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.



