transport Protocol
Description
rcp
rshd
514
TCP
Copy a file or directory bidirectionally from the local system to the remote system (or vice versa) or from one remote system to another. It works like the cp command on Linux but provides no warning to the user for overwriting existing files on a system.
rsh
rshd
514
TCP
Opens a shell on a remote machine without a login procedure. Relies upon the trusted entries in the /etc/hosts.equiv and .rhosts files for validation.
rexec
rexecd
512
TCP
Enables a user to run shell commands on a remote machine. Requires authentication through the use of a username and password through an unencrypted network socket. Authentication is overridden by the trusted entries in the /etc/hosts.equiv and .rhosts files.
rlogin
rlogind
513
TCP
Enables a user to log in to a remote host over the network. It works similarly to telnet but can only connect to Unix-like hosts. Authentication is overridden by the trusted entries in the /etc/hosts.equiv and .rhosts files.
/etc/hosts.equiv
The /etc/hosts.equiv file contains a list of trusted hosts and is used to grant access to other systems on the network. When users on one of these hosts attempt to access the system, they are automatically granted access without further authentication.
Access Control & Trusted Relationships
The primary concern for r-services, and one of the primary reasons SSH was introduced to replace it, is the inherent issues regarding access control for these protocols. R-services rely on trusted information sent from the remote client to the host machine they are attempting to authenticate to. By default, these services utilize Pluggable Authentication Modules (PAM) for user authentication onto a remote system; however, they also bypass this authentication through the use of the /etc/hosts.equiv and .rhosts files on the system. The hosts.equiv and .rhosts files contain a list of hosts (IPs or Hostnames) and users that are trusted by the local host when a connection attempt is made using r-commands. Entries in either file can appear like the following:
Note: The hosts.equiv file is recognized as the global configuration regarding all users on a system, whereas .rhosts provides a per-user configuration.
sample .rhosts
z3tssu@htb[/htb]$ cat .rhosts
htb-student 10.0.17.5
+ 10.0.17.10
+ +As we can see from this example, both files follow the specific syntax of <username> <ip address> or <username> <hostname> pairs. Additionally, the + modifier can be used within these files as a wildcard to specify anything. In this example, the + modifier allows any external user to access r-commands from the htb-student user account via the host with the IP address 10.0.17.10.
Misconfigurations in either of these files can allow an attacker to authenticate as another user without credentials, with the potential for gaining code execution. Now that we understand how we can potentially abuse misconfigurations in these files let's attempt to try logging into a target host using rlogin.
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