Mathstat Frequently Asked Questions


This posting contains frequently asked questions and answers about the Mathstat UNIX machines. All input is welcome, I can be reached at sysadmin @ mathstat.yorku.ca

  1. General Questions

  2. Account Administration

  3. Memory and Processes

  4. Email

  5. Shells, Commands, and Man Pages

  6. Software

  7. Windows 9x/NT Questions

  8. DNS Questions

  9. Modem Questions

  10. X Window Questions

  11. Choosing good passwords

  12. BACKSPACE/DELETE problem

General Questions

What is the Mathstat system?

The Mathstat system is comprised of the departmentally operated computers. The main graduate/faculty machine is pascal.math.yorku.ca, which is also known as loginsrv.math.yorku.ca, mathstat.yorku.ca and mailhub.math.yorku.ca. The main undergraduate machine is vector.gauss.yorku.ca. Other machines are operated by the department, including the web server (
www.math.yorku.ca), special-purpose machines for statistical computation (davinci.math.yorku.ca) and a Windows NT server.

As well as the Mathstat system, Math users have access to the services provided by CCS, such as e-mail and ACADLABS, and the services provided by ATSG (Novell YORKARTS tree).

What is UNIX? Where can I find more information?

I have placed UNIX help pages on the system: UNIX HELP PAGES
Another excellent Beginner's Guide is available here: Beginner's Guide to UNIX

Also, visit the Steacie Science Library, and search the catalogue under UNIX to find many excellent references. The time invested will be paid back many times.

How can I keep informed? (York Listserv)

If you want to subscribe to mailing lists, send a message to listserv @ yorku.ca with the message Info for further information about mailing lists at York. Note that subscriptions have to be approved by the list owner.

There are also departmental mailing lists, some of which you may already have been subscribed to (either automatically or manually). likely have been subscribed automatically. The manager of these lists is Susan Rainey, . More information can be obtained automatically by sending e-mail to with the body of the message being "info". A body of "lists" will send a "list-of-lists."

What does the quota information I see when I login mean?

It means that you approaching your maximum quota: by default, the information is not displayed otherwise.
Blocks in use:
The current number of 1K-byte file system blocks used by this user or group.

Inodes in use:
The current number of files used by this user or group.

Block soft limit:
The number of 1K-byte blocks the user or group will be allowed to use during normal operations.

Block hard limit:
The total amount of 1K-byte blocks the user or group will be allowed to use, including temporary storage during a quota grace period.

Inode soft limit:
The number of files the user or group will be allowed to create during normal operations.

Inode hard limit:
The total number of files the user or group will be allowed to create, including temporary files created during a quota grace period.
NOTE: A hard limit with a value of "1" indicates that no allocations are permitted. A soft limit with a value of 1, in conjunction with a hard limit with a value of 0, indicates that allocations are permitted only on a temporary basis. You may view your current quota usage and restrictions by using quota -v

How should the printer be used?

The general command for printing in UNIX is lpr - the pn604 command pre-processes the documents to make sure that they are of the right type, and then pipes them through to lpr. You may also print using lpr -P[printer-name] [filename.ps] where [printer-name] is the name of a print queue (such as "n604") and [filename.ps] is a PostScript file. To print plain-text files, enscript must be used, as in enscript -d[printer-name] [filename.txt] - note that -d is used instead of -P, and that this command sends the jobs directly to the printer. Similarly, for TeX-generated dvi files, the command dvips -P[printer-name] [filename.dvi] exists. Make sure to use the right command, as an error can cause the waste of a lot of paper.

You can cancel print jobs using lprm [job-number] where the [job-number] is what is displayed as "Job" by prtstat.

* 2 Up means that you get two pages of a text file printed on each 8.5 x 11 sheet. By using the enscript command manually, it is also possible to print pages 4 Up, if desired.

Why doesn't my YorkCard work?

This painful subject has a FAQ of its own, but an overview is presented here as well...

