tcpwrapper

hosts.allow and hosts.deny belongs tcpwrapper package (man hosts_config for details).

tcpwrapper first check rules in hosts.allow from top to bottom. If one rule is satisfied, then access is granted.

If all rules in hosts.allow are not satisfied, then tcpwrapper check the rules in hosts.deny from top the bottom. If one rule is satisfied, then access is denied.

If all rules in hosts.deny are not satisfied, then access is granted.

/etc/hosts.allow和/etc/hosts.deny

这两个文件都是控制访问来源IP

这两个文件是tcpd服务器的配置文件，tcpd服务器可以控制外部IP对本机服务的访问。这两个配置文件的格式如下：

#服务进程名:主机列表:当规则匹配时可选的命令操作
server_name:hosts-list[:command]

ALL关键字匹配所有情况，EXCEPT匹配除了某些项之外的情况，PARANOID匹配你想控制的IP地址和它的域名不匹配时(域名伪装)的情况。

限制所有的ssh，
除非从218.64.87.0——127上来。
hosts.deny:
sshd:ALL
hosts.allow:
ALL:127.0.0.1
sshd:218.64.87.0/255.255.255.128

其中IP地址范围的写法有若干种，主要的三种是：
1.网 络地址——子网掩码方式：
218.64.87.0/255.255.255.0
2.网络地址方式（我自己这样叫，呵呵）
218.64.（即以218.64打头的IP地址） 等价于 218.64.*
3.缩略子网掩码方式，既数一数二进制子网掩码前面有多少个“1”比如：
218.64.87.0/255.255.255.0 等价于 218.64.87.0/24

WILDCARDS

The access control language supports explicit wildcards:

ALL

The universal wildcard, always matches.

LOCAL

Matches any host whose name does not contain a dot character.

UNKNOWN

Matches any user whose name is unknown, and matches any host whose name or address are unknown. This pattern should be used with care: host names may be unavailable due to temporary name server problems. A network address will be unavailable when the software cannot figure out what type of network it is talking to.

KNOWN

Matches any user whose name is known, and matches any host whose name and address are known. This pattern should be used with care: host names may be unavailable due to temporary name server problems. A network address will be unavailable when the software cannot figure out what type of network it is talking to.

PARANOID

Matches any host whose name does not match its address. When tcpd is built with -DPARANOID (default mode), it drops requests from such clients even before looking at the access control tables. Build without -DPARANOID when you want more control over such requests.

OPERATORS

EXCEPT

Intended use is of the form: `list_1 EXCEPT list_2'; this construct matches anything that matches list_1 unless it matches list_2. The EXCEPT operator can be used in daemon_lists and in client_lists. The EXCEPT operator can be nested: if the control language would permit the use of parentheses, `a EXCEPT b EXCEPT c' would parse as `(a EXCEPT (b EXCEPT c))'.

SHELL COMMANDS

If the first-matched access control rule contains a shell command, that command is subjected to % substitutions (see next section). The result is executed by a /bin/sh child process with standard input, output and error connected to /dev/null. Specify an `&' at the end of the command if you do not want to wait until it has completed.

Shell commands should not rely on the PATH setting of the inetd. Instead, they should use absolute path names, or they should begin with an explicit PATH=whatever statement.

The (5) document describes an alternative language that uses the shell command field in a different and incompatible way.

% EXPANSIONS

The following expansions are available within shell commands:

%a (%A)

The client (server) host address.

%c

Client information: , , a host name, or just an address, depending on how much information is available.

%d

The daemon process name (argv[0] value).

%h (%H)

The client (server) host name or address, if the host name is unavailable.

%n (%N)

The client (server) host name (or "unknown" or "paranoid").

%p

The daemon process id.

%s

Server information: , , or just a daemon name, depending on how much information is available.

%u

The client user name (or "unknown").

%%

Expands to a single `%' character.

Characters in % expansions that may confuse the shell are replaced by underscores.

SERVER ENDPOINT PATTERNS

In order to distinguish clients by the network address that they connect to, use patterns of the form:

: client_list ...

Patterns like these can be used when the machine has different internet addresses with different internet hostnames. Service providers can use this facility to offer FTP, GOPHER or WWW archives with internet names that may even belong to different organizations. See also the `twist' option in the (5) document. Some systems (Solaris, FreeBSD) can have more than one internet address on one physical interface; with other systems you may have to resort to SLIP or PPP pseudo interfaces that live in a dedicated network address space.

The host_pattern obeys the same syntax rules as host names and addresses in client_list context. Usually, server endpoint information is available only with connection-oriented services.

