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Archive-name: computer-security/anonymous-ftp-faq
Post-Frequency: monthly
Last-modified: 1994/6/1
Version: 1.3
How to set up a Secure Anonymous FTP Site
The following is a FAQ on setting up a secure FTP Site. FTP sites
are known for much abuse by transferring illegal files. They also open many
oppurtunities for intruders to gain access via misconfigured setups. And
lastly many versions of ftp servers have had security holes. This FAQ is
intended to clean up this abuse by allowing administrators to go through this
check list of steps to make sure their FTP is correctly configured and that
they are running the most current ftp daemon.
This is organized in the following fashion, I am breaking into several parts
as follows:
Part 1 - General Description of Setting up an "anonymous" ftp server.
Part 2 - Setting up a chrooted Secure Anonymous ftp server.
Part 3 - OS Specific needed information and suggestions.
Part 4 - Where to get other FTP daemons
Part 5 - Archie
Part 6 - Acknowledgements.
Part 1 - General Description of Setting up an "anonymous" ftp server.
How do I setup "anonymous" ftp securely?
PLEASE READ ALL NOTES AND WARNINGS!!!
1) Create the user ftp in /etc/passwd. Use a misc group. The user`s home
directory will be ~ftp where ~ftp is the root you wish anonymous users to
see.
Use an invalid password and user shell for better security. The entry in the
passwd file should look something like:
ftp:*:400:400:Anonymous FTP:/home/ftp:/bin/true
2) Create the home directory ~ftp. Make the directory owned by root (NOT ftp)
with the same group as ftp. Thus, owner permissions are for root and group
permissions are for the anonymous users. Set the permissions for ~ftp to 555
(read, nowrite, execute).
3) Create the directory ~ftp/bin. This directory is owned by root (group
e.g. wheel) with permissions 111 (noread, nowrite, execute).
4) Copy the program ls into ~ftp/bin. ls is owned by root with permissions
111 (noread, nowrite, execute). Any other commands you put in ~ftp/bin
should have the same permissions as well.
5) Make the directory ~ftp/etc. This directory is owned by root with
permissions 111.
6) Create from scratch the files /etc/passwd and /etc/group in ~ftp/etc.
These files should be mode 444. The passwd file should only contain root,
daemon, uucp, and ftp. The group file must contain ftp's group. Use your
/etc/passwd and /etc/group files as a template for creating passwd and group
files going to ~ftp/etc. You may even change the user names in this file,
they are used only for 'ls' command. So for example if all files in your
~ftp/pub/linux hierarchy will be maintained by a real user 'balon' with
uid=156 you may put
linux:*:156:120:Kazik Balon::
in the ~ftp/etc/passwd file (regardless of his real username). Leave only
these users who will own files under ftp hierarchy (e.g. root, daemon,
ftp...) and definitely remove *ALL* passwords by replacing them with '*' so
the entry looks like:
root:*:0:0:Ftp maintainer::
ftp:*:400:400: Anonymous ftp::
For more security, you can just remove ~ftp/etc/passwd and
~ftp/etc/group (the effect is that ls -l will not show the directories' group
names). Wuarchive ftp daemon (and some others) have some extensions based on
the contents of the group/passwd files, so read the appropriate documentation.
7) Make the directory ~ftp/pub. This directory is owned by you and has the
same group as ftp with permissions 555. On most systems (like SunOS) you may
want to make this directory 2555, ie. set-group-id, in order to create new
files with the same group ownership.
Files are left here for public distribution. All folders inside ~ftp/pub
should have the same permissions as 555.
- ** Neither the home directory (~ftp) nor any directory below it should be
owned by ftp! No files should be owned by ftp either. Modern ftp daemons
support all kinds of useful commands, such as chmod, that allow outsiders to
undo your careful permission settings. They also have configuration options
like the following (WuFTP) to disable them:
# all the following default to "yes" for everybody
delete no guest,anonymous # delete permission?
overwrite no guest,anonymous # overwrite permission?
rename no guest,anonymous # rename permission?
chmod no anonymous # chmod permission?
umask no anonymous # umask permission?