Your properly enabled YorkCard has access to the S110 door (undergraduate and grads/faculty) and N604 (faculty and grads). Sadly the door card readers have been failing. If you swipe the card and the door doesn't open you can call security at x58000 and they will either come and open the door for you or ask you to go to security to check out a key. The YorkCard office is working on making the doors more reliable.

To have your card encoded to open the doors, you must *first* have it entered into the database at the YorkCard office in West Office Building if it has not been already. If the York Card office asks you for a letter of authorization, explain to them that you are only asking to have your card entered into the system, and that Math/ATSG will handle your access to specific labs.

Until this has been done, it is *pointless* to try to have your card added to the list of people allowed to access the lab. It will only waste our (and your!) time. If you are not sure whether your card has been encoded, please ask at the YorkCard office. Once your card has been encoded at the YorkCard office, try it again in the lab.

(This paragraph applies only to graduate students and faculty, including visiting faculty) Once your card has been encoded at West Office Building (York Card office), log in to pascal and type yorkcard at the prompt, and follow the instructions carefully. Your card should start working within the next few days if you followed all the steps correctly. The implication of this, of course, is that you must have a pascal account in order to be allowed to have swipe-card access to N604.

Where can I find out about York's computing policy?

Senate Computing Policies


Account Administration

How do I get an account?

Through the MAYA system, managed by CCS, which controls most of the accounts on campus. This is the control centre for most of your accounts at York, and the first place to turn to for an account in the case of grad students, full-time employees and undergraduates. Visiting faculty will generally not interface directly with the MAYA system, and will have received a letter with account information prior to their arrival or shortly after. If you are visiting faculty, and have not received such a letter, please visit North Ross 525 (N525), along with your host, or someone else who can confirm your status. Note: students cannot and will not obtain accounts this way, and must use MAYA. Public access MAYA terminals (as well as training sessions) are managed by CCS, or MAYA can be reached by telnet to maya.ccs.yorku.ca

CCS is located in the Steacie Science Building, along with the PC and Mac labs, the Helpdesk and MAYA/e-mail terminals. Visit the CCS Helpdesk if you have any difficulties using MAYA or other centrally managed services - the Math department does not have control over MAYA. Further details can be viewer on CCS' home page here

Requests for accounts to which you are not automatically entitled to via MAYA should be sent to raphael @ mathstat.yorku.ca for departmental machines, atsgtech @ yorku.ca for YORKARTS accounts (although these requests may have to be processed through Anna Cavaliere anna @ mathstat.yorku.ca) and to ccsaccts @ yorku.ca for any others. Requests for web space on the Math & Stats servers should go to webmaster @ mathstat.yorku.ca

How do I backup my files to another machine?

  1. Log into your account
  2. Tar (tape archive) your home directory:
    
              cd /users/students   (replace students by another path, as appropriate)
              tar -cvf /tmp/myfiles.tar yku00271
    
  3. FTP the tar file to another server:
    
              ftp your.new.account.somewhere.else.ca
              (login)
              bin (binary file transfer)
              lcd /tmp  (change to temporary directory)
              put myfiles.tar
              bye (quit)
    
  4. Extract the files on your other machine:
    
              tar -xvf myfiles.tar
    


Memory and Processes

How can I keep track of my processes?

It's not a bad idea (especially if you have lost a connection) to periodically look for old processes. You can do this with the command:

         ps aux | grep 
This will show you all the processes that belong to you. For example, if I give the command

         ps aux | grep georges
I get the following output:

      georges   9386  0.0  1.0  100  252  pts/0 R    03:40:00  0:00 ps aux 
      georges  13223  0.0  1.0  224  596  pts/0 S    03:39:59  0:00 sh -c sh 
      georges  25840  0.0  2.0  364  740  pts/0 S    03:10:45  0:00 -tcsh 
      georges  40563  0.3  1.0  272  516  pts/0 S    03:29:06  0:02 vi /usr/tmp/.ed0445451 
      georges  40872  0.0  1.0   92  280  pts/0 S    03:40:00  0:00 sh 
      georges  44457  0.0  0.0   88  140  pts/0 S    03:40:00  0:00 grep georges 
      georges  44545  0.5  6.0 1000 2516  pts/0 S    03:10:58  0:08 /usr/local/bin/zmail j 
All of these processes are good healthy ones and I wouldn't want to euthanize any of them but let's say I wanted to kill the first one. I would use the command:

