NAME
tcpd —
tcp wrappers access control facility
for internet services
DESCRIPTION
The tcpd program can be set up to monitor
incoming requests for
telnet(1),
finger(1), ftp(1),
rsh(1), tftp(1),
talk(1),
comsat(8), and other services that have a one-to-one mapping onto
executable files.
Operation is as follows: whenever a request for service arrives,
the inetd(8) daemon is tricked into running the
tcpd program instead of the desired server.
tcpd logs the request and does some additional
checks. When all is well, tcpd runs the appropriate
server program and goes away.
Optional features are: pattern-based access control, client username lookups with the RFC 931 etc. protocol, protection against hosts that pretend to have someone else's host name, and protection against hosts that pretend to have someone else's network address.
LOGGING
Connections that are monitored by tcpd are
reported through the
syslog(3) facility. Each record contains a time stamp, the client
host name and the name of the requested service. The information can be
useful to detect unwanted activities, especially when logfile information
from several hosts is merged.
In order to find out where your logs are going, examine the syslog configuration file, usually /etc/syslog.conf.
ACCESS CONTROL
Optionally, tcpd supports a simple form of
access control that is based on pattern matching. The access-control
software provides hooks for the execution of shell commands when a pattern
fires. For details, see the
hosts_access(5) manual page.
HOST NAME VERIFICATION
The authentication scheme of some protocols (rsh(1)) relies on host names. Some implementations believe the host name that they get from any random name server; other implementations are more careful but use a flawed algorithm.
tcpd verifies the client host name that is
returned by the address->name DNS server by looking at the host name and
address that are returned by the name->address DNS server. If any
discrepancy is detected, tcpd concludes that it is
dealing with a host that pretends to have someone elses host name.
HOST ADDRESS SPOOFING
Optionally, tcpd disables source-routing
socket options on every connection that it deals with. This will take care
of most attacks from hosts that pretend to have an address that belongs to
someone else's network. UDP services do not benefit from this protection.
This feature must be turned on at compile-time.
RFC 931
When RFC 931 etc. lookups are enabled (compile-time option)
tcpd will attempt to establish the name of the
client user. This will succeed only if the client host runs an RFC
931-compliant daemon. Client user name lookups will not work for
datagram-oriented connections, and may cause noticeable delays in the case
of connections from PCs.
FILES
The default locations of the host access control tables are:
- /etc/hosts.allow
- Access control table (allow list)
- /etc/hosts.deny
- Access control table (deny list)
EXAMPLES
This example applies when tcpd expects
that the network daemons are left in their original place, as it is
configured within OpenBSD.
In order to monitor access to the finger(1) service, perform the following edits on the inetd(8) configuration file, /etc/inetd.conf:
finger stream tcp nowait nobody /usr/libexec/fingerd fingerd
becomes:
finger stream tcp nowait nobody /usr/libexec/tcpd fingerd
Similar changes will be needed for the other services that are to
be covered by tcpd. Send a `kill -HUP´ to the
inetd(8) process to make the changes effective.
In the case of daemons that do not live in a common directory ("secret" or otherwise), edit the inetd(8) configuration file so that it specifies an absolute path name for the process name field. For example:
ntalk dgram udp wait root /usr/libexec/tcpd /usr/local/lib/ntalkd
Only the last component (ntalkd) of the
pathname will be used for access control and logging.
SEE ALSO
AUTHORS
Wietse Venema (wietse@wzv.win.tue.nl), Department of Mathematics and Computing Science, Eindhoven University of Technology Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
BUGS
Some UDP (and RPC) daemons linger around for a while after they have finished their work, in case another request comes in. In the inetd configuration file these services are registered with the wait option. Only the request that started such a daemon will be logged.
RPC broadcast requests (for example: rwall(1), rup(1), rusers(1)) always appear to come from the responding host. What happens is that the client broadcasts the request to all portmap(8) daemons on its network; each portmap(8) daemon forwards the request to a local daemon. As far as the rwalld(8) etc. daemons know, the request comes from the local host.