8) If you wish to have a place for anonymous users to leave files, create
the directory ~ftp/pub/incoming. This directory is owned by root with
permissions 733. Do a 'chmod +t ~ftp/pub/incoming'. The ftp daemon will
normally not allow an anonymous user to overwrite an existing file, but a
normal user of the system would be able to delete anything. By setting the
mode to '1733' you prevent this from happening. In wuftpd you may configure
the daemon to create new files with permissions '600' owned by root or any
other user. Many times, incoming directories are abused by exchanging pirated
and pornographic material. Abusers often create hidden directories there for
this purpose. Making the incoming directory unreadable by anonymous ftp helps
to some extent. With ordinary ftp severs there is no way to prevent
directories being created in incoming. The WUarchive ftp server can limit
uploads to certain directories and can restrict characters used in file names
like this:
# specify the upload directory information
upload /var/spool/ftp * no
upload /var/spool/ftp /incoming yes ftp staff 0600 nodirs
# path filters
# path-filter...
path-filter anonymous /etc/msgs/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^-
path-filter guest /etc/msgs/pathmsg ^[-A-Za-z0-9_\.]*$ ^\. ^-
Suggestion: Create an extra file-system for your ftp-area (or at least for
your incoming-area) to prevent a denial-of-service attack by filling your
disk with garbage (inside your incoming directory).
If you have wuftpd you may want to add some ftp extensions like
compression/decompression 'on the fly' or creation of tar files for the
directory hierarchies. Get the appropriate sources (gzip, gnutar, compress),
compile them and link statically, put in the ~ftp/bin directory and edit the
appropriate file containing the definitions of the allowed conversions.
/usr/bin/tar is already statically-linked. You may wish to use gnu tar
anyway.
Gary Mills wrote a small program to support the following:
I got compress from ftp.uu.net, in the root directory, I believe, and compiled
it. To do tar and compress, I wrote a tiny program called `pipe', and
statically-linked it. My /etc/ftpconversions file looks like this:
#strip prefix:strip postfix:addon prefix:addon postfix:external command:
#types:options:description
:.Z: : :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
:-z: : :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
: : :.Z:/bin/compress -c %s:T_REG:O_COMPRESS:COMPRESS
: : :.tar:/bin/tar cf - %s:T_REG|T_DIR:O_TAR:TAR
: : :.tar.Z:/bin/pipe /bin/tar cf - %s | /bin/compress -c:T_REG|T_DIR:O_COMP
RESS|O_TAR:TAR+COMPRESS
: : :.tar:/bin/gtar -c -f - %s:T_REG|T_DIR:O_TAR:TAR
: : :.tar.Z:/bin/gtar -c -Z -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRES
S
: : :.tar.gz:/bin/gtar -c -z -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+GZIP
Here it is:
-----------------8<-------------cut---------------
/* pipe.c: exec two commands in a pipe */
#define NULL (char *)0
#define MAXA 16
main(argc, argv) int argc; char *argv[]; {
char *av1[MAXA], *av2[MAXA];
int i, n, p[2], cpid;
i = 0; n = 0;
while ( ++i < argc && n < MAXA ) {
if ( *argv[i] == '|' && *(argv[i]+1) == '\0' ) break;
av1[n++] = argv[i];
}
if ( n == 0 ) uexit();
av1[n] = NULL;
n = 0;
while ( ++i < argc && n < MAXA )
av2[n++] = argv[i];
if ( n == 0 ) uexit();
av2[n] = NULL;
if ( pipe(p) != 0 ) exit(1);
if ( ( cpid = fork() ) == (-1) ) exit(1);
else if ( cpid == 0 ) {
(void)close(p[0]);
(void)close(1);
(void)dup(p[1]);
(void)close(p[1]);
(void)execv(av1[0], av1);
_exit(127);
}
else {
(void)close(p[1]);
(void)close(0);
(void)dup(p[0]);
(void)close(p[0]);
(void)execv(av2[0], av2);
_exit(127);
}
/*NOTREACHED*/
}
uexit() {
(void)write(2, "Usage: pipe <command> | <command>\n", 34);
exit(1);
}
9) Other things to do:
as root: touch ~ftp/.rhosts ~ftp/.forward
chmod 400 ~ftp/.rhosts ~ftp/.forward
ie. make these files zero-length and owned by root.