           kill -9 9386
(of course this process is already dead because it existed only while "ps aux" was actually executing). If not closed properly, Netscape on Unix has a particularly nasty tendency not to die properly, and to remain in the background, consuming a large percentage of available CPU power. This is detrimental to the system as a whole. Should you notice that some of your Netscape processes have gone haywire, please kill them off (with kill -9).

How does the quota system work?

The quota system will never wipe out any data you have saved into your account. When you have surpassed your quota useage the system will prevent you from saving more data. This could be a problem if you were in the middle of composing a document. However, you can always save a file into /tmp, quit the application, compress some unused files (using gzip) then move the /tmp file back into your home directory.
      Disk quotas for user jsmith (uid 12847):
      Filesystem  blocks   quota   limit   grace   files   quota   limit grace
           /home    1800    3500    4000             127       0       0
The quota system quotes you your quota in 512 K blocks (column 3) Column 4 (limit) is the absolute limit the system will allow you to use. That means that your quota is:
 
      3500 * 512 = 1792000
      1792000 / 1024 = 1750  (1.75 Meg.)
             (Kilobyes)


Email

How do I check my email?

One of the most popular product is Pine (UNIX). It is available in Phoenix and other central servers. To access Pine, simply telnet to mailsrv.yorku.ca and log in. Push ENTER when you see TERM = (vt100). When the prompt sign (%) appears, enter "pine" and the Pine Main Menu will appear.

How do I configure my IMAP client to work with MATHSTAT email?

Visit the MATHSTAT Netscape Messenger FAQ for Messenger-specific information.

For other IMAP clients, a brief summary: the incoming mail server is mailhub.math.yorku.ca, the outgoing mail server is mailhub.math.yorku.ca (if you're on campus or dialing in through York, otherwise, use your ISP's outgoing mail server). Your mail folders are in ~/Mail/ and your e-mail address is username@mathstat.yorku.ca. Should this not be enough information, contact raphael @ mathstat.yorku.ca for further assistance.

I strongly advise against using POP to retrieve your e-mail, and am even considering whether we should discontinue the service, on account of the many accidents (mail downloaded to a local machine, then "lost") which have resulted from the use of POP.

If you are using the central e-mail servers, visit the CCS Help Page for more information.

Vacation

setvac is the answer to setting a vacation message (on pascal) easily. Simply telnet in to pascal.math.yorku.ca, log in, and type "setvac" at the prompt. The rest should be self-explanatory.

Forwarding e-mail

Carefully! BE CAREFUL NOT TO CREATE MAIL LOOPS, BY FORWARDING ACCOUNT "A" TO ACCOUNT "B" AND VICE-VERSA!

Log in to the account that you want to forward the mail from, via telnet or ssh. Use a text editor (such as "pico" or "vi" to create a file called .forward (note the ".") in your home directory (the default location where you will be when you log in), containing the e-mail address of the account that you want to forward to. Before doing this, double-check that the account that you are forwarding to doesn't have a .forward file as well, as you may create a mail loop. This is a guaranteed way to greatly annoy at least two systems administrators, and I highly advise you not to attempts this procedure unless you understand what you are doing. It isn't complicated, but it is important to get it right.


Shells, Commands, and Man Page

How do UNIX file permissions work?

