{ "mode": "man", "parameter": "hosts_access", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/hosts_access/5/json", "generated": "2026-06-02T19:34:30Z", "sections": { "NAME": { "content": "hostsaccess - format of host access control files\n", "subsections": [] }, "DESCRIPTION": { "content": "This manual page describes a simple access control language that is based on client (host\nname/address, user name), and server (process name, host name/address) patterns. Examples\nare given at the end. The impatient reader is encouraged to skip to the EXAMPLES section for\na quick introduction.\n\nThe extended version of the access control language is described in the hostsoptions(5) doc‐\nument. Note that this language supersedes the meaning of shellcommand as documented below.\n\nIn the following text, daemon is the process name of a network daemon process, and client is\nthe name and/or address of a host requesting service. Network daemon process names are speci‐\nfied in the inetd configuration file.\n", "subsections": [] }, "ACCESS CONTROL FILES": { "content": "The access control software consults two files. The search stops at the first match:\n\n• Access will be granted when a (daemon,client) pair matches an entry in the\n/etc/hosts.allow file.\n\n• Otherwise, access will be denied when a (daemon,client) pair matches an entry in the\n/etc/hosts.deny file.\n\n• Otherwise, access will be granted.\n\nA non-existing access control file is treated as if it were an empty file. Thus, access con‐\ntrol can be turned off by providing no access control files.\n", "subsections": [] }, "ACCESS CONTROL RULES": { "content": "Each access control file consists of zero or more lines of text. These lines are processed\nin order of appearance. The search terminates when a match is found.\n\n• A newline character is ignored when it is preceded by a backslash character. This per‐\nmits you to break up long lines so that they are easier to edit.\n\n• Blank lines or lines that begin with a `#´ character are ignored. This permits you to\ninsert comments and whitespace so that the tables are easier to read.\n\n• All other lines should satisfy the following format, things between [] being optional:\n\ndaemonlist : clientlist [ : shellcommand ]\n\ndaemonlist is a list of one or more daemon process names (argv[0] values) or server port\nnumbers or wildcards (see below).\n\nclientlist is a list of one or more host names, host addresses, patterns or wildcards (see\nbelow) that will be matched against the client host name or address.\n\nThe more complex forms daemon@host and user@host are explained in the sections on server end‐\npoint patterns and on client username lookups, respectively.\n\nList elements should be separated by blanks and/or commas.\n\nWith the exception of NIS (YP) netgroup lookups, all access control checks are case insensi‐\ntive.\n", "subsections": [] }, "PATTERNS": { "content": "The access control language implements the following patterns:\n\n• A string that begins with a `.´ character. A host name is matched if the last compo‐\nnents of its name match the specified pattern. For example, the pattern `.tue.nl´\nmatches the host name `wzv.win.tue.nl´.\n\n• A string that ends with a `.´ character. A host address is matched if its first nu‐\nmeric fields match the given string. For example, the pattern `131.155.´ matches the\naddress of (almost) every host on the Eindhoven University network (131.155.x.x).\n\n• A string that begins with an `@´ character is treated as an NIS (formerly YP) netgroup\nname. A host name is matched if it is a host member of the specified netgroup. Net‐\ngroup matches are not supported for daemon process names or for client user names.\n\n• An expression of the form `n.n.n.n/m.m.m.m´ is interpreted as a `net/mask´ pair. An\nIPv4 host address is matched if `net´ is equal to the bitwise AND of the address and\nthe `mask´. For example, the net/mask pattern `131.155.72.0/255.255.254.0´ matches ev‐\nery address in the range `131.155.72.0´ through `131.155.73.255´. `255.255.255.255´\nis not a valid mask value, so a single host can be matched just by its IP.\n\n• An expression of the form `n.n.n.n/mm' is interpreted as a `net/masklength' pair,\nwhere `mm' is the number of consecutive `1' bits in the netmask applied to the\n`n.n.n.n' address.\n\n• An expression of the form `[n:n:n:n:n:n:n:n]/m´ is interpreted as a `[net]/prefixlen´\npair. An IPv6 host address is matched if `prefixlen´ bits of `net´ is equal to the\n`prefixlen´ bits of the address. For example, the [net]/prefixlen pattern\n`[3ffe:505:2:1::]/64´ matches every address in the range `3ffe:505:2:1::´ through\n`3ffe:505:2:1:ffff:ffff:ffff:ffff´.