Due to the last /bin/mail bugs in SunOS:
touch /usr/spool/mail/ftp; chmod 400 /usr/spool/mail/ftp
Consider an email-alias for the ftp-admin(s) to provide an email-address for
problems-reports.
If you are mounting some disks from other machines (or even your own) to the
~ftp hierarchy, mount it read-only. The correct entry for the /etc/fstab (on
the host with ftpd) is something like:
other:/u1/linux /home/ftp/pub/linux nfs ro,noquota,nosuid,intr,bg 1 0
This mounts under /home/ftp/pub/linux the disk from host 'other' with no
quota, no 'suid' programs (just in case), interruptible (in case 'other'
goes down) and 'bg' - so if 'other' is down when you reboot it will not stop
you trying to mount /home/ftp/pub/linux all over again.
Part 2 - Setting up a chrooted Secure Anonymous ftp server.
This part was contributed by Marcus J Ranum <mjr@tis.com>
Steps
-----
1) Build a statically linked version of ftpd and put it in ~ftp/bin.
Make sure it's owned by root.
2) Build a statically linked version of /bin/ls if you'll need one.
Put it in ~ftp/bin. If you are on a Sun, and need to build
one, there's a ported version of the BSD net2 ls command
for SunOs on ftp.tis.com: pub/firewalls/toolkit/patches/ls.tar.Z
Make sure it's owned by root.
3) Chown ~ftp to root and make it mode 755 THIS IS VERY IMPORTANT
4) Set up copies of ~ftp/etc/passwd and ~ftp/etc/group just as you would
normally, EXCEPT make 'ftp's home directory '/' -- make sure
they are owned by root.
5) Write a wrapper to kick ftpd off and install it in /etc/inetd.conf
The wrapper should look something like: (assuming ~ftp = /var/ftp)
main()
{
if(chdir("/var/ftp")) {
perror("chdir /var/ftp");
exit(1);
}
if(chroot("/var/ftp")) {
perror("chroot /var/ftp");
exit(1);
}
/* optional: seteuid(FTPUID); */
execl("/bin/ftpd","ftpd","-l",(char *)0);
perror("exec /bin/ftpd");
exit(1);
}
Options:
You can use 'netacl' from the toolkit or tcp_wrappers to achieve
the same effect.
We use 'netacl' to switch so that a few machines that connect to
the FTP service *don't* get chrooted first. This makes transferring
files a bit less painful.
You may also wish to take your ftpd sources and find all the places
where it calls seteuid() and remove them, then have the wrapper do
a setuid(ftp) right before the exec. This means that if someone
knows a hole that makes them "root" they still won't be. Relax and
imagine how frustrated they will be.
If you're hacking ftpd sources, I suggest you turn off a bunch of
the options in ftpcmd.y by unsetting the "implemented" flag in
ftpcmd.y. This is only practical if your FTP area is read-only.
6) As usual, make a pass through the FTP area and make sure that the files
are in correct modes and that there's nothing else in there that
can be executed.
7) Note, now, that your FTP area's /etc/passwd is totally separated from
your real /etc/passwd. This has advantages and disadvantages.
8) Some stuff may break, like syslog, since there is no /dev/log. Either
build a version of ftpd with a UDP-based syslog() routine or
run a second syslogd based on the BSD Net2 code, that maintains
a unix-domain socket named ~ftp/dev/log with the -p flag.
REMEMBER:
If there is a hole in your ftpd that lets someone get "root"
access they can do you some damage even chrooted. It's just
lots harder. If you're willing to hack some code, making the
ftpd run without permissions is a really good thing. The
correct operation of your hacked ftpd can be verified by
connecting to it and (while it's still at the user prompt)
do a ps-axu and verify that it's not running as root.
Part 3 - OS Specific needed information and suggestions.
Older SVR2 and SVR3 system, RTU 6.0 (Masscomp, now Concurrent Real Time UNIX),
AT&T 3B1 and 3B2 machines - May need dev/tcp:
[dev/tcp]
These ftpd implementations may require a ~ftp/dev/tcp in order for anonymous
ftp to work.