Unix file permissions are based on three attributes: read, write and execute. All of these attributes can be set for the user, group and others. Permissions can be set both on files, and on directories. Thus, if you make a file "-rw-rw-r--" you are allowing both yourself and members of your group read and write access to it, and all others read-only access. Be ware of granting others write access to your files or directories - write access also means delete access, and the ability to look through your files. Unless you have a specific reason, it is recommended that your files be left for your use only. The obvious exception is files on the web server, which should be set with world readeable permissions, and group (www) writeable permissions, like so: "-rw-rw-r--". Permissions can be changed using the chmod command (man chmod for more information). In a nutshell, chmod ug=rw,o=r [filename] will give your web files proper permissions. Note that if they were not created (but moved to) the web directory, they will not automatically be a part of group www, and giving write permissions to members of your default group is probably a bad idea. You can either work around this problem by creating the files in your web directory, copying the files into your web directory and then deleting the originals (rather than moving the files as such), or (and this is not recommended, for reasons of system consistency), by leaving your files as-is, but making them "-rw-r--r--" instead (chmod u=rw,go=r [filename]).

I downloaded a file with a .tar.Z. suffix. What do I do with it?

.Z is a UNIX compression format
.tar is a UNIX tape archive format
.tar.Z is a compressed tape archive format

To extract the data, you will need to:


Software

How do I run SAS from home?

You can run sas on the Gauss server (vector.gauss.yorku.ca - undergraduates) or on Pascal (pascal.math.yorku.ca - grads and faculty) by using a telnet application (such as NCSA-Telnet or Ewan) to connect to the server. Then you would run either:

sas .sas ( = your sas file)

or for interasctive sas:

sas -fsdevice ascii

How do I make a bookmar in Lynx?

As far as I know the easiest way to create a bookmark in lynx is to "go" to the page you want to add and then add the bookmark. Once you are in lynx you can go directly to a URL by using the command

          g

followed by the URL, in this case

         http://www.math.yorku.ca

Once you are in the desired page, you add a bookmark with the command


           a

(for "add", I guess). You then get a prompt that says:


           Save D)ocument or L)ink to bookmark file or C)ancel? (d,l,c): 

and you respond with a


           d

To view -- and use -- the bookmarks you type


           v

The file of bookmarks is shown to you as a regular html file. This means that you can edit directly with the command


           e

which invokes your default editor on the bookmark file.

Secure Shell (SSH)

A copy of Clarkson University's Windows SSH client has been placed in matrix.gauss.yorku.ca:/pub/windows/EZttermssh.exe (see also the file in the same directory EZttermssh.txt for installation instructions... the base TeraTerm is also available in the same directory as ttssh151.exe). Installing this on your Windows PC will often allow you to log in from some connections where you normally get "Access Denied" messages, as SSH does not pass information through unencrypted, meaning that it is safe to use over insecure networks. (Generally speaking, "Access Denied" messages are output when the host that you are trying to connect from has some problems, such as a missing reverse DNS entry. In most cases, our SSH is set up so as to allow connections in some cases where telnet doesn't, as all communication is encrypted when using SSH - in other words, as long as you trust your own machine and the server, what goes on in between is not a concern... this, of course, is a simplification, but should help to explain the situation.)

As a general rule, you should install and use SSH whenever possible, even if you do not get "Access Denied" messages, to guard against your account being hacked. You may not care about your account, but the hacking of your account is often the first step towards the hacking of my system... :-) -- something that I'm sure we'd all like to avoid... Also, remember that your accounts often contain e-mail of a confidential or sensitive nature, as well as files that are confidential material.

A web-based Secure Shell client is available here.

How do I include a postrscript file into a Latex file?

NOTE:

  1. If your postscript file is not in the same directory as your latex file, you need to indicate the path of the postscript file.
  2. You can change the picture environment in order to make less or more space for your ps file.
  3. You can also change the size of your ps file by modifying hscale and vscale parameters.

