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31 .Dd August 17, 2001
32 .Dt FTP-PROXY 8
33 .Os
34 .Sh NAME
35 .Nm ftp-proxy
36 .Nd Internet File Transfer Protocol proxy server
37 .Sh SYNOPSIS
38 .Nm
39 .Bk -words
40 .Op Fl AnrVw
41 .Op Fl a Ar address
42 .Op Fl D Ar debuglevel
43 .Op Fl g Ar group
44 .Op Fl M Ar maxport
45 .Op Fl m Ar minport
46 .Op Fl R Ar address[:port]
47 .Op Fl S Ar address
48 .Op Fl t Ar timeout
49 .Op Fl u Ar user
50 .Ek
51 .Sh DESCRIPTION
52 .Nm
53 is a proxy for the Internet File Transfer Protocol.
54 The proxy uses
55 .Xr pf 4
56 and expects to have the FTP control connection as described in
57 .Xr services 5
58 redirected to it via a
59 .Xr pf 4
60 .Em rdr
61 command.
62 An example of how to do that is further down in this document.
63 .Pp
64 The options are as follows:
65 .Bl -tag -width Ds
66 .It Fl A
67 Permit only anonymous FTP connections.
68 The proxy will allow connections to log in to other sites as the user
69 .Qq ftp
70 or
71 .Qq anonymous
72 only.
73 Any attempt to log in as another user will be blocked by the proxy.
74 .It Fl a Ar address
75 Specify the local IP address to use in
76 .Xr bind 2
77 as the source for connections made by
78 .Nm
79 when connecting to destination FTP servers.
80 This may be necessary if the interface address of
81 your default route is not reachable from the destinations
82 .Nm
83 is attempting connections to, or this address is different from the one
84 connections are being NATed to.
85 In the usual case this means that
86 .Ar address
87 should be a publicly visible IP address assigned to one of
88 the interfaces on the machine running
89 .Nm
90 and should be the same address to which you are translating traffic
91 if you are using the
92 .Fl n
93 option.
94 .It Fl D Ar debuglevel
95 Specify a debug level, where the proxy emits verbose debug output
96 into
97 .Xr syslogd 8
98 at level
99 .Dv LOG_DEBUG .
100 Meaningful values of debuglevel are 0-3, where 0 is no debug output and
101 3 is lots of debug output, the default being 0.
102 .It Fl g Ar group
103 Specify the named group to drop group privileges to, after doing
104 .Xr pf 4
105 lookups which require root.
106 By default,
107 .Nm
108 uses the default group of the user it drops privilege to.
109 .It Fl M Ar maxport
110 Specify the upper end of the port range the proxy will use for the
111 data connections it establishes.
112 The default is
113 .Dv IPPORT_HILASTAUTO
114 defined in
115 .In netinet/in.h
116 as 65535.
117 .It Fl m Ar minport
118 Specify the lower end of the port range the proxy will use for all
119 data connections it establishes.
120 The default is
121 .Dv IPPORT_HIFIRSTAUTO
122 defined in
123 .In netinet/in.h
124 as 49152.
125 .It Fl n
126 Activate network address translation
127 .Pq NAT
128 mode.
129 In this mode, the proxy will not attempt to proxy passive mode
130 .Pq PASV or EPSV
131 data connections.
132 In order for this to work, the machine running the proxy will need to
133 be forwarding packets and doing network address translation to allow
134 the outbound passive connections from the client to reach the server.
135 See
136 .Xr pf.conf 5
137 for more details on NAT.
138 The proxy only ignores passive mode data connections when using this flag;
139 it will still proxy PORT and EPRT mode data connections.
140 Without this flag,
141 .Nm
142 does not require any IP forwarding or NAT beyond the
143 .Em rdr
144 necessary to capture the FTP control connection.
145 .It Fl R Ar address:[port]
146 Reverse proxy mode for FTP servers running behind a NAT gateway.
147 In this mode, no redirection is needed.
148 The proxy is run from
149 .Xr inetd 8
150 on the port that external clients connect to (usually 21).
151 Control connections and passive data connections are forwarded
152 to the server.
153 .It Fl r
154 Use reverse host
155 .Pq reverse DNS
156 lookups for logging and libwrap use.
157 By default,
158 the proxy does not look up hostnames for libwrap or logging purposes.
159 .It Fl S Ar address
160 Source address to use for data connections made by the proxy.
161 Useful when there are multiple addresses (aliases) available
162 to the proxy.
