167 lines
4.2 KiB
Groff
167 lines
4.2 KiB
Groff
.TH SERV.ACCESS 5
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.SH NAME
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serv.access \- Internet service access list
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.SH SYNOPSIS
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.B /etc/serv.access
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.SH DESCRIPTION
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.de SP
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.if t .sp 0.4
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.if n .sp
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..
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The
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.B serv.access
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file contains a list of rules that guide the access checks made by the
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.BR servxcheck (3)
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function. The file is a text file containing entries that look as follows:
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.PP
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.RS
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.I service1 service2
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.RB ... :
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.I check1 check2
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.RB ... ;
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.RE
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.PP
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Each of the service names is a service name from the
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.B /etc/services
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file. The same names are used in the
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.B /etc/inetd.conf
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configuration file that guides
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.BR inetd (8).
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.PP
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The checks may look as follows:
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.PP
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.BI +
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.br
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.BI -
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.RS
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Allow all, or allow none. Used to explicitly set the initial state.
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.RE
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.PP
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.BI + name
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.RS
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Grant access to one of the services if the host name of the remote system
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matches
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.BR name .
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.RE
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.SP
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.BI \- name
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.RS
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Deny access to one of the services if the host name of the remote system
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matches
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.BR name .
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.RE
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.PP
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.BI + ipaddr
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.br
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.BI \- ipaddr
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.br
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.BI + netaddr / len
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.br
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.BI \- netaddr / len
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.RS
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Grants or denies access to a remote host with IP address
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.IR ipaddr ,
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or the remote host whose IP address is within the network
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.IR netaddr .
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.I Len
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tells the number of bits used for the network address, i.e. the top
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.I len
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bits of the network address must equal the host address.
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.RE
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.PP
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.BR log
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.RS
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This is not a check, but a flag that instruct
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.B servxcheck()
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to log the result of the access check whether it succeeds or not to
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.BR /usr/adm/log .
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By default only failure is logged.
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.RE
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.PP
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The first "+" or "\-" access check sets the tone. Read it as "access denied
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unless +...", or "access granted unless \-...". An access check will
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therefore almost always start with a "+" check. To make the initial state
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clear you can start with a lone "+" or "\-". Checks are done from left
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to right. A check that doesn't match does not change the outcome. A check
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that can't change the outcome is skipped.
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.PP
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Both the service and the host names may contain the
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.B "\(**"
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wildcard that matches any number of characters including none. Letters are
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compared ignoring case. A service name may appear in more than one rule,
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but a service mentioned explicitly is not matched by wildcard patterns in
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later rules.
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.PP
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A check for a hostname causes
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.B servxcheck()
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to do a reverse lookup on the IP address of the remote host to find its
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name. This name is then looked up to find the host's IP address(es).
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If those lookups fail then all
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.BI \- name
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checks cause access to be denied, and no
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.BI + name
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check grants access.
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The DNS lookup failures may be a
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misconfiguration, but could indicate a break-in attempt from a badly
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maintained host. You can use a simple "+*" in an otherwise empty list to
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just deny misconfigured hosts.
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.PP
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An IP or network address check is simply done on the remote hosts IP
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address. Such a check has no overhead, but a
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.B log
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flag will cause a reverse lookup anyway.
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.PP
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Comments start with "#" and continue until end of line.
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.SH EXAMPLES
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Example access file on a machine that offers most services only to hosts within
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the cs.vu.nl domain, and news (nntp) only to two machines and a specific
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network.
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.PP
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.RS
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.nf
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.ta +2.2i +.4i
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# Service # Access list
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login shell: +*.cs.vu.nl log;
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telnet pop smtp finger: + log;
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nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
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+172.16.102.0/24 log;
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*: +*.cs.vu.nl;
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.fi
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.RE
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.PP
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More paranoid example that limits all services by default, but allows ftp and
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http to the world:
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.PP
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.RS
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.nf
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.ta +2.2i +.4i
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# Service # Access list
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ftp http: +;
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smtp finger: + log;
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nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
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+172.16.102.0/24 log;
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*: +*.cs.vu.nl log;
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.fi
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.RE
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.PP
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(Note that the last rule doesn't match any of the services mentioned
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explicitly earlier.)
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.SH FILES
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.TP 25n
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.B /etc/serv.access
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The service access check file.
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.SH "SEE ALSO"
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.BR servxcheck (3),
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.BR services (5),
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.BR inetd.conf (5).
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.SH NOTES
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It may be wise not to put checks on telnet. It is reasonably secure, since
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it always requires a password, and your only way in if things are seriously
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hosed.
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.SH BUGS
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IP and DNS based access checks will stop most crackers, but not the really
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determined ones. Luckily MINIX 3 is sufficiently strange to thwart the well
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known cracking schemes. But don't ever allow yourself to feel secure.
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.SH AUTHOR
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Kees J. Bot <kjb@cs.vu.nl>
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