     \documentstyle[12pt]{article}

     \begin{document}

     The text before the picture

     \begin{picture}(10,150)(0,0)
     \put(-300,-700){\special{psfile=pic1.ps hscale=120 vscale=120}}
     \end{picture}

     The text after the picture

     \end{document}

     OR: 

     \documentstyle[epsf]{article}

     \begin{document}


     \centerline{
     \epsffile{yourfile.ps}
     }


     % modify the parameters in %%BoundingBox in ps file
     % in order to change the size of picture

     \end{document}

     OR: (under LaTeX2e, at least) 

     \documentclass{article}

     \usepackage{graphicx}

     \begin{document}

     \includegraphics{file.eps}
     % Optionally, options in [] brackets, such as height= and width= can
     % be included. See the relevant documentation.

     \end{document}

Note that the \includegraphics directive can (and probably should) be included between \begin{figure} and \end{figure} statements. EPS graphics can be can created using xfig.


Windows 9x/NT Questions

SAMBA and Pascal directories onWindows

SAMBA is software that allows UNIX home directories to be mounted as Windows drives. It functions by making the UNIX server emulate an NT server. Samba mounts are available to most of our major fileservers, and some of the laboratory machines use it as a default login authentication / shared drive. If you're trying to set up your Windows machine to mount a directory via SAMBA, you must first install Microsoft NetBEUI (and would probably do well to install the Client for Microsoft Networks at the same time, even if you don't plan to use it). Right-click "Network Neighbourhood" and then choose properties. Ask to add a new Protocol (for NetBEUI) or Client (for Microsoft Networks).

You should then be able to find the sample file C:\WINDOWS\LMHOSTS.SAM - along the lines of the format suggested by this file, you can create a file c:\WINDOWS\LMHOSTS and enter an entry referring to the host that you are interested in... for pascal, this would be:

130.63.194.91 PASCAL #PRE

after which you would run the command "nbtstat -R". These two operations are one-time events. (If you are in the same subnet, it may not be necessary to create the LMHOSTS file, but it won't hurt, so you may as well do it, in case you later change subnets).

In order to be able to access Samba volumes, you also need to add a registry key.

For Windows 95/98: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\VNETSUP\EnablePlainTextPassword should be set to DWORD=1.

For Windows NT: HKEY_LOCAL_MACHINE\system\CurrentControlSet\Services\Rdr\Parameters\EnablePlainTextPassword should be set to DWORD=1.

If you haven't worked with the registry before, please ask someone who has: making a mistake here can render the Windows operating system unusable.

Alternatively, you can use anonymous FTP to matrix.gauss.yorku.ca:/pub/windows/samba , pick the appropriate .reg file, FTP it to your Windows desktop, and double-click on it in order to insert it into your registry.

At this point, you should reboot your computer, and make sure to log in to it with the same username that you use on pascal. If your computer doesn't ask you to log in, then specifically log out (of Windows) and log back in, specifying the user name which you use on pascal. When asked for a password, type in your pascal password.

This done, you will be able to mount your pascal home directory by referring to "\\pascal\username" where "username" is your user name on pascal.

Note that SAMBA access is blocked from outside our usual subnets. If you require a specific IP to be added, contact me. Likewise if you don't feel absolutely comfortable with this procedure.

It is also possible to obtain a Samba mount of the web server, but this will require the assistance of the system administrators.

Other Windows 9x/NT questions

Help setting up my PC with York's network, setting up Netscape to work with York's central mail servers, and other York-specific PC questions are best answered by visiting CCS' web site, where pre-prepared, step-by-step documents are available. Select "Online help" for rapid access to the information that you need.

CCS Help Services

Should you still have questions, visit the Math Sysadmins' office, NRoss525 or ATSG in NRoss125.


DNS Questions

Often, we are asked why the "159.226.253.131 -- access denied" message shows up on attempts to telnet in to pascal (and other York machines).

