13 .TH noip 1 "5 May 2005" "Straylight/Edgeware" "Preload hacks"
15 noip \- run programs without the ability to use IP sockets
25 (by default, the user's shell, as determined by the
27 environment variable) in an environment where attempts to use TCP/IP
28 networking are (mostly) transparently translated into the use of
29 Unix-domain sockets in a private directory.
31 There are many programs which use TCP/IP for their own internal
32 communications needs, largely unnecessarily. This can present security
33 problems: even if a program binds its listening sockets to
35 other users on the same system can still connect, and many such programs
36 don't seem to have authentication systems.
39 addresses this problem by intercepting a program's networking calls and
40 making it use Unix-domain sockets in a private directory instead of
41 TCP/IP. Now its communications are truly private to the running user.
42 .SS The socket directory
45 program keeps its sockets in a directory whose name can be configured,
55 environment variables, or is
58 in the temporary directory, which in turn is determined by the
62 environment variables, or is
64 The sockets in this directory are simply named
66 after the Internet sockets they represent.
68 If the socket directory does not exist when a program running under
70 starts up, it is created and made readable and writable only by the
71 current user. Also, it is scanned and any apparently stale sockets are
76 is controlled by a configuration file. By default,
78 reads configuration from
80 in the calling user's home directory, as determined by the
82 environment, or, failing that, looking up the
84 (not effective) user id in the password database. However, if the
87 is set, then the file it names is read instead (assuming it exists; if
88 it doesn't, no configuration is read).
90 The configuration file has a simple line-based format. A line is
91 ignored if it consists only of whitespace, or if its first whitespace
94 Otherwise, the first whitespace-delimited word is a keyword and the
95 remainder of the line is a value. The following keywords are
98 .BR "debug " [\fInumber\fR]
101 is nonzero, turn debugging on; if it is zero, turn debugging off. The
104 is given, is to turn debugging on. Debugging is written to standard
105 error. Some debugging is produced before the configuration file is
106 read; the environment variable
108 can be used to control this.
110 .BI "socketdir " directory
111 Store the Unix-domain sockets in
113 rather than the default. The environment variable
115 can also be used to control which directory is used for sockets.
117 .BI "autoports " min "\-" max
118 Select which ports are used for implicit binding. Allocating ports can
119 be a bit slow, since checking whether a Unix domain socket is in use is
120 difficult. A wide range makes things easier, because
122 starts by trying ports at random from the given range. The environment
125 can also be used to control which ports are assigned automatically.
127 .BI "realbind " acl-entry
130 access control list (ACL). When a program attempts to
132 a socket to an address, the
134 ACL is consulted. If the address is matched, then the program is
135 allowed to bind a real Internet socket to that address; otherwise, the
136 socket is bound to a Unix-domain socket. Three environment variables
138 .BR NOIP_REALBIND_BEFORE ,
141 .BR NOIP_REALBIND_AFTER .
144 rules are inserted at the front of the list; the
146 rules are appended on the end. Currently, the rules in
148 are also put at the end (before the
150 rules), though this may change later.
152 .BI "realconnect " acl-entry
155 access control list (ACL). When a program attempts to
157 a socket to an address, or to contact another socket using
163 ACL is consulted. If the destination address is matched, then the
164 program is allowed to contact the real Internet socket; otherwise, the
165 attempt is made to contact a Unix-domain socket. Three environment variables
167 .BR NOIP_REALCONNET_BEFORE ,
168 .BR NOIP_REALCONNECT ,
170 .BR NOIP_REALCONNECT_AFTER .
173 rules are inserted at the front of the list; the
175 rules are appended on the end. Currently, the rules in
177 are also put at the end (before the
179 rules), though this may change later.
181 (Aside: An attempt to connect to a remote host may not be a hopeless failure,
182 even if a real IP socket is denied:
184 deliberately makes no attempt to check that addresses being bound to
185 sockets correspond to locally available addresses; and besides, sockets
186 can be introduced into the directory by other programs simulating remote
191 is a comma-separated list of entries of the form:
205 (The spaces in the above are optional.)
207 The leading sign says whether
208 matching addresses should be
215 The IP-address portion may be any of the following
218 Matches all addresses.
221 Matches the address of one of the machine's network interfaces.
224 Matches just the given address
226 .IB address \- address
227 Matches any address which falls in the given range. Addresses are
228 compared lexicographically, with octets to the left given precedence
229 over octets to the right.
232 Matches an address in the given network. The
234 may be a netmask in dotted-quad form, or a one-bit-count.
236 The port portion may be omitted (which means `match any port'), or may
243 Range comparisons are always inclusive of both endpoints.
245 ACL entries are processed in the order they appear in the configuration
246 file. The default action of an ACL, used if none of its entries match,
247 is the opposite of the last actual entry in the list: if the last entry
248 says `accept', then the default is to deny, and vice-versa. If the ACL
249 is empty, the default is to deny all addresses.
251 For example, it may be useful to allow access at least to a DNS server.
252 This can be accomplished by adding a line
254 realconnect +1.2.3.4:52
256 to the configuration file, where 1.2.3.4 is the IP address of one of
258 .SS Example applications
259 SLIME is an Emacs extension for doing interactive programming with Lisp
260 systems. It communicates with the Lisp system using TCP sockets, since
261 Unix-domain sockets are unavailable on Windows, and besides, they are
262 less well supported by Lisp implementations. Unfortunately, when the
263 author wrote this program, SLIME applied no authentication on its TCP
264 port, allowing any local user to take over the running Lisp. Worse,
265 some Lisps are unable to bind a listening socket to a particular
266 address, leaving the socket potentially available to anyone on the
267 network. By running Emacs under
269 the security hole is closed completely and no messing with
270 authentication secrets is needed.
272 SSH is an excellent tool for secure communications over hostile
273 networks. In particular, its ability to forward TCP connections to a
274 port on one side of an SSH tunnel to the other side is very useful.
275 However, such a forwarded port is available to all users on the source
276 side of the tunnel. Using
278 and a suitable configuration, a user can restrict access to a forwarded
279 port to himself or a small group.
284 hack. It won't work on setuid programs. Also, perhaps more
285 importantly, it can't do anything a
287 program use of networking: a program could theoretically issue sockets
288 system calls directly instead of using the C library calls that
290 intercepts. It is intended only as a tool for enhancing the security of
291 software written by well-meaning programmers who don't understand the
292 security aspects of writing networking code.
294 It's very hard to tell exactly what state a Unix-domain socket is in.
295 If the filesystem object isn't there, it's not active, but if it
297 then the socket might be in use or it might be stale.
301 to decide whether a socket is in use, but this can fail in two ways.
302 Firstly, if the socket is created and renamed, the kernel doesn't
305 will think that the new name is stale. Secondly, if the socket is
306 created, used, unlinked while it's still in use, and recreated, then
308 will think that it's in use when in fact it's gone stale. Don't mess
311 sockets unless you know what you're doing.
313 The procedure to replace a Unix-domain socket by an Internet one is
314 fairly thorough, but there are some missing cases. In particular, if
315 the socket being bound or connected is a duplicate (using
317 then only one of the copies will be fixed. Similarly, copies owned by
318 child processes will be unaffected.
320 This manual is surprisingly long and complicated for such a simple hack.
322 Mark Wooding, <mdw@nsict.org>
~mdw / preload-hacks / blob