CLIENT USERNAME LOOKUP

When the client host supports the RFC 931 protocol or one of its descendants (TAP, IDENT, RFC 1413) the wrapper programs can retrieve additional information about the owner of a connection. Client username information, when available, is logged together with the client host name, and can be used to match patterns like:

daemon_list : ... ...

The daemon wrappers can be configured at compile time to perform rule-driven username lookups (default) or to always interrogate the client host. In the case of rule-driven username lookups, the above rule would cause username lookup only when both the daemon_list and the host_pattern match.

A user pattern has the same syntax as a daemon process pattern, so the same wildcards apply (netgroup membership is not supported). One should not get carried away with username lookups, though.

*

The client username information cannot be trusted when it is needed most, i.e. when the client system has been compromised. In general, ALL and (UN)KNOWN are the only user name patterns that make sense.

*

Username lookups are possible only with TCP-based services, and only when the client host runs a suitable daemon; in all other cases the result is "unknown".

*

A well-known UNIX kernel bug may cause loss of service when username lookups are blocked by a firewall. The wrapper README document describes a procedure to find out if your kernel has this bug.

*

Username lookups may cause noticeable delays for non-UNIX users. The default timeout for username lookups is 10 seconds: too short to cope with slow networks, but long enough to irritate PC users.

Selective username lookups can alleviate the last problem. For example, a rule like:

daemon_list : @pcnetgroup ALL@ALL

would match members of the pc netgroup without doing username lookups, but would perform username lookups with all other systems.

DETECTING ADDRESS SPOOFING ATTACKS

A flaw in the sequence number generator of many TCP/IP implementations allows intruders to easily impersonate trusted hosts and to break in via, for example, the remote shell service. The IDENT (RFC931 etc.) service can be used to detect such and other host address spoofing attacks.

Before accepting a client request, the wrappers can use the IDENT service to find out that the client did not send the request at all. When the client host provides IDENT service, a negative IDENT lookup result (the client matches `') is strong evidence of a host spoofing attack.

A positive IDENT lookup result (the client matches `') is less trustworthy. It is possible for an intruder to spoof both the client connection and the IDENT lookup, although doing so is much harder than spoofing just a client connection. It may also be that the client's IDENT server is lying.

Note: IDENT lookups don't work with UDP services.

EXAMPLES

The language is flexible enough that different types of access control policy can be expressed 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 network/netmask information, to reduce the impact of temporary name server lookup failures.

MOSTLY CLOSED

In this case, access is denied by default. Only explicitly authorized hosts are permitted access.

The default policy (no access) is implemented with a trivial deny file:

/etc/hosts.deny: ALL: ALL

This denies all service to all hosts, unless they are permitted access by entries in the allow file.

The explicitly authorized hosts are listed in the allow file. For example:

/etc/hosts.allow: ALL: LOCAL @some_netgroup
ALL: .foobar.edu EXCEPT terminalserver.foobar.edu

The first rule permits access from hosts in the local domain (no `.' in the host name) and from members of the some_netgroup netgroup. The second rule permits access from all hosts in the foobar.edu domain (notice the leading dot), with the exception of terminalserver.foobar.edu.

MOSTLY OPEN

Here, access is granted by default; only explicitly specified hosts are refused service.

The default policy (access granted) makes the allow file redundant so that it can be omitted. The explicitly non-authorized hosts are listed in the deny file. For example:

/etc/hosts.deny: ALL: some.host.name, .some.domain
ALL EXCEPT in.fingerd: other.host.name, .other.domain

The first rule denies some hosts and domains all services; the second rule still permits finger requests from other hosts and domains.

BOOBY TRAPS

The next example permits tftp requests from hosts in the local domain (notice the leading dot). Requests from any other hosts are denied. Instead of the requested file, a finger probe is sent to the offending host. The result is mailed to the superuser.

/etc/hosts.allow:

in.tftpd: LOCAL, .my.domain

/etc/hosts.deny:
in.tftpd: ALL: spawn (/some/where/safe_finger -l @%h | \
        /usr/ucb/mail -s %d-%h root) &

The safe_finger command comes with the tcpd wrapper and should be installed in a suitable place. It limits possible damage from data sent by the remote finger server. It gives better protection than the standard finger command.

The expansion of the %h (client host) and %d (service name) sequences is described in the section on shell commands.

Warning: do not booby-trap your finger daemon, unless you are prepared for infinite finger loops.

On network firewall systems this trick can be carried even further. The typical network firewall only provides a limited set of services to the outer world. All other services can be "bugged" just like the above tftp example. The result is an excellent early-warning system.

SEE ALSO

(8) tcp/ip daemon wrapper program.
tcpdchk(8), tcpdmatch(8), test programs.