With very few exceptions, this is due to improperly configured DNS (Domain Name Service: the Internet's way of mapping names (pascal.math.yorku.ca to numbers [130.63.194.91] and vice-versa) at the originating site, not at pascal's end. Blocking incoming connections from improperly configured machines (i.e., lacking reverse DNS entries) is standard at York (and not just in the Math dep't). Most cases occur with lower-end startup ISPs (Internet Service Providers), and not with academic institutions (such as the CAS in the example below, below). It is quite typical to find that IP numbers assigned to dial-up customers, as well as DHCP assigned IPs (dynamically assigned IPs, on a LAN or cable-modem) have this problem. Do note that York NetOps and CCS *do* have IPs properly mapped, and that this problem shouldn't occur when using a legitimately connected computer within York or via CCS' dialup access.

The reason that York systems impose this restriction is to provide a measure of safety against getting hacked: (a) Improperly configured ISPs are more likely than average to be hacked (and to be corrupted in such a way that your password can be "sniffed" while you type it in) and (b) another large source of unregistered IPs is "grabbed" IPs on a local area network (i.e., IPs which have not been assigned by the local systems admin). Someone setting up a machine on a LAN which he intends to hack from will typically "grab" an IP, as this makes it very much harder to relate the trouble back to its source, and lowers the likelihood of being caught. Indeed, the vast majority of portscans coming our way are from such machines, which serves as a frequent reminder that this policy is very sound from a security point of view. As an example, if (say) ftpd were to be discovered to have a security hole (which has happened in the past), it is likely that hackers will try to exploit it--but fortunately, most such hackers come from machines without reverse DNS entries (since their IPs were not obtained legitimately), and are thus stopped by our TCP wrappers' restrictions, dramatically lowering the chance of our being hacked, and buying us additional time to upgrade the ftpd package (obviously, finding out about a remotely exploitable root compromise is a "drop everything and deal with it" situation).

Should you be faced with this problem, the "perfect" solution is to contact your *local* systems admins (institution) or technical support (ISP) and to ask that they fix their Reverse DNS properly. Feel free to point them to this page for more information as to what is wrong and how to fix it (do note that York systems administrators do not exist to solve the problems of third-party ISPs or to provide them with technical support: the recommended solution would be for you to use York's dialup service, or a *competent* third-party ISP).

As a temporary solution, if you are stuck, it *may* be possible to bypass the security for a limited number of incoming machines and for a limited time--this is certainly not ideal, and should not be viewed as a permanent solution, as it decreases our system's security.

A further alternative which we have provided is the possibility of using Secure Shell, which (because of the way we have set it up) is not as restrictive as to which machines it accepts connections from. We feel that this is reasonably safe, as the connection is encrypted and safe from the intermediaries (who could otherwise "sniff" your password). A Java-based SSH connection to pascal is also available via http://www.math.yorku.ca/telnet --contact Louis for a password.

Please remember: System security is everyone's responsibility... hacking *your* account is the first step to hacking *our* system-- while *you* may not care if your files are seen (how about deleted or modified?) by someone else, others do...

For further information (this is particularly directed at errant ISPs), I recommend reading the following RFCs:

ftp://ftp.isi.edu/in-notes/rfc1912.txt
ftp://ftp.isi.edu/in-notes/rfc1537.txt
ftp://ftp.isi.edu/in-notes/rfc1536.txt
ftp://ftp.isi/edu/in-notes/rfc1034.txt
ftp://ftp.isi/edu/in-notes-rfc1035.txt

and the book DNS and BIND by Paul Albitz & Cricket Liu, published by O'Reilly, which is the best volume which I have found on the subject, and well worth its $50 cost.

Below is the transcript of a typical case (names removed):

On Mon, 17 Jul 2000, XXX XXX wrote:

> Dear Louis and Rohinie:
>
> I am visiting the Chinese Academy of Sciences now. But I could not telnet or
> ftp my pascal account. I don't know what happened.
> The following error message was displayed.
>
> 159.226.253.131 -- access denied
>
> For information contact sysadmin @ mathstat.yorku.ca
> Connection closed by foreign host.
>
> Please help me with this as I will stay here for a month.
>
> Best regards,
>
> XXX XXX
>

From raphael @ mathstat,yorku.ca Thu Aug 3 16:52:51 2000
Date: Mon, 17 Jul 2000 07:07:17 -0400 (EDT)
From: Louis Raphael
To: XXX XXX
Cc: sysadmin@pascal.math.yorku.ca, xxxx@yorku.ca,
Rohinie Bisesar
Subject: Re: Hi, Louis and Rohinie.