\n\n• A string that begins with a `/´ character is treated as a file name. A host name or\naddress is matched if it matches any host name or address pattern listed in the named\nfile. The file format is zero or more lines with zero or more host name or address\npatterns separated by whitespace. A file name pattern can be used anywhere a host\nname or address pattern can be used.\n\n• Wildcards `*´ and `?´ can be used to match hostnames or IP addresses. This method of\nmatching cannot be used in conjunction with `net/mask´ matching, hostname matching be‐\nginning with `.´ or IP address matching ending with `.´.\n", "subsections": [] }, "WILDCARDS": { "content": "The access control language supports explicit wildcards:\n\nALL The universal wildcard, always matches.\n\nLOCAL Matches any host whose name does not contain a dot character.\n\nUNKNOWN\nMatches any user whose name is unknown, and matches any host whose name or address are\nunknown. This pattern should be used with care: host names may be unavailable due to\ntemporary name server problems. A network address will be unavailable when the soft‐\nware cannot figure out what type of network it is talking to.\n\nKNOWN Matches any user whose name is known, and matches any host whose name and address are\nknown. This pattern should be used with care: host names may be unavailable due to\ntemporary name server problems. A network address will be unavailable when the soft‐\nware cannot figure out what type of network it is talking to.\n\nPARANOID\nMatches any host whose name does not match its address. When tcpd is built with\n-DPARANOID (default mode), it drops requests from such clients even before looking at\nthe access control tables. Build without -DPARANOID when you want more control over\nsuch requests.\n", "subsections": [] }, "OPERATORS": { "content": "EXCEPT Intended use is of the form: `list1 EXCEPT list2´; this construct matches anything\nthat matches list1 unless it matches list2. The EXCEPT operator can be used in dae‐\nmonlists and in clientlists. The EXCEPT operator can be nested: if the control lan‐\nguage would permit the use of parentheses, `a EXCEPT b EXCEPT c´ would parse as `(a\nEXCEPT (b EXCEPT c))´.\n", "subsections": [] }, "SHELL COMMANDS": { "content": "If the first-matched access control rule contains a shell command, that command is subjected\nto % substitutions (see next section). The result is executed by a /bin/sh child\nprocess with standard input, output and error connected to /dev/null. Specify an `&´ at the\nend of the command if you do not want to wait until it has completed.\n\nShell commands should not rely on the PATH setting of the inetd. Instead, they should use\nabsolute path names, or they should begin with an explicit PATH=whatever statement.\n\nThe hostsoptions(5) document describes an alternative language that uses the shell command\nfield in a different and incompatible way.\n", "subsections": [ { "name": "% EXPANSIONS", "content": "The following expansions are available within shell commands:\n\n%a (%A)\nThe client (server) host address.\n\n%c Client information: user@host, user@address, a host name, or just an address, depend‐\ning on how much information is available.\n\n%d The daemon process name (argv[0] value).\n\n%h (%H)\nThe client (server) host name or address, if the host name is unavailable.\n\n%n (%N)\nThe client (server) host name (or \"unknown\" or \"paranoid\").\n\n%r (%R)\nThe clients (servers) port number (or \"0\").\n\n%p The daemon process id.\n\n%s Server information: daemon@host, daemon@address, or just a daemon name, depending on\nhow much information is available.\n\n%u The client user name (or \"unknown\").\n\n%% Expands to a single `%´ character.\n\nCharacters in % expansions that may confuse the shell are replaced by underscores.\n" } ] }, "SERVER ENDPOINT PATTERNS": { "content": "In order to distinguish clients by the network address that they connect to, use patterns of\nthe form:\n\nprocessname@hostpattern : clientlist ...\n\nPatterns like these can be used when the machine has different internet addresses with dif‐\nferent internet hostnames. Service providers can use this facility to offer FTP, GOPHER or\nWWW archives with internet names that may even belong to different organizations. See also\nthe `twist´ option in the hostsoptions(5) document. Some systems (Solaris, FreeBSD) can have\nmore than one internet address on one physical interface; with other systems you may have to\nresort to SLIP or PPP pseudo interfaces that live in a dedicated network address space.