- The Internet Super Server inetd
- Allowing and denying hosts - /etc/hosts.allow, /etc/hosts.deny
The internet super server, or
available on all Unix(like) systems, providing many of the basic network
services available. This chapter describes the relationship between the daemon
and several of the config files in the
In this document we will look at a simple definition of inetd(8), how several files that relate to inetd(8) work (not that these files are not related to other software), how to add a service to inetd(8) and some considerations both to use inetd(8) for a particular service and times when a service might be better off running outside of inetd(8).
In traditional Unix scenarios, one server (daemon) process watches for connections on a particular port, and handles incoming requests. Now if a machine offers many services, many daemon processes would be needed, mostly running idle but still wasting resources like memory. The internet super server, inetd, is an approach to this problem. It listens on a number of ports, and when it receives a request it then determines which program to run to handle the request and starts an instance of that program.
Following is a very simple diagram to illustrate inetd(8):
pop3 -------| | ftpd --------| INETD | ---- Internet / DMZ / Switch / Whatever . . . | cvsupserver -|
In the above diagram you can see the general idea. The inetd(8) process receives a request and then starts the appropriate server process. What inetd(8) is doing is software multiplexing. An important note here, regarding security: On many other UNIX-like systems, a package called tcpwrappers is used as a security enhancement for inetd(8). On NetBSD the tcpwrapper functionality is built into inetd(8) using libwrap.
The operation of
controlled by its own config file, surprisingly named
inetd.conf file basically provides enabling and mapping of services the
systems administrator would like to have multiplexed through
indicating which program should be started for incoming requests on which port.
inetd.conf(5) is an ascii file containing one service per line, and several fields per line. The basic field layout is:
service-name socket-type protocol wait/nowait user:group server-program arguments
service-name: The service name indicates the port inetd(8) should listen on. It is either a decimal number, or a name matching a service name given in
socket-type: The communications socket type, the different types are
streamfor a TCP stream,
dgramfor an UDP service,
rawfor a raw socket,
rdmfor reliably delivered message and "seqpacket
for a sequenced packet socket. The most common socket types arestream
protocol: The protocol used, mostly
udp6for stream-oriented services via the Transmission Control Protocol, or datagram-oriented services via the User Datagram Protocol. It is worth noting that
udpmean they use the default (currently IPv4),
tcp4specifically means communication via IPv4 only, and
udp6are IPv6-only. In addition to those, protocols based on Remote Procedure Calls (RPC) can be specified as either
wait/nowait: This field tells inetd(8) if it should wait for a server program to return or to continue processing new connections immediately. Many connections to server processes require answers after data transfers are complete, where other types can keep transmitting on a connection continuously, the latter is a
nowaitand the former
wait. In most cases, this entry corresponds to the socket-type, for example a streaming connection would (most of the time) have a
nowaitvalue in this field.
user[:group]: This field gives the user name and optionally a group name that the server process which inetd(8) starts up runs as.
server-program: This field is the full path of the program that gets started.
program-arguments: This field contains the argument vector argv of the program started, including the program name and additional arguments the systems administrator may need to specify for the server program that is started.
That is all a lot to digest and there are other things the systems administrator
can do with some of the fields. Here is a sample line from an
ftp stream tcp nowait root /usr/libexec/ftpd ftpd -ll
From the left, the service-name is
ftp, socket-type is
stream, protocol is
wait for the server process to terminate (
nowait), the process runs as user
root, path is
/usr/libexec/ftpd and program name and arguments are
ftpd -ll. Notice in the last field, the program name is different from the
The next file to consider is the service name data base that can be found in
/etc/services. This file basically contains information mapping a service name
to a port number. The format of the
/etc/services file is:
service-name port-number/protocol-name [aliases]
service-name is the name of the service,
port-number is the port number
assigned to the service,
protocol-name is either
udp, and if alias
names for a port are needed, they can be added as
aliases, separated by white
spaces. Comments may be added after a hash mark (
Let's take a look at the
ssh entries as an example:
ssh 22/tcp # Secure Shell ssh 22/udp
As we can see, from the left, the service name is
ssh, the port number is 22,
the protocols are both tcp and udp. Notice that there is a separate entry for
every protocol a service can use (even on the same port).
Another file read by
/etc/protocols. This file has the information pertaining to DARPA Internet
protocols. The format of the protocols name data base is:
protocol-name number [aliases]
protocol-name describes the payload of an IP packet, e.g.
number is the official protocol number assigned by IANA, and optional
alias names can be added after that.
Let's look at the seventh entry in the
/etc/protocols db as an example:
tcp 6 TCP # transmission control protocol
Starting from the left, we see that the protocol name is tcp, the number is 6 and the only aliases listed is TCP, belonging to the Transmission Control Protocol as indicated by the comment in that line.