The problem is that the Chinese Academy of Sciences hasn't set up their Domain Name Service properly.

What do I mean? Every machine on the network should have both a name and a numerical address. As an example, let us take the machine vector.gauss.yorku.ca... we can go from its name to its numerical address:

---
% nslookup vector.gauss.yorku.ca
Server: deadtech.ccs.yorku.ca
Address: 130.63.168.21

Name: vector.gauss.yorku.ca
Address: 130.63.68.243
---

and we find out that the numerical address is 130.63.68.243. Nice... but what if we wanted to go the other way around, for example, to log an incoming connection? Easy... since York has set up its Reverse DNS properly:

---
% nslookup 130.63.68.243
Server: deadtech.ccs.yorku.ca
Address: 130.63.168.21

Name: vector.gauss.yorku.ca
Address: 130.63.68.243
---

The reason that this works are fairly complicated, but the short version is that York's Network Operations has set things up properly, and address-to-name mapping is in effect, as required by Internet standards.

Now, what happens if I do:

---
% nslookup 159.226.253.131
*** deadtech.ccs.yorku.ca can't find 159.226.253.131:Non-existent host/domain
Server: deadtech.ccs.yorku.ca
Address: 130.63.168.21
---

Aha... so there's no Reverse DNS available: the connecting network doesn't want to tell us which machine is attempting to connect. There are usually two reasons for this: (a) the originating network didn't set things up properly (as in this case) or (b) the IP# has been "grabbed" by a hacker who is attempting to break in to the machine. (b) is the reason why the connection is automatically denied (on pascal & most York machines).

In other words, the problem isn't at this end, but should be fixed by CAS' equivalent of York's Network Operations. Likely as not, they won't care. The only thing which I can suggest is for you to telnet in to a CAS server that has proper Reverse DNS set up for its IP number, and then to connect from there.

As a side note, it appears that forward DNS (name-to-address mapping) has been set up properly by CAS:

---
% nslookup mcm01.mcm.ac.cn
Server: deadtech.ccs.yorku.ca
Address: 130.63.168.21

Name: mcm01.mcm.ac.cn
Address: 159.226.253.131
---

... but the only way I was able to get this result was by guessing that you were most likely trying to connect from the same machine as you sent e-mail from.

Since it appears to be a server at the other end (running SunOS) and not a workstation being dynamically assigned an IP (a typical source of IPs without proper Reverse DNS), I tried to specifically allow it to connect, regardless of its DNS status, in order to allow you to telnet back home. That being said, this may or may not work, and (in this case) the problem is not at this end, but with CAS' broken Reverse DNS, which is really what should be getting fixed.

Louis


Modem Questions

A few tidbits about modems (this may not work with WinModems)...

WinModems don't work with Linux, although there are attempts being made http://www.linmodems.org

Call-waiting can/will cause modem calls to disconnect. Prefix the number to be dialed by "1170," and you will avoid the problem. "1170" instructs Bell to disable call-waiting for the duration of the call. The comma inserts a half-second pause.

Stutter-dialtone prevents most modems from dialing, because they don't recognise it as dialtone. If your modem is Hayes SmartModem compatible, you can add "X0" (zero, not oh) to the init string (which should look something like "AT X0 ...") in order to make it ignore the absence of what it recognises as dialtone and dial anyways.


X Window Questions

X Windows programs can be displayed remotely (i.e., not on the machine which they are running on), which is one of the really nice things about X.

