As service clients, we basically need to make sure that we do not put our users in danger (or they put themselves in danger) by using insecure services. Avoid the use of services that do not use data encryption and passwords (FTP, telnet, non-secure mail). Use encrypted connection techniques, such as SSH and SSL.
Another important point concerns the potential substitution of servers for other false ones or session hijacking techniques. In these cases, we will need to have powerful authentication mechanisms that allow us to verify the servers' authenticity (for example, SSH and SSL have some of these mechanisms). And we will also have to verify the network searching for intruders who try to replace servers, as well as to apply correct package filtering services using firewalls, which allow us to remove our packages from a request and use the right servers, controlling the incoming packages that we receive as a response.
As we have seen, network services [Mou01] are configured from various places [Ano99][Hat01][Peń]:
In /etc/inetd.conf or the equivalent directory in /etc/xinetd.d: these systems are sort of "superservers" since they control subsidiary services and start up conditions. The inetd service is used in Debian and xinetd in Fedora (in Debian it can be installed as an option to replace inetd).
Servers initiated during start up: depending on the runlevel we will have a number of servers initiated. The start up will originate in the directory associated to the runlevel. For example, in Debian, the default runlevel is 2, the services will be started up from the /etc/rc2.d directory, certainly with links to the scripts contained in /etc/init.d, which will run with the parameter start, stop, restart, as applicable.
Other RPC type services: associated to remote calls between machines are used, for example in NIS and NFS. We can examine which ones with the rpcinfo -p command.
Other support files (with useful information) include: /etc/services, which consists of a list of known local or network services together with the protocol name, (tcp, udp or others), used for the service and the port that it uses; /etc/protocols is a list of known protocols; and /etc/rpc is a list of RPC servers together with the used ports. These files come with the distribution and are a sort of database used by the network tools and commands in order to determine the name of services and their associated protocols or rpc and ports. We should mention that they are more or less historical files, which do not necessarily contain all the definitions of protocols and services; likewise we can search different Internet lists of known ports.
One of the administrator's first actions will be to disable all services that are not being used or that are not scheduled to be used, reading up on the use of services [Mou01] and what software may need them. [Neu]
In the case of /etc/inetd.conf, we just have to comment the service line that has to be disabled, by placing a number symbol (#) as the first character on the line.
In the other model of services, used by default in Fedora (and optionally in Debian), xinetd, the configuration lies in the /etc/xinetd.conf file, where some of the default values of log, control are configured and then the configuration of each subsidiary service is done through a file within the /etc/xinetd.d directory. In each file, the service information is defined, equivalent to what appears in the inetd.conf, in this case, to disable a service, we just have to enter the line "disable = yes" within the service file. Xinetd has a more flexible configuration than inetd, since it separates the configuration of the different services into different files and has a fair number of options for limiting connections to a service, their number or capabilities; all of which allows for a better control of the service and with the right configuration we can avoid some of the attacks by denying the service (DoS o DDoS).
With regard to the handling of runlevel services from the distribution's commands, we have already mentioned several tools that allow services to be enabled or disabled in the unit on local administration. There are also graphic tools such as ksysv of KDE, or the system-config-services and ntsysv in Fedora (in Debian, we recommend sysv-rc-conf, rcconf or bum). And at a lower level, we can go to the runlevel that we want (/etc/rcx.d) and deactivate the services we wish by changing the initial S or K of the script for other text: for example, one method would be: changing S20ssh, for STOP_S20ssh, and it will no longer start up; the next time we need it, we can remove the prefix and it will be active again. Or perhaps the recommended use of simple utilities to place, remove or activate a specific service (like service and chkconfig in Fedora or similar ones in Debian, such as update-rc.d and invoke-rc.d).
Another aspect is closing down insecure services. Traditionally, in the world of UNIX file transfer systems such as FTP were used with remote connection, such as telnet, and remote run commands (login or copy), many of which started with the letter "r" (for example, rsh, rcp, rexec...). Other potential dangers are finger and rwhod services, which allowed information to be obtained from the network of the machine users; here the danger lay in the information that an attacker could obtain that would make the attacker's job easier. All of these services should not be used currently due to the potential dangers that they entail. In relation to the first group:
a) in network transmissions, ftp and telnet do not encrypt passwords and anyone can obtain pde service passwords or the associated accounts (for example, by using a sniffer).
b) rsh, rexec, rcp also have the problem that, under certain conditions, passwords are not even necessary (for example, if run from places validated in the .rhosts file), which means that once again they are insecure and leave the doors wide open to attacks.
The alternative is to use secure clients and servers that support message encryption and the authentication of participants. There are secure alternatives to the classical servers, but currently the most commonly used solution is the OpenSSH package (which can also be combined with OpenSSL for web environments). OpenSSH offers solutions based on the ssh, scp and sftp commands, allowing old clients and servers to be replaced (using a daemon called sshd). The ssh command allows the old functionalities of telnet, rlogin and rsh among others, scp would be the secure equivalent of rcp and sftp the equivalent of ftp.
With regards to SSH, we also have make sure we use ssh version 2. The first version has some known exploits; we need to take care when we install OpenSSH and, if we do not need the first version, install only the support for ssh2 (see the option Protocol in the /etc/ssh/ssh_config configuration file).
Besides, most services that we leave active on our machines would have to be filtered afterwards by a firewall to make sure that they are not used or attacked by people to whom they are not directed.