You have to create a character special device with the appropriate major and
minor device numbers. The appropriate major and minor numbers of ~ftp/dev/tcp
are what the major and minor numbers of /dev/tcp are.
The ~ftp/dev is a directory and ~ftp/dev/tcp is a character special device.
Make them owned and grouped by root. Permissions for ~ftp/dev is root
read/write/exec and other & group read and exec. The permissions for
~ftp/dev/tcp is root read/write, other & group read.
HPUX
[Logging] If you're using HP's native ftpd, the line in /etc/inetd.conf
should execute ftpd -l, which does extra logging.
SunOS
[Libraries] To set up SunOS to use its shared dynamic libraries, follow these
steps:
1) Create the directory ~ftp/usr. This directory is owned by root with
permissions 555.
2) Create the directory ~ftp/usr/lib. This directory is owned by root with
permissions 555.
3) Copy the runtime loader ld.so into ~ftp/usr/lib for use by ls. ld.so is
owned by root with permissions 555.
4) Copy the latest version of the shared C library, libc.so.* into
~ftp/usr/lib for use by ls.
libc.so.* is owned by root with permissions 555.
- **4.1.2(or above) users: you also need to copy /usr/lib/libdl.so.* to
~ftp/lib.
5) Create the directory ~ftp/dev. This directory is owned by root with
permissions 111.
6) ~ftp/dev/zero is needed by the runtime loader. Move into the directory
~ftp/dev and create it with the command:
mknod zero c 3 12
chown ~ftp/dev/zero to root. Make sure it's readable.
- **For novices: WARNING!! Don't try to copy /dev/zero to ~ftp/dev/zero!!
This is an endless file of zeroes and it will completely fill your filesystem!
7) If you want to have the local time showing when people connect, create the
directory ~ftp/usr/share/lib/zoneinfo and copy
/usr/share/lib/zoneinfo/localtime
8) If you are bothered by the need for copying your libraries so that you can
use Sun's 'ls', which is dynamically linked, you can try to get a statically
linked copy of 'ls' instead. The CD-ROM that contains Sun's OS has a
statically-linked version of ls. In this case, you can dispense with steps
#6-8.
Statically linked versions may be available from the following sources:
If you want a statically linked "ls" get the GNU fileutils off a
archive site near you and statically link it.
[Logging] Sun's standard ftpd logs *all* password information. To correct it,
install patch:
101640-03 SunOS 4.1.3: in.ftpd logs password info when -d option is
used.
In /etc/inetd.conf find the line that starts with "ftp". At the
end of that line, it should read "in.ftpd". Change that to "in.ftpd -dl".
In /etc/syslog.conf, add a line that looks like:
daemon.* /var/adm/daemonlog
Note that the whitespace between the two columns must include at least one
TAB character, not just spaces, or it won't work. Of course your log file
could be anything you want. Then, create the logfile (touch
/var/adm/daemonlog should do). Finally, restart inetd and syslogd, either
individually, or by rebooting the system. You should be good to go. If you
do not install the patch, make sure the log file is owned by root and mode
600, as the ftp daemon will log *everything*, including users' passwords.
Part 4 - Where to get other FTP daemons
Wuarchive FTP 2.4- A secure FTP daemon that allows improved access-control,
logging, pre-login banners, and is very configurable:
Can be ftp'd from ftp.uu.net in "/networking/ftp/wuarchive-ftpd"
directory. Be certain to verify the checksum information to confirm that you
have retrieved a valid copy. [Warning: Older versions of Wu-FTP are extremely
insecure and in some cases have been trojaned.]
BSD SVR4
File Checksum Checksum MD5 Digital Signature
----------------- -------- --------- --------------------------------
wu-ftpd-2.4.tar.Z 38213 181 20337 362 cdcb237b71082fa23706429134d8c32e
patch_2.3-2.4.Z 09291 8 51092 16 5558a04d9da7cdb1113b158aff89be8f
For DECWRL ftpd, sites can obtain version 5.93 via anonymous FTP
from gatekeeper.dec.com in the "/pub/misc/vixie" directory.