In order to know where to display themselves, X programs check for an environment variable DISPLAY. X windows emulator software and SSH set this automatically, but in many cases, it can be useful to set it manually. This can be done as follows:

csh and variants:
setenv DISPLAY yourmachine.yorku.ca:0.0

sh and variants:
DISPLAY=yourmachine.yorku.ca:0.0
export DISPLAY

yourmachine.yorku.ca should be set to the machine that you want to display to (not necessarily in yorku, either). The ":0.0" part is necessary to tell the application what screen to display to (it is possible to have more than one screen attached to the same machine, hence the need to specify it).

With some applications, such as xterm, it is possible to shorten this entire process to one line:

xterm -display yourmachine.yorku.ca:0.0 &

would usually work.

How to find your machine name... on Windows, "winipcfg" will show your current IP (you can use the IP instead of the name). Once logged in, you can use the "hostwhence" command (if it is installed: it is only installed on a few machines). If hostwhence is installed, you could do the following (assuming csh):

setenv DISPLAY `hostwhence`:0.0

to have it do everything automatically...

eXceed

eXceed and variants can be set up to REXEC into the server and to start an xterm that way. In cases where this cannot be done (from cable modems / Sympatico, for example), it is still possible to connect to the server, set the DISPLAY variable and to run X software normally... but you must start eXceed manually (the actual eXceed program, in Start/Programs/eXceed/eXceed).

Other X Servers

Under Unix/Linux (and some X emulation software), there are controls on which hosts are allowed to display to the local screen (unlike with eXceed). This is for obvious (and not so obvious) security reasons. If you actually *want* to allow displaying to your local screen on a Linux/Unix machine, you can use "xhost +machinename.yorku.ca" where machinename.yorku.ca is the name of the machine that you want to allow displaying *from*. "xhost +" will allow displaying from anywhere, and isn't a very safe command to run. "xhost -machinename.yorku.ca" can be used to reverse the action, as can "xhost -".

I can't log into X Window but I can log into telnet

In almost 100% cases, this is due to being over quota: the system can't create the .Xauthority file necessary for you to display graphical applications (including the window manager), and you thus end up logged out and looking at another login screen.

The solution is to telnet in to the server and remove files as appropriate, until you are back under quota (you can check this with "quota -v").

Users typically go over quota when an application crashes and leaves a "core" file in their home directory, filling it up. Just go ahead and delete such files: generally speaking, they're useless.

The other way in which it happens "suddenly" is due to the nature of the quota system, which has both a hard and a soft quota. The hard quota cannot be exceeded for any reasons, but the soft quota can be exceeded (up to the limit of the hard quota) for up to one week. After one week, no more files can be added, *including* the .Xauthority file, hence the "sudden" inability to log in.

Note that the quota system will never delete files, only prevent the creation of new ones.

Storing MP3s in one's home directory seems to be a fairly common cause of users going over quota...


BACKSPACE/DELETE problem

In Unix,

stty erase "^h"
and
stty erase "^?"

can be used to set whether the Backspace or Delete key will erase the previous character. Some keyboards/OSes have the key positions inverted, or the Backspace key sending a DEL.

If you are seeing "^?" when you try to erase, this is because you are sending DELs, and the server is expecting Backspaces (^H). If you are seeing "^H" when you try to erase, the reverse is happening. As a one-time solution, try using the DEL key or CTRL-H (as appropriate) to erase... then you can go back and "stty" the correct "erase" character.

This history of this problem goes back to DEC vt100 terminals, which put the DEL key where the Backspace normally is. Actually, it goes back even further, to the days of teletype (tty) terminals and punch cards / paper tape, which had both a Backspace ASCII code (to go back one position) and a DEL ASCII code (to overwrite the current character). With the coming of more modern terminals with CRT-type displays, the question of which to use as the erase character didn't get settled properly, and we are unfortunately still living with the results :-(.


Archive-name: mathstat-faq
Last-modified: 29 May 2001
Version: 2.0

Modifications by: Louis Raphael-Beliveau (original author: Ryan McRonald)
raphael @ yorku.ca