| [9] | 1 | .TH SERV.ACCESS 5
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| 2 | .SH NAME
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| 3 | serv.access \- Internet service access list
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| 4 | .SH SYNOPSIS
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| 5 | .B /etc/serv.access
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| 6 | .SH DESCRIPTION
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| 7 | .de SP
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| 8 | .if t .sp 0.4
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| 9 | .if n .sp
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| 10 | ..
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| 11 | The
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| 12 | .B serv.access
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| 13 | file contains a list of rules that guide the access checks made by the
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| 14 | .BR servxcheck (3)
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| 15 | function. The file is a text file containing entries that look as follows:
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| 16 | .PP
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| 17 | .RS
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| 18 | .I service1 service2
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| 19 | .RB ... :
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| 20 | .I check1 check2
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| 21 | .RB ... ;
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| 22 | .RE
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| 23 | .PP
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| 24 | Each of the service names is a service name from the
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| 25 | .B /etc/services
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| 26 | file. The same names are used in the
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| 27 | .B /etc/inetd.conf
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| 28 | configuration file that guides
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| 29 | .BR inetd (8).
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| 30 | .PP
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| 31 | The checks may look as follows:
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| 32 | .PP
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| 33 | .BI +
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| 34 | .br
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| 35 | .BI -
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| 36 | .RS
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| 37 | Allow all, or allow none. Used to explicitly set the initial state.
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| 38 | .RE
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| 39 | .PP
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| 40 | .BI + name
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| 41 | .RS
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| 42 | Grant access to one of the services if the host name of the remote system
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| 43 | matches
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| 44 | .BR name .
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| 45 | .RE
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| 46 | .SP
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| 47 | .BI \- name
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| 48 | .RS
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| 49 | Deny access to one of the services if the host name of the remote system
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| 50 | matches
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| 51 | .BR name .
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| 52 | .RE
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| 53 | .PP
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| 54 | .BI + ipaddr
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| 55 | .br
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| 56 | .BI \- ipaddr
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| 57 | .br
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| 58 | .BI + netaddr / len
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| 59 | .br
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| 60 | .BI \- netaddr / len
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| 61 | .RS
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| 62 | Grants or denies access to a remote host with IP address
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| 63 | .IR ipaddr ,
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| 64 | or the remote host whose IP address is within the network
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| 65 | .IR netaddr .
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| 66 | .I Len
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| 67 | tells the number of bits used for the network address, i.e. the top
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| 68 | .I len
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| 69 | bits of the network address must equal the host address.
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| 70 | .RE
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| 71 | .PP
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| 72 | .BR log
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| 73 | .RS
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| 74 | This is not a check, but a flag that instruct
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| 75 | .B servxcheck()
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| 76 | to log the result of the access check whether it succeeds or not to
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| 77 | .BR /usr/adm/log .
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| 78 | By default only failure is logged.
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| 79 | .RE
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| 80 | .PP
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| 81 | The first "+" or "\-" access check sets the tone. Read it as "access denied
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| 82 | unless +...", or "access granted unless \-...". An access check will
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| 83 | therefore almost always start with a "+" check. To make the initial state
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| 84 | clear you can start with a lone "+" or "\-". Checks are done from left
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| 85 | to right. A check that doesn't match does not change the outcome. A check
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| 86 | that can't change the outcome is skipped.
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| 87 | .PP
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| 88 | Both the service and the host names may contain the
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| 89 | .B "\(**"
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| 90 | wildcard that matches any number of characters including none. Letters are
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| 91 | compared ignoring case. A service name may appear in more than one rule,
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| 92 | but a service mentioned explicitly is not matched by wildcard patterns in
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| 93 | later rules.
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| 94 | .PP
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| 95 | A check for a hostname causes
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| 96 | .B servxcheck()
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| 97 | to do a reverse lookup on the IP address of the remote host to find its
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| 98 | name. This name is then looked up to find the host's IP address(es).
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| 99 | If those lookups fail then all
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| 100 | .BI \- name
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| 101 | checks cause access to be denied, and no
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| 102 | .BI + name
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| 103 | check grants access.
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| 104 | The DNS lookup failures may be a
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| 105 | misconfiguration, but could indicate a break-in attempt from a badly
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| 106 | maintained host. You can use a simple "+*" in an otherwise empty list to
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| 107 | just deny misconfigured hosts.
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| 108 | .PP
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| 109 | An IP or network address check is simply done on the remote hosts IP
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| 110 | address. Such a check has no overhead, but a
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| 111 | .B log
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| 112 | flag will cause a reverse lookup anyway.
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| 113 | .PP
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| 114 | Comments start with "#" and continue until end of line.
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| 115 | .SH EXAMPLES
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| 116 | Example access file on a machine that offers most services only to hosts within
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| 117 | the cs.vu.nl domain, and news (nntp) only to two machines and a specific
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| 118 | network.
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| 119 | .PP
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| 120 | .RS
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| 121 | .nf
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| 122 | .ta +2.2i +.4i
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| 123 | # Service # Access list
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| 124 | login shell: +*.cs.vu.nl log;
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| 125 | telnet pop smtp finger: + log;
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| 126 | nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
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| 127 | +172.16.102.0/24 log;
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| 128 | *: +*.cs.vu.nl;
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| 129 | .fi
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| 130 | .RE
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| 131 | .PP
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| 132 | More paranoid example that limits all services by default, but allows ftp and
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| 133 | http to the world:
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| 134 | .PP
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| 135 | .RS
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| 136 | .nf
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| 137 | .ta +2.2i +.4i
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| 138 | # Service # Access list
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| 139 | ftp http: +;
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| 140 | smtp finger: + log;
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| 141 | nntp: +flotsam.cs.vu.nl +jetsam.cs.vu.nl
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| 142 | +172.16.102.0/24 log;
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| 143 | *: +*.cs.vu.nl log;
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| 144 | .fi
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| 145 | .RE
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| 146 | .PP
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| 147 | (Note that the last rule doesn't match any of the services mentioned
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| 148 | explicitly earlier.)
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| 149 | .SH FILES
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| 150 | .TP 25n
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| 151 | .B /etc/serv.access
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| 152 | The service access check file.
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| 153 | .SH "SEE ALSO"
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| 154 | .BR servxcheck (3),
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| 155 | .BR services (5),
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| 156 | .BR inetd.conf (5).
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| 157 | .SH NOTES
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| 158 | It may be wise not to put checks on telnet. It is reasonably secure, since
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| 159 | it always requires a password, and your only way in if things are seriously
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| 160 | hosed.
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| 161 | .SH BUGS
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| 162 | IP and DNS based access checks will stop most crackers, but not the really
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| 163 | determined ones. Luckily MINIX 3 is sufficiently strange to thwart the well
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| 164 | known cracking schemes. But don't ever allow yourself to feel secure.
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| 165 | .SH AUTHOR
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| 166 | Kees J. Bot <kjb@cs.vu.nl>
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