BSD SVR4
File Checksum Checksum MD5 Digital Signature
----------------- -------- --------- --------------------------------
ftpd.tar.gz 38443 60 1710 119 ae624eb607b4ee90e318b857e6573500
For BSDI systems, patch 005 should be applied to version 1.1 of
the BSD/386 software. You can obtain the patch file via
anonymous FTP from ftp.bsdi.com in the "/bsdi/patches-1.1"
directory.
BSD SVR4
File Checksum Checksum MD5 Digital Signature
----------------- -------- --------- --------------------------------
BU110-005 35337 272 54935 543 1f454d4d9d3e1397d1eff0432bd383cf
Public Domain Sources:
ftp.uu.net ~ftp/systems/unix/bsd-sources/libexec/ftpd
gatekeeper.dec.com ~ftp/pub/DEC/gwtools/ftpd.tar.Z
Part 5 - Archie
Searches FTP sites for programs. Login into these sites as archie
or use client software for faster access. To get your own anonymous
site added to Archie's search list, e-mail archie-updates@bunyip.com.
archie.ac.il 132.65.20.254 (Israel server)
archie.ans.net 147.225.1.10 (ANS server, NY (USA))
archie.au 139.130.4.6 (Australian Server)
archie.doc.ic.ac.uk 146.169.11.3 (United Kingdom Server)
archie.edvz.uni-linz.ac.at 140.78.3.8 (Austrian Server)
archie.funet.fi 128.214.6.102 (Finnish Server)
archie.internic.net 198.49.45.10 (AT&T server, NY (USA))
archie.kr 128.134.1.1 (Korean Server)
archie.kuis.kyoto-u.ac.jp 130.54.20.1 (Japanese Server)
archie.luth.se 130.240.18.4 (Swedish Server)
archie.ncu.edu.tw 140.115.19.24 (Taiwanese server)
archie.nz 130.195.9.4 (New Zealand server)
archie.rediris.es 130.206.1.2 (Spanish Server)
archie.rutgers.edu 128.6.18.15 (Rutgers University (USA))
archie.sogang.ac.kr 163.239.1.11 (Korean Server)
archie.sura.net 128.167.254.195 (SURAnet server MD (USA))
archie.sura.net(1526) 128.167.254.195 (SURAnet alt. MD (USA))
archie.switch.ch 130.59.1.40 (Swiss Server)
archie.th-darmstadt.de 130.83.22.60 (German Server)
archie.unipi.it 131.114.21.10 (Italian Server)
archie.univie.ac.at 131.130.1.23 (Austrian Server)
archie.unl.edu 129.93.1.14 (U. of Nebraska, Lincoln (USA))
archie.uqam.ca 132.208.250.10 (Canadian Server)
archie.wide.ad.jp 133.4.3.6 (Japanese Server)
Part 6 - Acknowledgements
Thanks to the following people for suggestions that help shape this FAQ:
Tomasz Surmacz (tsurmacz@asic.ict.pwr.wroc.pl)
Wolfgang Ley (Ley@rz.tu-clausthal.de)
Russel Street (russells@ccu1.auckland.ac.nz)
Gary Mills (mills@CC.UManitoba.CA)
Morten Welinder (terra@diku.dk)
Nick Christenson (npc@minotaur.jpl.nasa.gov)
Mark Hanning-Lee (markhl@romoe.caltech.edu)
Marcus J Ranum <mjr@tis.com>
Copyright
This paper is Copyright (c) 1994
by Christopher Klaus of Internet Security Systems, Inc.
Permission is hereby granted to give away free copies. You may
distribute, transfer, or spread this paper. You may not pretend that you
wrote it. This copyright notice must be maintained in any copy made.
Disclaimer
The information within this paper may change without notice. Use of
this information constitutes acceptance for use in an AS IS condition.
There are NO warranties with regard to this information. In no event shall
the author be liable for any damages whatsoever arising out of or in
connection with the use or spread of this information. Any use of this
information is at the user's own risk.
Address of Author
Please send suggestions, updates, and comments to:
Christopher Klaus <cklaus@shadow.net>
of Internet Security Systems, Inc. <iss@shadow.net>
--
Christopher William Klaus <cklaus@shadow.net> <iss@shadow.net>
Internet Security Systems, Inc.
2209 Summit Place Drive,
Atlanta,GA 30350-2430. (404)998-5871.