\n\nThe hostpattern obeys the same syntax rules as host names and addresses in clientlist con‐\ntext. Usually, server endpoint information is available only with connection-oriented ser‐\nvices.\n", "subsections": [] }, "CLIENT USERNAME LOOKUP": { "content": "When the client host supports the RFC 931 protocol or one of its descendants (TAP, IDENT, RFC\n1413) the wrapper programs can retrieve additional information about the owner of a connec‐\ntion. Client username information, when available, is logged together with the client host\nname, and can be used to match patterns like:\n\ndaemonlist : ... userpattern@hostpattern ...\n\nThe daemon wrappers can be configured at compile time to perform rule-driven username lookups\n(default) or to always interrogate the client host. In the case of rule-driven username\nlookups, the above rule would cause username lookup only when both the daemonlist and the\nhostpattern match.\n\nA user pattern has the same syntax as a daemon process pattern, so the same wildcards apply\n(netgroup membership is not supported). One should not get carried away with username\nlookups, though.\n\n• The client username information cannot be trusted when it is needed most, i.e. when\nthe client system has been compromised. In general, ALL and (UN)KNOWN are the only\nuser name patterns that make sense.\n\n• Username lookups are possible only with TCP-based services, and only when the client\nhost runs a suitable daemon; in all other cases the result is \"unknown\".\n\n• A well-known UNIX kernel bug may cause loss of service when username lookups are\nblocked by a firewall. The wrapper README document describes a procedure to find out\nif your kernel has this bug.\n\n• Username lookups may cause noticeable delays for non-UNIX users. The default timeout\nfor username lookups is 10 seconds: too short to cope with slow networks, but long\nenough to irritate PC users.\n\nSelective username lookups can alleviate the last problem. For example, a rule like:\n\ndaemonlist : @pcnetgroup ALL@ALL\n\nwould match members of the pc netgroup without doing username lookups, but would perform\nusername lookups with all other systems.\n", "subsections": [] }, "DETECTING ADDRESS SPOOFING ATTACKS": { "content": "A flaw in the sequence number generator of many TCP/IP implementations allows intruders to\neasily impersonate trusted hosts and to break in via, for example, the remote shell service.\nThe IDENT (RFC931 etc.) service can be used to detect such and other host address spoofing\nattacks.\n\nBefore accepting a client request, the wrappers can use the IDENT service to find out that\nthe client did not send the request at all. When the client host provides IDENT service, a\nnegative IDENT lookup result (the client matches `UNKNOWN@host´) is strong evidence of a host\nspoofing attack.\n\nA positive IDENT lookup result (the client matches `KNOWN@host´) is less trustworthy. It is\npossible for an intruder to spoof both the client connection and the IDENT lookup, although\ndoing so is much harder than spoofing just a client connection. It may also be that the\nclient´s IDENT server is lying.\n\nNote: IDENT lookups don´t work with UDP services.\n", "subsections": [] }, "EXAMPLES": { "content": "The language is flexible enough that different types of access control policy can be ex‐\npressed with a minimum of fuss. Although the language uses two access control tables, the\nmost common policies can be implemented with one of the tables being trivial or even empty.\n\nWhen reading the examples below it is important to realize that the allow table is scanned\nbefore the deny table, that the search terminates when a match is found, and that access is\ngranted when no match is found at all.\n\nThe examples use host and domain names. They can be improved by including address and/or net‐\nwork/netmask information, to reduce the impact of temporary name server lookup failures.\n", "subsections": [] }, "MOSTLY CLOSED": { "content": "In this case, access is denied by default. Only explicitly authorized hosts are permitted ac‐\ncess.\n\nThe default policy (no access) is implemented with a trivial deny file:\n\n/etc/hosts.deny:\nALL: ALL\n\nThis denies all service to all hosts, unless they are permitted access by entries in the al‐\nlow file.\n\nThe explicitly authorized hosts are listed in the allow file. For example:\n\n/etc/hosts.allow:\nALL: LOCAL @somenetgroup\nALL: .foobar.edu EXCEPT terminalserver.foobar.edu\n\nThe first rule permits access from hosts in the local domain (no `.