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 (Aside: An attempt to connect to a remote host may not be a hopeless failure,
180 even if a real IP socket is denied:
182 deliberately makes no attempt to check that addresses being bound to
183 sockets correspond to locally available addresses; and besides, sockets
184 can be introduced into the directory by other programs simulating remote
189 is a comma-separated list of entries of the form:
196 .IR prefix-length ]| \c
203 (The spaces in the above are optional.)
205 The leading sign says whether
206 matching addresses should be
213 The IP-address portion may be any of the following
216 Matches all addresses.
219 Matches the address of one of the machine's network interfaces.
222 Matches just the given IPv4 or IPv6 address. An
224 may be enclosed in square brackets; IPv6 addresses must be so enclosed,
225 because colons are significant in the rest of the ACL syntax.
227 .IB address \- address
228 Matches any address which falls in the given range. Addresses are
229 compared lexicographically, with octets to the left given precedence
230 over octets to the right.
232 .IB address / prefix-length
233 Matches an address in the given network.
235 The port portion may be omitted (which means `match any port'), or may
242 Range comparisons are always inclusive of both endpoints.
244 ACL entries are processed in the order they appear in the configuration
245 file. The default action of an ACL, used if none of its entries match,
246 is the opposite of the last actual entry in the list: if the last entry
247 says `accept', then the default is to deny, and vice-versa. If the ACL
248 is empty, the default is to deny all addresses.
250 For example, it may be useful to allow access at least to a DNS server.
251 This can be accomplished by adding a line
253 realconnect +1.2.3.4:53
255 to the configuration file, where 1.2.3.4 is the IP address of one of
257 .SS Example applications
258 SLIME is an Emacs extension for doing interactive programming with Lisp
259 systems. It communicates with the Lisp system using TCP sockets, since
260 Unix-domain sockets are unavailable on Windows, and besides, they are
261 less well supported by Lisp implementations. Unfortunately, when the
262 author wrote this program, SLIME applied no authentication on its TCP
263 port, allowing any local user to take over the running Lisp. Worse,
264 some Lisps are unable to bind a listening socket to a particular
265 address, leaving the socket potentially available to anyone on the
266 network. By running Emacs under
268 the security hole is closed completely and no messing with
269 authentication secrets is needed.
271 SSH is an excellent tool for secure communications over hostile
272 networks. In particular, its ability to forward TCP connections to a
273 port on one side of an SSH tunnel to the other side is very useful.
274 However, such a forwarded port is available to all users on the source
275 side of the tunnel. Using
277 and a suitable configuration, a user can restrict access to a forwarded
278 port to himself or a small group.
283 hack. It won't work on setuid programs. Also, perhaps more
284 importantly, it can't do anything to prevent a
286 program's use of networking: a program could theoretically issue sockets
287 system calls directly instead of using the C library calls that
289 intercepts. It is intended only as a tool for enhancing the security of
290 software written by well-meaning programmers who don't understand the
291 security aspects of writing networking code.
293 It's very hard to tell exactly what state a Unix-domain socket is in.
294 If the filesystem object isn't there, it's not active, but if it
296 then the socket might be in use or it might be stale.
300 to decide whether a socket is in use, but this can fail in two ways.
301 Firstly, if the socket is created and renamed, the kernel doesn't
304 will think that the new name is stale. Secondly, if the socket is
305 created, used, unlinked while it's still in use, and recreated, then
307 will think that it's in use when in fact it's gone stale. Don't mess
310 sockets unless you know what you're doing.
312 The procedure to replace a Unix-domain socket by an Internet one is
313 fairly thorough, but there are some missing cases. In particular, if
314 the socket being bound or connected is a duplicate (using
316 then only one of the copies will be fixed. Similarly, copies owned by
317 child processes will be unaffected.
319 This manual is surprisingly long and complicated for such a simple hack.
321 Mark Wooding, <mdw@distorted.org.uk>
~mdw / preload-hacks / blob