serv.access(5)
NAME
serv.access - Internet service access list
SYNOPSIS
/etc/serv.access
DESCRIPTION
The serv.access file contains a list of rules that guide the access
checks made by the servxcheck(3) function. The file is a text file
containing entries that look as follows:
service1 service2 ...: check1 check2 ...;
Each of the service names is a service name from the /etc/services file.
The same names are used in the /etc/inetd.conf configuration file that
guides inetd(8).
The checks may look as follows:
+
-
Allow all, or allow none. Used to explicitly set the initial state.
+name
Grant access to one of the services if the host name of the remote
system matches name.
-name
Deny access to one of the services if the host name of the remote
system matches name.
+ipaddr
-ipaddr
+netaddr/len
-netaddr/len
Grants or denies access to a remote host with IP address ipaddr, or
the remote host whose IP address is within the network netaddr. Len
tells the number of bits used for the network address, i.e. the top
len bits of the network address must equal the host address.
log
This is not a check, but a flag that instruct servxcheck() to log
the result of the access check whether it succeeds or not to
/usr/adm/log. By default only failure is logged.
The first "+" or "-" access check sets the tone. Read it as "access
denied unless +...", or "access granted unless -...". An access check
will therefore almost always start with a "+" check. To make the initial
state clear you can start with a lone "+" or "-". Checks are done from
left to right. A check that doesn't match does not change the outcome.
A check that can't change the outcome is skipped.
Both the service and the host names may contain the * wildcard that
matches any number of characters including none. Letters are compared
ignoring case. A service name may appear in more than one rule, but a
service mentioned explicitly is not matched by wildcard patterns in later
rules.
A check for a hostname causes servxcheck() to do a reverse lookup on the
IP address of the remote host to find its name. This name is then looked
up to find the host's IP address(es). If those lookups fail then all
-name checks cause access to be denied, and no +name check grants access.
The DNS lookup failures may be a misconfiguration, but could indicate a
break-in attempt from a badly maintained host. You can use a simple "+*"
in an otherwise empty list to just deny misconfigured hosts.
An IP or network address check is simply done on the remote hosts IP
address. Such a check has no overhead, but a log flag will cause a
reverse lookup anyway.
Comments start with "#" and continue until end of line. The "\" is a
line continuation character. It, any whitespace or a comment following
it and a newline are treated as a single space.
EXAMPLES
Example access file on a machine that offers most services only to hosts
within the cs.vu.nl domain, and news (nntp) only to two machines and a
specific network.
# Service # Access list
login shell: +*.cs.vu.nl log;
telnet pop smtp finger: + log;
nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
+172.16.102.0/24 log;
*: +*.cs.vu.nl;
More paranoid example that limits all services by default, but allows ftp
and http to the world:
# Service # Access list
ftp http: +;
smtp finger: + log;
nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
+172.16.102.0/24 log;
*: +*.cs.vu.nl log;
(Note that the last rule doesn't match any of the services mentioned
explicitly earlier.)
FILES
/etc/serv.access The service access check file.
SEE ALSO
servxcheck(3), services(5), inetd.conf(5).
NOTES
It may be wise not to put checks on telnet. It is reasonably secure,
since it always requires a password, and your only way in if things are
seriously hosed.
BUGS
IP and DNS based access checks will stop most crackers, but not the
really determined ones. Luckily Minix is sufficiently strange to thwart
the well known cracking schemes. But don't ever allow yourself to feel
secure.
AUTHOR
Kees J. Bot <kjb@cs.vu.nl>