163 Clients may expect data connections to have the same source
164 address as the control connections, and reject or drop other
165 connections.
166 .It Fl t Ar timeout
167 Specifies a timeout, in seconds.
168 The proxy will exit and close open connections if it sees no data
169 for the duration of the timeout.
170 The default is 0, which means the proxy will not time out.
171 .It Fl u Ar user
172 Specify the named user to drop privilege to, after doing
173 .Xr pf 4
174 lookups which require root privilege.
175 By default,
176 .Nm
177 drops privilege to the user
178 .Em proxy .
179 .Pp
180 Running as root means that the source of data connections the proxy makes
181 for PORT and EPRT will be the RFC mandated port 20.
182 When running as a non-root user, the source of the data connections from
183 .Nm
184 will be chosen randomly from the range
185 .Ar minport
186 to
187 .Ar maxport
188 as described above.
189 .It Fl V
190 Be verbose.
191 With this option the proxy logs the control commands
192 sent by clients and the replies sent by the servers to
193 .Xr syslogd 8 .
194 .It Fl w
195 Use the tcp wrapper access control library
196 .Xr hosts_access 3 ,
197 allowing connections to be allowed or denied based on the tcp wrapper's
198 .Pa /etc/hosts.allow
199 and
200 .Pa /etc/hosts.deny
201 files.
202 The proxy does libwrap operations after determining the destination
203 of the captured control connection, so that tcp wrapper rules may
204 be written based on the destination as well as the source of FTP connections.
205 .El
206 .Pp
207 .Nm
208 is run from
209 .Xr inetd 8
210 and requires that FTP connections are redirected to it using a
211 .Em rdr
212 rule.
213 A typical way to do this would be to use a
214 .Xr pf.conf 5
215 rule such as
216 .Bd -literal -offset 2n
217 int_if = \&"xl0\&"
218 rdr pass on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021
219 .Ed
220 .Pp
221 .Xr inetd 8
222 must then be configured to run
223 .Nm
224 on the port from above using
225 .Bd -literal -offset 2n
226 127.0.0.1:8021 stream tcp nowait root /usr/libexec/ftp-proxy ftp-proxy
227 .Ed
228 .Pp
229 in
230 .Xr inetd.conf 5 .
231 .Pp
232 .Nm
233 accepts the redirected control connections and forwards them
234 to the server.
235 The proxy replaces the address and port number that the client
236 sends through the control connection to the server with its own
237 address and proxy port, where it listens for the data connection.
238 When the server opens the data connection back to this port, the
239 proxy forwards it to the client.
240 The
241 .Xr pf.conf 5
242 rules need to let pass connections to these proxy ports
243 (see options
244 .Fl u , m ,
245 and
246 .Fl M
247 above) in on the external interface.
248 The following example allows only ports 49152 to 65535 to pass in
249 statefully:
250 .Bd -literal -offset indent
251 block in on $ext_if proto tcp all
252 pass in on $ext_if inet proto tcp from any to $ext_if \e
253 port > 49151 keep state
254 .Ed
255 .Pp
256 Alternatively, rules can make use of the fact that by default,
257 .Nm
258 runs as user
259 .Qq proxy
260 to allow the backchannel connections, as in the following example:
261 .Bd -literal -offset indent
262 block in on $ext_if proto tcp all
263 pass in on $ext_if inet proto tcp from any to $ext_if \e
264 user proxy keep state
265 .Ed
266 .Pp
267 These examples do not cover the connections from the proxy to the
268 foreign FTP server.
269 If one does not pass outgoing connections by default additional rules
270 are needed.
271 .Sh SEE ALSO
272 .Xr ftp 1 ,
273 .Xr pf 4 ,
274 .Xr hosts_access 5 ,
275 .Xr inetd.conf 5 ,
276 .Xr pf.conf 5 ,
277 .Xr inetd 8 ,
278 .Xr pfctl 8 ,
279 .Xr syslogd 8
280 .Sh BUGS
281 Extended Passive mode
282 .Pq EPSV
283 is not supported by the proxy and will not work unless the proxy is run
284 in network address translation mode.
285 When not in network address translation mode, the proxy returns an error
286 to the client, hopefully forcing the client to revert to passive mode
287 .Pq PASV
288 which is supported.
289 EPSV will work in network address translation mode, assuming a
290 .Xr pf.conf 5
291 setup which allows the EPSV connections through to their destinations.
292 .Pp
293 IPv6 is not yet supported.