computer-security/anonymous-ftp FAQ
Archive-name: computer-security/anonymous-ftp-faq
Post-Frequency: monthly
Last-modified: 1995/4/05
Version: 1.4
Anonymous FTP FAQ
Version: 2.01
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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:
1. General Description of Setting up an "Anonymous" FTP server.
2. Setting up a chrooted Secure Anonymous FTP server.
3. OS Specific needed information and suggestions.
o Older SVR2 and SVR3 system, RTU 6.0 (Masscomp, now Concurrent Real
Time UNIX), and AT&T 3B1 and 3B2 machines
o HPUX
o Solaris 2.x
o SunOS
4. Where to get other FTP daemons
5. How to Know if your Anonymous FTP Server is Secure
6. Archie
-------------------------------------------------------------------------------
1. General Description of Setting up an "anonymous" ftp server.
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. Creating this user turns on anonymous ftp.
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).
[Warning:] Some MAN pages recommend making the ~ftp directory owned by
ftp. This is a big NO-NO, if you want any type of security on your system.
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.
[Warning:] 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:
To do tar and compress, he wrote a tiny program called `pipe', and
statically-linked it. His /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_COMPRESS|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+COMPRESS
: : :.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 | \n", 34);
exit(1);
}
-------- CUT HERE ------------
9. Other things to do:
as root:
touch ~ftp/.rhosts
touch ~ftp/.forward
chmod 400 ~ftp/.rhosts
chmod 400 ~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.
-------------------------------------------------------------------------------
2. Setting up a chrooted Secure Anonymous ftp server.
This part was contributed by Marcus J Ranum <mjr@tis.com>
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.
-------------------------------------------------------------------------------
3. OS Specific needed information and suggestions.
These machines may need dev/tcp:
* Older SVR2 and SVR3 system
* RTU 6.0 (Masscomp, now Concurrent Real Time UNIX),
* AT&T 3B1 and 3B2 machines
[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.
Solaris 2.x
[Script] Solaris' man page contains a script for installing anonymous ftpd
which saves time. You may still want to check over your anonymous ftpd for
vulnerabilities.
Command for reading the man page is:
$ man ftpd
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.
[Note:] 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.
[Warning:] For novices: 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
The information can be separated (or like SunOs4.1.1 does not recognize
daemon.* so it requires the following form), such as:
daemon.info /var/adm/daemon.info
daemon.debug /var/adm/daemon.debug
daemon.err /var/adm/daemon.err
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.
[Warning:] You want to make all logs root only readable for security
reasons If a user mistypes his password for his username, it could be
compromised if anyone can read the log files.
-------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------
5. How to Know if your Anonymous FTP Server is Secure
This section is intended for the administrator to go down a small check list of
things to make sure his server is not easily compromised.
1. Check to make sure your ftp server does not have SITE EXEC command by
telneting to port 21 and typing SITE EXEC. If your ftp daemon has SITE
EXEC make sure it is the most current version (ie, Wu-FTP 2.4). In older
versions this allows anyone to gain shell via port 21.
2. Check to make sure no one can log in and make files or directories in the
main directory. If anyone can log in as anonymous FTP and make files such
as .rhosts and .forward, instant access is granted to any intruder.
3. Check to make sure the main directory is NOT owned by ftp. If it is owned
by FTP, an intruder could SITE CHMOD 777 the main directory and then plant
files to give him instant access. SITE CHMOD command should be removed
because anonymous users do not need any extra priviledges.
4. Check to make sure NO files or directories are owned by ftp. If they are,
it is possible an intruder could replace them with his own trojan
versions.
5. There were several bugs in old daemons, so it is very important to make
sure you are running the most current ftp daemons.
-------------------------------------------------------------------------------
6. 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.univ-rennes1.fr (French Server)
archie.uqam.ca 132.208.250.10 (Canadian Server)
archie.wide.ad.jp 133.4.3.6 (Japanese Server)
-------------------------------------------------------------------------------
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)
Mirsad Todorovac (mirsad.todorovac@etf.hr)
Nicholas Ironmonger (ndi@sam.math.ethz.ch)
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, 1995
by Christopher Klaus of Internet Security Systems, Inc.
Permission is hereby granted to give away free copies electronically. You may
distribute, transfer, or spread this paper electronically. You may not pretend
that you wrote it. This copyright notice must be maintained in any copy made.
If you wish to reprint the whole or any part of this paper in any other medium
excluding electronic medium, please ask the author for permission.
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@iss.net> of Internet Security Systems, Inc.
<iss@iss.net>
Internet Security Systems, Inc.
Internet Security Systems, Inc, located in Atlanta, Ga., specializes in the
developement of security scanning software tools. Its flagship product,
Internet Scanner, is software that learns an organization's network and probes
every device on that network for security holes. It is the most comprehensive
"attack simulator" available, checking for over 100 security vulnerabilities.
--
Christopher William Klaus Voice: (404)441-2531. Fax: (404)441-2431
Internet Security Systems, Inc. Computer Security Consulting
2000 Miller Court West, Norcross, GA 30071