´ in the host name) and\nfrom members of the somenetgroup netgroup. The second rule permits access from all hosts in\nthe foobar.edu domain (notice the leading dot), with the exception of terminalserver.foo‐\nbar.edu.\n", "subsections": [] }, "MOSTLY OPEN": { "content": "Here, access is granted by default; only explicitly specified hosts are refused service.\n\nThe default policy (access granted) makes the allow file redundant so that it can be omitted.\nThe explicitly non-authorized hosts are listed in the deny file. For example:\n\n/etc/hosts.deny:\nALL: some.host.name, .some.domain\nALL EXCEPT in.fingerd: other.host.name, .other.domain\n\nThe first rule denies some hosts and domains all services; the second rule still permits fin‐\nger requests from other hosts and domains.\n", "subsections": [] }, "BOOBY TRAPS": { "content": "The next example permits tftp requests from hosts in the local domain (notice the leading\ndot). Requests from any other hosts are denied. Instead of the requested file, a finger\nprobe is sent to the offending host. The result is mailed to the superuser.\n\n/etc/hosts.allow:\nin.tftpd: LOCAL, .my.domain\n\n/etc/hosts.deny:\nin.tftpd: ALL: (/usr/sbin/safefinger -l @%h | \\\n/usr/bin/mail -s %d-%h root) &\n\nThe safefinger command comes with the tcpd wrapper and should be installed in a suitable\nplace. It limits possible damage from data sent by the remote finger server. It gives better\nprotection than the standard finger command.\n\nThe expansion of the %h (client host) and %d (service name) sequences is described in the\nsection on shell commands.\n\nWarning: do not booby-trap your finger daemon, unless you are prepared for infinite finger\nloops.\n\nOn network firewall systems this trick can be carried even further. The typical network\nfirewall only provides a limited set of services to the outer world. All other services can\nbe \"bugged\" just like the above tftp example. The result is an excellent early-warning sys‐\ntem.\n", "subsections": [] }, "DIAGNOSTICS": { "content": "An error is reported when a syntax error is found in a host access control rule; when the\nlength of an access control rule exceeds the capacity of an internal buffer; when an access\ncontrol rule is not terminated by a newline character; when the result of % expansion\nwould overflow an internal buffer; when a system call fails that shouldn´t. All problems are\nreported via the syslog daemon.\n", "subsections": [] }, "FILES": { "content": "/etc/hosts.allow, (daemon,client) pairs that are granted access.\n/etc/hosts.deny, (daemon,client) pairs that are denied access.\n", "subsections": [] }, "SEE ALSO": { "content": "hostsoptions(5) extended syntax.\ntcpd(8) tcp/ip daemon wrapper program.\ntcpdchk(8), tcpdmatch(8), test programs.\n", "subsections": [] }, "BUGS": { "content": "If a name server lookup times out, the host name will not be available to the access control\nsoftware, even though the host is registered.\n\nDomain name server lookups are case insensitive; NIS (formerly YP) netgroup lookups are case\nsensitive.\n", "subsections": [] }, "AUTHOR": { "content": "Wietse Venema (wietse@wzv.win.tue.nl)\nDepartment of Mathematics and Computing Science\nEindhoven University of Technology\nDen Dolech 2, P.O. Box 513,\n5600 MB Eindhoven, The Netherlands\n\n\n\n\nHOSTSACCESS(5)", "subsections": [] } }, "summary": "hostsaccess - format of host access control files", "flags": [], "examples": [ "The language is flexible enough that different types of access control policy can be ex‐", "pressed with a minimum of fuss. Although the language uses two access control tables, the", "most common policies can be implemented with one of the tables being trivial or even empty.", "When reading the examples below it is important to realize that the allow table is scanned", "before the deny table, that the search terminates when a match is found, and that access is", "granted when no match is found at all.", "The examples use host and domain names. They can be improved by including address and/or net‐", "work/netmask information, to reduce the impact of temporary name server lookup failures." ], "see_also": [ { "name": "hostsoptions", "section": "5", "url": "https://www.chedong.com/phpMan.php/man/hostsoptions/5/json" }, { "name": "tcpd", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/tcpd/8/json" }, { "name": "tcpdchk", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/tcpdchk/8/json" }, { "name": "tcpdmatch", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/tcpdmatch/8/json" } ] }