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The advantage of this system is that software packages can be added and removed transparently just by adding or removing a file. No special editing is required, as is the case for BSD unix. 8.4.2
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The BSD style in it program is quite simple. It starts executing a shell script called /etc/ r c which then generally calls other child-scripts. These scripts start important daemons and configure the system. To add our own local modifications, we have to edit the file / e t c / r e , local. This is a Bourne shell script. The BSD approach has a simpler structure than the System 5 inittab directories, but it is harder to manipulate package-wise.
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inetd Configuration
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The internet daemon is a service demultiplexer. In English, that means that it is a daemon which listens on the network for messages to several services simultaneously. When it receives a message intended for a specific port, it starts the relevant daemon to handle the request just long enough to handle one request, inetd saves the system some resources by starting daemons only when they are required, rather than having the clutter up the process table all the time. The format this file can differ slightly on older systems. The best way to gleaning its format is to look at the entries which are already there. Here is a common example for the format:
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# S e r v i c e | t y p e | p r o t o c o l | w a i t | u s e r | d a e m o n - f i l e | c o m m a n d line # # NB wu-ftpd needs -a now # ftp stream tcp nowait root /usr/sbin/in.ftpd in.ftpd-a telnet stream tcp nowait root /usr/sbin/in.telnetd in.telnetd f i n g e r stream tcp nowait f i n g e r / l o c a l / e t c / i n . f i n g e r d i n . f i n g e r d c finger stream tcp nowait f i n g e r / l o c a l / e t c / i n . cf ingerd in. c f i n g e r d
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The first column is the name of the service from /etc/services. The next column is the type of connection (stream or dgram or tli), then comes the protocol type (tcp/udp, etc.). The wait column indicates whether the service is to be single or multi-threaded, i.e. whether new requests should wait for an existing request to complete or whether a new daemon should be started in parallel. The last two columns contain the location of the program which should handle the request and the actual command line (including options) which should be executed. Notice that the finger daemon runs as a special user with no privileges. To add a new service, we edit the file / e t c / i n e t d . conf and then send the inetd process the HUP signal. To do this, we find the process id: ps aux | grep inetd Then we type: kill -HUP process-id
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Services
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When a daemon is started, it creates a listening socket or port with a specific port number, which then gets 'bound' to the host running the service concerned. The act of binding a socket to a host's IP address identifies a fixed port service with that host. This has a specific consequence. It is only possible to bind a socket port to an address once. If we try to start another daemon, we will often see the error message host: Couldn't bind to socket bind: Address already in use This means that another daemon is already running. This error can occur if two copies of inet d are started, or if we try to start a daemon twice, or indeed if we try to place a service in inet d and start a daemon at the same time. The error can also occur within a finite timewindow after a service has crashed, but the problem should right itself within a few minutes. 8.4.5 TCP Wrapper Security
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One of the problems with inetd is that it accepts connections from any host and passes them to services registered in its configuration file without question. In today's network climate this is a dangerous step, and it is usually desirable to limit the availability of certain services. For instance, services which are purely local (like RPC) should never be left open so that outside users could try to exploit them. In short, services should only be made available to those who need them. If they are left open to those who do not need them, we invite attacks on the system. TCP wrappers is a solution to this problem. In short, it gives us the possibility of adding Access Control Lists (ACLs) to network services. TCP wrappers exists in two forms: as the tcpd daemon, and as a library which standalone programs can link to, called libwr ap . a. Services which are not explicitly compiled with the library can use the daemon as a wrapper, if the services can be started from inetd. See section 10.3.5 for more details. TCP wrapper expects to find the daemons it proxies for in a special directory. It requires two configuration files, one which grants access to services and one which denies access. If services are not listed explicitly, TCP wrappers does nothing to prevent connection. The file to allow access to a service overrides the file to deny access, thus one normally denies access to all services as a default measure, and opens specific services one by one (see below). The hosts. allow file contains the names of daemons followed by a list of hosts or IP addresses, or domains or network series. The word LOCAL matches any host which has an unqualified host name. If we are opening a service to our local domain, it is often necessary to have both the domain suffix and the word LOCAL, since different operating systems employ different name services in different ways. # hosts.allow in.fingerd: .domain, country LOCAL in.cfingerd: .domain.country LOCAL sendmail: ALL cfd: .domain, country LOCAL in.rlogin: ALL in.telnetd: ALL sshd: ALL sshdfwd-Xll: . domain. country # Portmapper doesn't understand DNS for security
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