The rpc program number data base used by services with the
rpc protocol type
is kept in
/etc/rpc and contains name mappings to rpc program numbers. The
format of the file is:
server-name program-number aliases
For example, here is the nfs entry:
nfs 100003 nfsprog
As mentioned above, NetBSD's
the tcpwrapper package built in via the libwrap library. As such,
allow or deny access to each service on a more fine-grained base than just
allowing a service to everyone, or not enabling it at all. The access control is
defined in the files
/etc/hosts.deny, see the
Each of the two files contains several lines that describe access restrictions
for a certain server. Access is allowed if permission is given in
/etc/hosts.allow. If the service is not listened in
/etc/hosts.allow but in
/etc/hosts.deny, it is denied. If a service is listed in neither file, it is
allowed, giving standard
Each line in
/etc/hosts.deny contains a service either
by name (as given in the field for argv in
ftp), or the special service
ALL which obviously applies to all
services. Following the service name is - separated by a colon - a number of
access restrictions, which can be hostnames, domains, single IP addresses, whole
IP subnets or some other restrictions, please check
for all the details.
An example configuration that is mostly open but denies access to services to a certain host and all machines from a certain domain would look like this:
# /etc/hostname.deny: ALL: some.host.name, .some.domain
Another example that would be mostly closed, denying access to all but very few
machines would need entries in both
The entry for
/etc/hosts.deny would be:
# /etc/hosts.deny ALL: ALL
The entry to allow a few hosts would be put into
# /etc/hosts.allow ALL: friend.host.domain otherfriend.otherhost.otherdomain
Many times a systems administrator will find that they need to add a service to their system that is not already in inetd(8) or they may wish to move a service to it because it does not get very much traffic. This is usually pretty simple, so as an example we will look at adding a version of POP3 on a NetBSD system.
In this case we have retrieved and installed the
cucipop package, which can be
pkgsrc/mail/cucipop. This server is pretty simple to use, the only
oddities are different path locations. Since it is POP3 we know it is a stream
oriented connection with
nowait. Running as
root will be fine, the only item
that is different is the location of the program and the name of the program
So the first half of the new entry in
/etc/inetd.conf looks like this:
pop3 stream tcp nowait root
After installation, pkgsrc deposited cucipop in
/usr/pkg/sbin/cucipop. So with
the next field we have:
pop3 stream tcp nowait root /usr/pkg/sbin/cucipop
Last, we want to use the Berkeley mailbox format, so our server program must be
called with the
-Y option. This leaves the entire entry looking like so:
pop3 stream tcp nowait root /usr/pkg/sbin/cucipop cucipop -Y
We have added the service named
/etc/inetd.conf. Next item to check
is that the system can map the service name to a port number in
# grep ^pop3 /etc/services pop3 110/tcp # POP version 3 pop3 110/udp pop3s 995/tcp # pop3 protocol over TLS/SSL (was spop3) pop3s 995/udp # pop3 protocol over TLS/SSL (was spop3)
pop3 entries here are of interest, i.e. they are already contained in the
/etc/services file shipped with NetBSD.
Now, to have inetd(8) use the new entry, we simply restart it using the rc script:
# sh /etc/rc.d/inetd restart
All done, in most cases, the software you are using has documentation that will specify the entry, in the off case it does not, sometimes it helps to try and find something similar to the server program you will be adding. A classic example of this is a MUD server which has built-in telnet. You can pretty much borrow the telnet entry and change parts where needed.
The decision to add or move a service into or out of inetd(8) is usually based on server load. As an example, on most systems the telnet daemon does not require as many new connections as say a mail server. Most of the time the administrator has to feel out if a service should be moved.
A good example I have seen is mail services such as smtp and pop. I had setup a mail server in which pop3 was in inetd(8) and exim was running in standalone, I mistakenly assumed it would run fine since there was a low amount of users, namely myself and a diagnostic account. The server was also setup to act as a backup MX and relay in case another heavily used one went down. When I ran some tests I discovered a huge time lag for pop connections remotely. This was because of my steady fetching of mail and the diagnostic user constantly mailing diagnostics back and forth. In the end I had to move the pop3 service out of inetd(8).
The reason for moving the service is actually quite interesting. When a particular service becomes heavily used, of course, it causes a load on the system. In the case of a service that runs within the inetd(8) meta daemon the effects of a heavily loaded service can also harm other services that use inetd(8). If the multiplexor is getting too many requests for one particular service, it will begin to affect the performance of other services that use inetd(8). The fix, in a situation like that, is to make the offending service run outside of inetd(8) so the response time of both the service and inetd(8) will increase.
Following is some additional reading and information about topics covered in this document.
NetBSD manual pages: