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Поиск:  Каталог документации | Безопасность, защита информации and frequently asked questions

VERY frequently asked questions which have straightforward answers
Archive-name: computer-security/most-common-qs
Posting-frequency: monthly
Last-modified: October 2002
Last-seriously-modified: January 2001

This is a faq file for and  It is
cross-posted to because I think it will also be useful there.

Please check whether your question is in this file before posting.
Also, unix-specific questions should be posted to, not; so if they're in here, there are now TWO reasons not to
post them to


Subject: Table of contents

- This faq

- Can anyone here tell me how to exploit the [whatever] bug?
  or Can anyone here tell me how to break in to my ISP?

- What do the "identd" lines in my syslog mean?  Is this a security
  exposure?  Can I turn off identd?

- I just noticed that [something].  Has my machine been compromised?

- What does port number [whatever] mean?

- Here's new, unbreakable encryption software.

- What should I read to learn how to secure my computers?  What should I read
  to learn about computer security?

- Is there a newer version of cops?

- Tripwire fails the self-test, dumps core when building the database, and
  dumps core when verifying.

- Cops won't "make" in some versions of linux (GNU).

- Various problems with building anything under Solaris, especially
  "/usr/ucb/cc:  language optional software package not installed".

- What's that weird URL with SATAN/SAINT?  I'm not running a web server!
  or SATAN says "Can't find my own hostname".

- SATAN doesn't display right in my web browser; it asks me to save the file.

- How do I find all setuid and setgid files?

- Tcp wrappers (tcpd) thinks all hosts are in Solaris 8 or in some
  versions of AIX.

- I can't get .rhosts/.shosts to work with ssh.
(Note: there is a newsgroup

- Should I block all ICMP at my firewall/router?

- How do I prevent my machine from announcing OS version, daemon version,
  etc in the banner message?

- How do I recover from forgetting my root password?  (Similarly: I messed up
  the root line in /etc/passwd and can't su or login as root; what do I do?)

- Is a portscan of a machine malicious/illegal/unfriendly?

- Can my ISP/employer monitor [various things I'm doing]?

- Why do some people get so upset when system penetration is called "hacking"?


Subject: This faq

This is not supposed to be a statement of group consensus.  This is simply
supposed to be a few VERY frequently asked questions and their answers, so
that we can snidely say "see the faq" when people ask them.  The answers
supplied are supposed to be completely uncontroversial amongst people who
know what they're talking about.  (My first answer might be a bit borderline
in this respect but I don't recall ever having seen a contrary opinion here.)
Except for the portscan question, in which I've attempted to present ALL of
the major views.

Contributions of questions are welcome (with or without answers); however,
the idea is that they are supposed to be things which have straightforward
answers and which we see very frequently (at least prior to their inclusion
in this document).  If your answer is long, it might not belong in this
document, at least as I see the purpose of this document.  For example, it is
intentional that this document doesn't contain firewall recommendations, even
though that's a frequently-asked question here.  (But see the firewall faq at

Thanks to Juan Gallego, Lamont Granquist, and Martin Ouwehand for additional
suggestions re finding setuid files on different versions of unix.  Thanks
to Dan Farmer for making me aware of cops 1.04+ (cf 1.04).  Thanks to Dan
Niles and Jyrki Havia for tripwire bug details as posted to the newsgroup.
Thanks to Scott Barman for a Windows NT security book reference.  Thanks to
Robert Graham for suggesting I cite his good firewall-seen.html file.
Thanks to Denis McKeon and Olaf Schreck for improvements to my bit about
editing the SATAN perl file (to avoid newbie errors).

Disclaimer: The posting of this file is not to be construed as a commitment
to provide free consulting to people I don't know.  Post your questions to
the newsgroup and I might answer them there, or someone else might do it
better.  (Although if you say "please send e-mail copies", I'm going to
ignore your message.)

Disclaimer 2: There ARE errors in this file, but at the time of writing, I
didn't know what they were.  (If I knew, I would have fixed them.)  This
document is offered on an "as-is" basis, no warranty is implied, blah blah blah.

The metafaqs say you should choose a random day of the month to post monthly
faqs on, so I just used random() and got the number 22 (I don't think it's
necessary for it to be a cryptographic random number).

Yes, I know that syntactically, these are not all "questions".


Subject: Can anyone here tell me how to exploit the [whatever] bug?
	or Can anyone here tell me how to break in to my ISP?

No.  We're security professionals.  We try to secure systems.  We think that
securing systems and fixing bugs are more intellectual activities than running
a program which someone else wrote which you don't understand.

You should only attempt "penetration testing" of a system with the consent
of its administrators and/or owners.  They will only be interested in your
services if you know something.  You can start your education by learning
some general computer science and computer programming, and by reading
computer security textbooks and/or newsgroups.


Subject: What do the "identd" lines in my syslog mean?  Is this a security
	exposure?  Can I turn off identd?

Discarding the timestamp and hostname, the lines look something like this:

    identd[10362]: from: ( ) for: 20546, 25
    identd[10362]: Successful lookup: 20546 , 25 : flaps.users

This states that the machine asked your machine who was
connecting from port 20546 on your machine to port 25 on
And your machine responded that the user was "flaps", and that flaps's group
is "users".  (10362 is the process id number of this particular invocation of
identd; for example, if two identd requests happened at about the same time
and the two lines were interleaved, it would help you sort them out.)

Theoretically, this is a security-sensitive data exposure, although the
practical effect of this is arguably nil.  And it can be very helpful to the
admin of a machine which often has more than a few simultaneous users.  When
one of your users does something untoward, this allows the remote machine to
log the username, and then the remote sysadmin's complaint to you will
contain information useful to you.  A linux machine at home connected to the
internet via ppp and with only one user should not be running identd because
it does not contribute to this process.  Very few things on the net REQUIRE
the sender to be running identd, because many machines don't have it and
because many people turn it off.

Your identd program probably has various options to configure what
information it discloses; see the man page.  You might want to run it with
options to minimize data OTHER than the above (-o and -e in the common
implementation), and/or perhaps run it with the option to report numeric uids
rather than lognames (-n), which is just as useful for tracking down
offenders from your point of view.  On the other hand, if you report numeric
uids, then in some cases the remote people will be able to gain logname<->uid
translation info (e.g. the outgoing connection is a mail message bearing
'from' information), so it's hard to say which discloses less data.

If you feel that this data is sensitive but still want to run identd, there
are some identd servers out there which report the data encrypted, so that
all the target sysadmins can do with the information they get is to send the
token back to you for your own use.  This facility might be available as -C.

You specify these options on the identd command-line, wherever it appears,
which is usually in /etc/inetd.conf.

The identd protocol is documented in RFC 1413.  It is the same as "auth".
The query specifies the port numbers only; the two IP addresses implied
are the sender and target of the identd query.  Thus you cannot query about
IP connections to other machines, although you can query about connections
which don't concern you but are to a machine you have an account on.

RFC 1413 states, "If you wouldn't run a 'finger' server due to privacy
considerations you may not want to run this protocol."  I agree with this but
suggest that it might not apply to a cryptographic identd (e.g. -C).


Subject: I just noticed that [something].  Has my machine been compromised?

Maybe.  You probably don't know whether it always was like this.  You should
look around your system enough of the time that you get used to how things
look BEFORE you get broken into.  And you should make a practice of following
up oddities you find, so that your judgement as to what is and is not weird
improves with experience.

If it's too late for that, before posting to* ask at least
one local expert in the OS you're running, or in the case of unix/linux/gnu,
one local unix expert.  There may be a straightforward, happy explanation
for the behaviour you observe.  Or there may not.  Not all anomalies are the
result of an intrusion; to some extent "My machine has been broken into!" has
replaced the "I have a virus!" default explanation of a few years ago.
On the other hand, machine breakins are very common these days, too.


Subject: What does port number [whatever] mean?

RFC 1700 is obsolete.  The standard current reference is

However, you can write a program which uses any port number, whether it has a
standard meaning or not; and similarly you can write a program which uses a
port number in a way contrary to its standard meaning.

If you notice an attempted connection to a weird port number on your machine,
the connection might have been meant for some other machine running an
idiosyncratic service (perhaps someone typoed the IP address or hostname),
it might be a probe for a widely-spread trojan horse program, it might be
part of some kind of portscan, or plenty of other possibilities.  Some notes
about what a particular port access might mean in practical terms (as opposed
to the intended purpose of that port number assignment) can be found at
And a list of some non-standard ports used by various strange programs is at

If you notice your machine listening on an unexpected port, you may have
been broken into, or it may be a "feature" of your OS distribution or some
third-party software you're running.  In unix, most ports your OS distribution
will use will be listed in /etc/services, along with MANY you don't use.
/etc/inetd.conf lists services whose daemons are started on demand by inetd,
the internet "super-server" (see the man page).  (/etc/inetd.conf entries
cause services to be offered; /etc/services entries basically just map names
to and from numbers.)  In different ways depending on OS version, /etc/rc*
specifies some standalone daemons to be started up on boot (or initlevel
change); see man pages (including man init).  These are conventional ways to
start services but any program can listen on a port (unprivileged processes
can only listen on port numbers >=1024 in most multiuser OSes).

Some port numbers are not fixed.  There are several possibilities here, but
in unix these most notably include port numbers bearing services registered
under the "portmapper", which listens on port 111.  Type "rpcinfo -p hostname"
for a list of services for which the portmapper is serving as a directory.
(Some of these port numbers may in fact be fixed, in which case client
programs have two different ways to find the port number (hardcode the port
number or use the portmapper).)

To see what listeners you have running (open ports), the canonical incantation
is "netstat -an".  But doing a portscan from a remote machine might be more
reliable if you suspect your machine has been compromised, because the netstat
program could have been replaced.  (But do keep in mind the tricky "malware"
technique of only accepting connections with certain *source* port numbers.)
To find out what process is doing the listening, try something like lsof.
Again, once your machine has been compromised, this might report the
wrong answer; the purpose of using lsof would be to investigate the normal
behaviour of your machine, not to check whether it's been compromised.


Subject: Here's new, unbreakable encryption software.

It's probably not substantially new, and I'm sure it's not unbreakable.
Read the snake oil FAQ at


Subject: What should I read to learn how to secure my computers?  What should
	I read to learn about computer security?

The number one thing to do is to install all of your vendor's security
patches and to disable unused services (in unix, comment things out of
/etc/inetd.conf, and remove daemon invocations from /etc/rc* (details
depend on OS version)).  See some other basic information in
Subscribe to the CERT advisory list and to your vendor's security alert list
to keep current in future.

If you're trying to learn your way around unix and internet security in
general, I suggest you want to start with a good grasp of unix basics, e.g.
from the Kernighan & Pike book.  You'll also want to be strong in C, which
education you can begin with the Kernighan & Ritchie book.  (Of course
there are alternatives to both.)

If you're feeling strong after that and want to go for the details, read
Farmer & Venema's "Improving the Security of Your Site by Breaking Into
it" at , and
the Cheswick & Bellovin firewalls book.  For a gentler approach covering
a broader range of security issues, read Spafford & Garfinkel's "Practical
Unix and Internet Security".  A more hands-on-oriented book about firewalls
is Chapman & Zwicky.

If you're interested in cryptography, the canonical book is Schneier's
"Applied Cryptography", and you might be interested in RFC 1750.

I've received a recommendation for "Windows NT Security" by Rutstein.

Some URLs with security notes for particular systems (in addition to those
above, and don't forget your vendor's security patch list):

Linux security:

Irix (out of date but contains notes which are still important):

Improve assorted file permissions for solaris 2.2 through 2.6, changing
the pkg database to match:

Solaris security:

Unix versus Windows NT:
	[ is now a domain squatter; does this page
	 have a new home, anyone?]

(Canonical URLs for additional platforms solicited!  Non-vendor URLs preferred.)


Subject: Is there a newer version of cops?

No.  Version 1.04+ is a bit old but performs some functions which are still as
useful as they ever were.  (And the message "/usr/lib/sendmail could have a
hole/bug!" is still right although the cert advisory quoted could be changed.)

(1.04+ contains an assortment of minor fixes and enhancements to 1.04,
and was released in 1993 by the original author (Dan Farmer).)


Subject: Tripwire fails the self-test, dumps core when building the database,
	and dumps core when verifying.

Fails the self-test (on fast machines):

You have to slow it down (just the self-test scripts, not the tripwire binary
itself).  The test scripts create and then update a file, and then fail to
detect that the timestamp has changed.  But this is ok, because the timestamp
has indeed not changed, because this all happens within a second on some
modern machines.  This occurs in a few places in the test scripts.  If a
second-boundary happens to be crossed during this brief interval, then that
particular test will succeed, but another one might fail soon.

In the tests directory, edit 3 of the 4 files named test.*.sh:
in, add "sleep 1" on line 46 (in the cert version), just before
running tripwire; in inter and update, un-comment-out the "sleep 1".
If this isn't good enough (obscure but can happen), use "sleep 2".  See

Dumps core when building the database (if you have 8-bit chars in filenames):

Tripwire 1.2 contains a bug relating to octal printing of 8-bit chars in file
names.  The bug occurs in filename_escape() in src/utils.c.  Double the size
of the "octal_array" to contain all 256 possible entries, and change
octal_array[(int)(*pcin)] to octal_array[*pcin & 255] farther down.
(This only works if you have eight-bit bytes, of course, but most of us do.)

Dumps core when verifying (this bug surfaces on some systems only):

In config.parse.c just before the end of configfile_read(), on line 356 in
the tripwire 1.2 distribution, there is a "rewind(fpout);".  It should be
conditional on "specified_configmode" as in the previous 'if' statement:
at this point the values "fpin" and "fpout" are the same (see line 184), so
it is actually rewinding the fp it might have closed in the previous line.
So simply add the word "else" before the "rewind".  (Perhaps change "fpout"
to "fpin" for clarity, although this won't affect its behaviour.)


Subject: Cops won't "make" in some versions of linux (GNU).

Remove the '#' from "BRAINDEADFLAGS" in the makefile.
(This adds a "-lcrypt" to the compilation of pass.c.)


Subject: Various problems with building anything under Solaris, especially
	"/usr/ucb/cc:  language optional software package not installed".

This is not a security question.  Please ask in a solaris newsgroup instead,
or ask someone near you because it's a detail, and easy to diagnose in
person but sometimes hard to diagnose over the net (depending on the problem).

If you get the message "language optional software package not installed",
this means that the compiler is not installed.  (Like, duh.)  Sun doesn't
include a C compiler with Solaris.  Get gcc.  Put it in your path and/or in
the makefile (e.g. CC=gcc).  Perhaps do "ln -s gcc /usr/local/bin/cc" so that
/usr/local/bin/cc points to /usr/local/bin/gcc, and make sure /usr/local/bin
is near the front of your path.  This is still not a security question.

If you are using the Sun compiler tools, or having problems with other missing
commands such as "make" or "ar", perhaps you need to add /usr/ccs/bin to
your path.  This is not a security question either.


Subject: What's that weird URL with SATAN/SAINT?  I'm not running a web server!
	or SATAN says "Can't find my own hostname".

SATAN acts as a web server so that it can use HTML conveniently.  The main
thing it gets out of HTML is its hypertext capabilities (you can click on

The web browser communicates with it using the HTTP protocol.  This allows
it to generate responses to queries dynamically, rather than having to
generate a huge number of static files (to be accessed via file://).  It
includes a cryptographic random number at the beginning of the URL so that
others can't contact your copy of SATAN and retrieve the information it's

If SATAN claims it "can't find my own hostname" or if the web browser can't
resolve your hostname in the URL, try adding your hostname to /etc/hosts.
You can list multiple hostnames for a given IP address in /etc/hosts; among
them should be the output from the "hostname" command and also your
fully-qualified domain name ("" rather than
"myname" or "myname.dept").


Subject: SATAN doesn't display right in my web browser; it asks me to save
	the file.

Newer web browsers seem to use different algorithms in guessing mime types
when the web server doesn't supply them.  Anyway, web servers are supposed to
supply the correct mime type and it's easy to fix SATAN to do so.

Add, in perl/, in process_html_request before it sends anything
(actually I see I put it just before the "Make sure they gave us the right
magic number"):

	# local bug fix: must send http response code and content type header
	print CLIENT "HTTP/1.0 200 Ok\nContent-Type: text/html\n\n";

There's some bad advice out there about adding a handler with the ".pl"
suffix in your netscape preferences.
1) This is wrong.  What's relevant about the satan response is that it is
indeed html code, not the fact that the requesting URL ends in .pl.  A web cgi
URL might end in .pl but the program might return a gif.  Unlike with e-mail,
mime types are an integral part of the http protocol.
2) This is dangerous (the version of the advice which says to set it to
invoke the perl interpreter).  You don't want to execute arbitrary perl code
off the net.  It also won't work, because the satan response is html code, not
a perl program.

The recommendation to deactivate an existing ".pl" handler is ok, but the
above is better imho; it fixes the real problem, and the fix won't go away
when you switch web browsers or use a different account.


Subject: How do I find all setuid and setgid files?

find / -local -type f \( -perm -4000 -o -perm -2000 \) -print

or to do an "ls -l" of them:

find / -local -type f \( -perm -4000 -o -perm -2000 \) -exec ls -ld '{}' \;

You may want to add the "-u" option to ls to see last-accessed times rather
than last-modified times (esp to help gauge how harmful it would be to
unsetuid the file).

Some versions of "find" don't have the "-local" option.  Its purpose is to
avoid searching nfs volumes.  If you don't have any nfs mounts, you can omit
the "-local".  If you do, here are some other possibilities:
    * On some systems you can do something like
	    find / -fstype nfs -prune -o -type f \( -perm -4000 ...
    * Some systems have "-xdev" or "-mount", which prevent find from
      traversing mounts.  But then you have to run it for each local
      filesystem separately.
    * Do the check with nfs filesystems unmounted (e.g. single-user mode).
    * As an alternative to find, "ncheck -s" will tell you all setuid and
      setgid files, plus all device files (which is something of equal
      interest, although usually much less problematic in OS distributions).
      It too must be run separately for each filesystem.

Please note that this is insufficient if you suspect backdoors have been
installed on your system.  The backdoor installation activity could have
included modifying the "find" command.  The purpose of the above is to find
locally-installed or vendor-supplied security bugs waiting to happen, not to
find backdoors.

Also note that on some systems, "-local" doesn't do what you'd think, because
it still traverses the entire remote filesystem, and rejects all nodes in it
as non-local.  In this case you want "! -local -prune -o", i.e. if not local
prune the search, else ... .


Subject: Tcp wrappers (tcpd) thinks all hosts are in Solaris 8 or in
	some versions of AIX.

This is because the line for that service in inetd.conf still says "tcp6".
The vendor-supplied application you are wrapping can handle IPv6 (aka
"IP-NG") connections, but the version of tcp wrappers you are using cannot.
Change "tcp6" to "tcp" on inetd.conf lines which you edit to invoke the
standard version of tcp wrappers.

For ftpd for AIX, I've heard that you then need to add the '-f' option to
the ftpd invocation.  (Confirmation requested.)

Alternatively, use a "IPv6-aware" version of tcp wrappers from


Subject: I can't get .rhosts/.shosts to work with ssh.

If ssh doesn't do what you want, the output of "ssh -v" may be helpful.

For .rhosts or .shosts (or hosts.equiv or shosts.equiv) to take effect with
ssh with the default configuration, a few somewhat unobvious things must be
the case.  These are all good restrictions and the rationale is included here.

    * The request must be coming in from a "privileged port"; thus, the ssh
      client must be setuid.  Without this restriction, any user could
      masquerade (for the purposes of passwordless login) as any other on the
      same source machine.  (Even with it, root can; but there's no way to
      restrict THAT without the user typing something or involving a third
      machine (i.e. some hardware which root doesn't have access to).)  Also,
      the ssh client must be able to read /etc/ssh_host_key (the private one)
      to be able to do the public key authentication thing to prove you're on
      the host whose IP address you're using.  N.B. that the 1.2.25 makefile
      sometimes turns off the setuid bit on ssh when doing a "make install"
      (it's a bug in the makefile, fixed in 1.2.26).

    * .rhosts or .shosts must be owned by the appropriate user and not be
      writable by group or others.  Sshd does not check for the situation of
      single-user groups common on some versions of unix these days (esp some
      versions of GNU/linux); you have to chmod g-w .rhosts/.shosts if your
      umask is 2.  (There is no way for sshd to detect the single-user group
      situation; current membership of size one doesn't tell you its history.)
      Similarly, your home directory should not be writable by group or others.

    * The source host must be in /etc/ssh_known_hosts or
      ~user/.ssh/known_hosts on the target machine.
      This is the difference between "RhostsRSAAuthentication" (allowed by
      default) and "RhostsAuthentication" (disallowed by default).  Without
      this, ssh is not gaining you any login security, although it is still
      gaining you anti-sniffing security.

When all else fails, try "ssh -v".
Take further questions to


Subject: Should I block all ICMP at my firewall/router?

No.  You need to allow the "can't fragment" message through or you will lose
connectivity to some number of sites with wacky packet sizes on their local
nets (notably token ring).  See

Less crucially but still somewhat important, if you block the "destination
unreachable" message then you'll get timeouts, after a long wait, in some
cases when you could have received immediate "no route to host" messages.

But blocking some of the rest might not be a bad idea, especially "redirect".


Subject: How do I prevent my machine from announcing OS version, daemon
	version, etc in the banner message?

In unix, find the daemon in question, possibly by finding its line
in /etc/inetd.conf, and read its man page.  For complex config files
(e.g. sendmail), search in the config file for the constant portions of the
string it's outputting (e.g. in find the string "Sendmail" with
a capital 'S').  For telnetd, some systems have "-h" to suppress the greeting
and other systems' banners come from a file called something like /etc/issue.
(Note that in redhat linux, you really want to modify /etc/rc.d/rc.local
rather than (or in addition to) /etc/issue*, because it regenerates
/etc/issue* upon boot.)  For Solaris 2.6 and greater, put "BANNER=" (without
the quotes) in /etc/default/telnetd and/or /etc/default/ftpd.  The telnetd
included with Solaris <2.6 and SunOS can't suppress the banner, but there's
no need to use that particular software; you could use GNU telnetd or wu-ftpd,
for example; or you might edit the binary, as the strings appear in it.

But this might not really be a security issue and it might not be worth
your effort.  Suppressing banners probably doesn't restrict any information
which is genuinely useful to an attacker.  If an attacker has some "exploit"
program for sendmail 1.2.3 only, then rather than checking the banner to see
if your machine is in fact running sendmail 1.2.3, they might as well just run
the exploit program, which is a direct check of whether you're vulnerable.
Whereas the banner suppression *will* interfere with some kinds of checking
of daemon versions which you yourself may want to do occasionally.


Subject: How do I recover from forgetting my root password?  (Similarly:
	I messed up the root line in /etc/passwd and can't su or login as
	root; what do I do?)

Basically, you want to boot from CD/floppy or in single-user mode.
Single-user mode in some versions of unix still prompts for the root
password, but can nevertheless be used to recover from messing up the root
line in /etc/passwd farther along, e.g. changing the shell to something
inappropriate.  And in some versions of unix it doesn't ask for the password.

To boot in single-user mode, in a prom monitor (e.g. L1-A on a Sun, or press
ESC while booting an SGI), you want a command like "single" or "boot -s"
or "b -s".  At the linux LILO prompt, you want something like "linux s".
If "linux s" gives you problems, "linux init=/bin/sh" might bypass the
normal boot sequence and just give you a shell, but you'll have to remount
the root filesystem (see below).

After single-user mode, it's cleaner to reboot rather than to press ^D to
do the multiuser boot, because the init "runlevel" mechanism is hacky.

It might be more rewarding to boot from OS installation media.  They usually
give you the opportunity to run a shell (e.g. in irix inst, type "sh"; in
redhat linux, press ctrl-alt-F2; in solaris, get a menu with the right button
in the background and select "command tool" in the "utilities" submenu).
In this case, do a "df" to find your root partition on something like /root
or /mnt (or, in solaris, /a).

Sometimes it's easier to make like a "cracker" and break in to it.
I imagine that most people who forget their root password have machines
which can easily be broken into...

Once you're in, you can edit the password file (or /etc/shadow as
appropriate), or you can change the password without supplying the old one
as root by typing "passwd root".  (Depending on how you got there, a plain
"passwd" might not know it's root's password you're trying to change.)

If you clear the password entry, be disconnected from the internet until
you've set a new root password (probably after a normal reboot).

If the above doesn't answer your question, please look for a faq specific
to your version of unix; if you end up posting here, please state precise
version of unix including version number (e.g. "irix 5.3", not just "5.3").

Problems editing the password file or running "passwd root" include:

/usr might not be mounted in single-user mode (and /bin might be a symlink to
/usr/bin, so most things might be on /usr).  You can probably just type "mount
/usr" or "/sbin/mount /usr".  Other filesystems might also be unavailable
but probably aren't needed just to change the password (and you're about
to reboot to get things back to normal after you change root's password).

The root filesystem might be mounted read-only, depending on how you
got there.  "mount / -o remount,rw" might fix this.


Subject: Is a portscan of a machine malicious/illegal/unfriendly?

This is included here because it's a recurring flamefest.  Please avoid
simply repeating the same old basic statements, because we've heard 'em all.

First of all, what a portscan is:  Basically (there are myriad variants),
it's an attempted connection to every single port number on a given machine
or range of machines.  Suppose you want to break in to a particular machine.
First thing you might do is to run a port-scanner to find out what all the
"open ports" are (ports with a listener).  Then you see, aha there's an
imapd, let's try the imapd exploit program.  Rather than just trying all
sorts of programs which wouldn't even connect let alone break in.

Portscanning your own machine is valuable; you may find listeners you didn't
know were there, and then you can close them off and/or check their security.

Since you have to secure each service on its own anyway, some people argue
that there's no need to defend against portscanning itself.  On the other
hand, you might configure your system to page you, or delete all your files,
or perform some such useful action when it detects a port-scanning in
progress.  Some defense systems cease accepting connections of any kind from
that IP address when they detect a portscan, and some sysadmins write to your
ISP and try to get you kicked off.  This leads to "stealth port-scanners"
which try to avoid triggering the alarms by various means.

Some people argue that there's not too much in the way of useful action you
can take automatically when you detect a port-scanning in progress, and
"counterattack" software is unwise and can be used via forgery to launch
attacks from your machine.

Now, the basic portscanning arguments.  (The discussion is only about machines
you don't admin, obviously; there is an additional, finer dispute about the
situation with machines you have some legitimate association with but not as
sysadmin, but I don't propose to address that intermediate situation here.)
I might be willing to add other statements to this list if they're similarly
terse, and certainly do let me know if you feel I've inadequately represented
a viewpoint, except that I reserve the right to apply a sense of humour.

Portscanning has been argued to be malicious/illegal/satanic because (see
rebuttals in subsequent section):
- a portscan is always/usually a prelude to or part of an attack, like testing
doorknobs to see if they're unlocked
- my pager beeps when I get portscanned, which takes my time unfairly (aka my
machine crashes, sends lots of e-mail, changes the root password to "soup")
- if everyone portscans a few machines for fun, in total there will be a
constant barrage of portscans to all/many/some machines, overloading them
- an attempt to commit a criminal offence is itself a criminal offence
- portscanning someone ELSE's machine is a completely different matter than
portscanning one's own
- a stealth portscan shows criminal intent even if you argue that a
traditional portscan does not
- any connection to a machine you're not explicitly authorized to use
constitutes the criminal offence of unauthorized access to a computer (i.e.
it's already a breakin)
- various lame analogies

Portscanning has been argued to be innocent/salutory/pure because (see
rebuttals in previous section):
- the "hands-on imperative": people should be curious, people should explore,
people should think
- a portscan uses a negligible amount of resources on the target machine
- if a portscan is a prelude to an attack, the ATTACK is what's wrong; the
portscan is not wrong, and is not usually a prelude to an attack anyway
- legally, "mere preparation" does not constitute an attempt
- having a listener on a port solicits connections; you can't complain that
someone makes the connection to the advertised port; port numbers are
"well-known" for a reason
- if a portscan crashes your system, it's crappy anyway; if your pager beeps
when you get portscanned, that's a stupid configuration
- if your machine is connected to the internet, it's your job as sysadmin to
deal with network activity and you shouldn't complain if you don't like it
- various lame analogies

NOTE!  The above two sections are a pair.  Don't cite a point from one
section in your favour without examining its rebuttal in the other section!
I am not attempting to resolve this issue here, just to decrease repetition.

Note re analogies:  Analogies are a good way to express a point of view but
usually the attempt to draw conclusions from them is essentially circular.
I recommend using an analogy to express views but not to argue.  Example of a
useful analogy:  "Looking through someone's protected files without their
permission is like looking through their desk drawers."  Very connotative;
conveys concepts of reasonable expectations of privacy despite organizational
ownership of the infrastructure; shows what the speaker thinks some of the
fundamental issues are; but note it's still not a proof of anything.  Example
of a useless analogy:  "Portscanning isn't like trying to turn the doorknob,
it's like looking at the doorknob while passing by on the street." Conveys no
information other than "I think portscanning is ok".  Neither is an argument,
but one of them gives a wealth of information as to the basic perspective of
the speaker and the other is useless.


Subject: Can my ISP/employer monitor [various things I'm doing]?

Do they have the technical ability?  Yes.  Your packets go through their
equipment.  Your packets are identified with your IP address, and they
contain the IP address of the destination; your ISP's routers need to know
the destination IP addresses to be able to route your packets.

The data you send (e.g. passwords, mail message contents, URLs) is all also
easily available, in the body of the packets.  If you use
(in its non-cryptographic mode), the URL you request is still just as
available in the outgoing packets.

If you use some form of encryption, e.g. ssh, they could still at least tell
the destination, even if the contents are unreadable.  In general, encryption
is the only way to render at least the contents unsnoopable, and only then if
the encryption and decryption are both done on machines which the overseers
DON'T control, and plain text not transmitted on any networks on which the
overseers have machines or are able to attach machines.  If you use their
computers (including if you run the encryption program on a unix machine
operated by them), then everything you do is available to them, theoretically.

But perhaps you were asking "*May* my ISP monitor X": is it allowed, is it

I think most sysadmins would feel that once there is reasonable suspicion that
you are acting improperly (breaking into computers, violating the acceptable
use policy, etc), that it is ethical for the admins to take a closer look.

It's unlikely that it's *illegal* for them to look at your stuff or what
you're doing, although there are some exceptions.  Under certain court orders
or subpoenas, it may be illegal for them *not* to look at your files or what
you're doing.

Many believe inquiry not in a case of suspicion and not under a court order is
unethical.  This is a potential topic for discussion in and, but posters should refrain from the argument that "they
paid for it, they can do what they like with it" (which is sometimes advanced
in the case of employers or educational institutions).  This is surely false
in general and thus not the basis for a convincing argument.  For example,
if they do something criminal with their equipment, it's a criminal act,
they can be charged criminally, it doesn't matter that they paid for it.
Similarly, if they do something unethical, it's unethical even though they
paid for it.  The whole concept of professional ethics is based on the
idea that ethics transcends legality: the idea that an action can be legal
but unethical.  If "they paid for it" means that all legal acts are ethical,
then you've pretty much defined away the whole idea of professional ethics.
*That* is probably best not attempted on


Subject: Why do some people get so upset when system penetration is called

The word "hacker" has a long and honourable tradition of referring to a
certain category of skilled computer programmer.  For example, see Eric
Raymond's "How To Become A Hacker" at

Some people who break into computers definitely *are* hackers: they discover
interesting security flaws through enthusiastic exploration of technical
information and artifacts, combined with skilled computer programming.
Many people here would quite reasonably call them hackers, while lamenting
their choice of focus.

However, most people who compromise computer systems are not "hackers" in
this sense.  Numerically speaking, these days most people who break into
computers use canned "exploit" programs or otherwise follow procedures
formulated by others.  So many people, on this newsgroup and elsewhere,
try to observe a distinction between the terms "cracking" and "hacking".
"Hacking" is not typically destructive, and its basic outlook is responsible
for the creation of a lot of the computer software we all use, whereas
"cracking" involves breaking or compromising something.  Also see

Other people say they're just words, and for better or for worse, the
media has conflated them in almost all people's minds, so let's give up.
I personally disagree with that view; and in particular I think that for
people who stray across the borderline between acceptable and unacceptable
system "exploration", it is helpful and can turn them into productive
citizens and maybe even keep them out of jail to discuss the difference
between hacking and cracking.

All these are the reasons that some people get upset when system penetration
is called "hacking".  And some people don't.

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