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 effective user id in the
83 password database. However, if the environment variable
85 is set, then the file it names is read instead (assuming it exists; if
86 it doesn't, no configuration is read).
88 The configuration file has a simple line-based format. A line is
89 ignored if it consists only of whitespace, or if its first whitespace
92 Otherwise, the first whitespace-delimited word is a keyword and the
93 remainder of the line is a value. The following keywords are
96 .BR "debug " [\fInumber\fR]
99 is nonzero, turn debugging on; if it is zero, turn debugging off. The
102 is given, is to turn debugging on. Debugging is written to standard
103 error. Some debugging is produced before the configuration file is
104 read; the environment variable
106 can be used to control this.
108 .BI "socketdir " directory
109 Store the Unix-domain sockets in
111 rather than the default. The environment variable
113 can also be used to control which directory is used for sockets.
115 .BI "autoports " min "\-" max
116 Select which ports are used for implicit binding. Allocating ports can
117 be a bit slow, since checking whether a Unix domain socket is in use is
118 difficult. A wide range makes things easier, because
120 starts by trying ports at random from the given range. The environment
123 can also be used to control which ports are assigned automatically.
125 .BI "realbind " acl-entry
128 access control list (ACL). When a program attempts to
130 a socket to an address, the
132 ACL is consulted. If the address is matched, then the program is
133 allowed to bind a real Internet socket to that address; otherwise, the
134 socket is bound to a Unix-domain socket. Three environment variables
136 .BR NOIP_REALBIND_BEFORE ,
139 .BR NOIP_REALBIND_AFTER .
142 rules are inserted at the front of the list; the
144 rules are appended on the end. Currently, the rules in
146 are also put at the end (before the
148 rules), though this may change later.
150 .BI "realconnect " acl-entry
153 access control list (ACL). When a program attempts to
155 a socket to an address, or to contact another socket using
161 ACL is consulted. If the destination address is matched, then the
162 program is allowed to contact the real Internet socket; otherwise, the
163 attempt is made to contact a Unix-domain socket. Three environment variables
165 .BR NOIP_REALCONNET_BEFORE ,
166 .BR NOIP_REALCONNECT ,
168 .BR NOIP_REALCONNECT_AFTER .
171 rules are inserted at the front of the list; the
173 rules are appended on the end. Currently, the rules in
175 are also put at the end (before the
177 rules), though this may change later.
179 .BI "impbind " bind-rule
180 Add an entry to the implicit-bind rule list. When a program attempts to
182 a socket without binding its local address first,
184 consults this list to decide on the correct local address to assign.
185 Each entry in the list has the form
192 The rules are tried in order: if the remote address matches (in the same
193 way as in an ACL entry) the address range on the left side of the rule,
194 then the socket is bound to the address from the right side; if the
195 address on the right is
197 then the remote address is used.
199 Three environment variables
201 .BR NOIP_IMPBIND_BEFORE ,
204 .BR NOIP_IMPBIND_AFTER .
207 rules are inserted at the front of the list; the
209 rules are appended on the end. Currently, the rules in
211 are also put at the end (before the
213 rules), though this may change later.
216 (Aside: An attempt to connect to a remote host may not be a hopeless failure,
217 even if a real IP socket is denied:
219 deliberately makes no attempt to check that addresses being bound to
220 sockets correspond to locally available addresses; and besides, sockets
221 can be introduced into the directory by other programs simulating remote
226 is a comma-separated list of entries of the form:
233 (The spaces in the above are optional.)
235 The leading sign says whether matching addresses should be
244 portion may be any of the following.
247 Matches all addresses.
250 Matches the address of one of the machine's network interfaces.
253 Matches just the given IPv4 or IPv6 address. An
255 may be enclosed in square brackets; IPv6 addresses must be so enclosed,
256 because colons are significant in the rest of the ACL syntax.
258 .IB address \- address
259 Matches any address which falls in the given range. Addresses are
260 compared lexicographically, with octets to the left given precedence
261 over octets to the right.
263 .IB address / prefix-length
264 Matches an address in the given network.
268 may be omitted (which means `match any port'), or may be a single
274 Range comparisons are always inclusive of both endpoints.
276 ACL entries are processed in the order they appear in the configuration
277 file. The default action of an ACL, used if none of its entries match,
278 is the opposite of the last actual entry in the list: if the last entry
279 says `accept', then the default is to deny, and vice-versa. If the ACL
280 is empty, the default is to deny all addresses.
282 For example, it may be useful to allow access at least to a DNS server.
283 This can be accomplished by adding a line
285 realconnect +1.2.3.4:53
287 to the configuration file, where 1.2.3.4 is the IP address of one of
289 .SS Example applications
290 SLIME is an Emacs extension for doing interactive programming with Lisp
291 systems. It communicates with the Lisp system using TCP sockets, since
292 Unix-domain sockets are unavailable on Windows, and besides, they are
293 less well supported by Lisp implementations. Unfortunately, when the
294 author wrote this program, SLIME applied no authentication on its TCP
295 port, allowing any local user to take over the running Lisp. Worse,
296 some Lisps are unable to bind a listening socket to a particular
297 address, leaving the socket potentially available to anyone on the
298 network. By running Emacs under
300 the security hole is closed completely and no messing with
301 authentication secrets is needed.
303 SSH is an excellent tool for secure communications over hostile
304 networks. In particular, its ability to forward TCP connections to a
305 port on one side of an SSH tunnel to the other side is very useful.
306 However, such a forwarded port is available to all users on the source
307 side of the tunnel. Using
309 and a suitable configuration, a user can restrict access to a forwarded
310 port to himself or a small group.
315 hack. It won't work on setuid programs. Also, perhaps more
316 importantly, it can't do anything to prevent a
318 program's use of networking: a program could theoretically issue sockets
319 system calls directly instead of using the C library calls that
321 intercepts. It is intended only as a tool for enhancing the security of
322 software written by well-meaning programmers who don't understand the
323 security aspects of writing networking code.
325 It's very hard to tell exactly what state a Unix-domain socket is in.
326 If the filesystem object isn't there, it's not active, but if it
328 then the socket might be in use or it might be stale.
332 to decide whether a socket is in use, but this can fail in two ways.
333 Firstly, if the socket is created and renamed, the kernel doesn't
336 will think that the new name is stale. Secondly, if the socket is
337 created, used, unlinked while it's still in use, and recreated, then
339 will think that it's in use when in fact it's gone stale. Don't mess
342 sockets unless you know what you're doing.
344 The procedure to replace a Unix-domain socket by an Internet one is
345 fairly thorough, but there are some missing cases. In particular, if
346 the socket being bound or connected is a duplicate (using
348 then only one of the copies will be fixed. Similarly, copies owned by
349 child processes will be unaffected.
351 This manual is surprisingly long and complicated for such a simple hack.
353 Mark Wooding, <mdw@distorted.org.uk>
~mdw / preload-hacks / blob