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• Opening firewall to incoming port 443

  - by jrdioko

  I recently set up the ufw firewall on a Linux machine so that outgoing connections are allowed, incoming connections are denied, and denied connections are logged. This seems to work fine for most cases, but I see many denied connections that are incoming on port 443 (many with IPs associated with Facebook). I can open that port to incoming connections, but first wanted to ask what these could be. Shouldn't HTTPS requests be initiated by me and be treated as outbound, not inbound connections? Is it typical to open incoming port 443 on consumer firewalls?

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• How do I allow users to execute commands via ssh without allocating a pseudo-terminal

  - by Dani El

  I need to allow users to run a limited set of commands. But not to allow them to create interactive sessions. Just like GitHub does. If you try to ssh without a command it greetings you and close the session. I can acquire this by using ForceCommand some-script But getting in some-script i then need to eval user's input. Perhaps any other NoTTY-like option in sshd_config? --- UPDATE --- i'm looking for a pure SSH / Bash solution, not Perl/Python/etc. hacks.

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• How do you synchronise huge sparse files (VM disk images) between machines?

  - by chrisdew

  Is there a command, such as rsync, which can synchronise huge, sparse, files from one linux server to another? It is very important that the destination file remains sparse. It may be longer (but not bigger) than the drive which contains it. Only changed blocks should be sent across the wire. I have tried rsync, but got no joy. groups.google.com/group/mailing.unix.rsync/browse_thread/thread/94f39271980513d3 If I write a programme to do this, am I just reinventing the wheel? http://www.finalcog.com/synchronise-block-devices Thanks, Chris.

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• How to login without entering username and password

  - by torbengb

  I'm a newbie running Ubuntu 9.10. I have two users (wife and me), and each user's screensaver is set to lock so that on wakeup, we get to choose which user's desktop to go to. However, Ubuntu requires a password, so this is pretty tedious. I'd like to switch users without entering any password. I know about this trick that works for the boot login, but it doesn't deal with multiple users. Is it possible to set empty passwords for users in Ubuntu, or skip the password in other ways? (I'm expecting real Linux users to suggest that passwordless users must not get any rights and there be an admin user with a strong password. Yes, you're right. But that's not what this question is about. Thanks.)

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• Why is "chmod -R 777 /" destructive?

  - by samwise

  This is a Canonical Question about File Permission and Why 777 is "destructive". I'm not asking how to fix this problem, as there are a ton of references of that already on Server Fault (reinstall OS). Why does it do anything destructive at all? If you've ever ran this command you pretty much immediately destroy your operating system. I'm not clear why removing restrictions has any impact on existing processes. For example, if I don't have read access to something and after a quick mistype in the terminal suddenly I now have access well... why does that cause Linux to break?

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• Issues with hard disk secure erase

  - by John Watson

  I want to completely wipe all the data and both OSes (Ubuntu and Windows 7) from my hard drive. I tried DBAN but it gives me an error and does not run. I am looking for an alternative. After reading some articles online, I came to know that, using a Linux live CD, it can be done using either of following commands. a) sudo dd if=/dev/zero of=/dev/sda b) sudo shred -vfz -n 1 /dev/sda My questions are 1) Which option (a or b) is more secure (wipes everything) and faster? 2) Does either of the options damage the hard drive or anything? I want to use hard drive again i.e. installing Windows and Ubuntu again.

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• cdrom drive doesn't work on laptop

  - by bozdoz

  Here's as best as I can describe it: When starting up, the boot order doesn't recognize the cdrom drive, but I can open and close the drive during this time. In Windows 7, I can't open the cdrom drive, and it doesn't recognize it in device manager, disk management, or my computer. In Ubuntu Linux, I can open the cdrom drive, but it still doesn't recognize the cds, and it won't mount. If I reformat everything, would my cdrom drive work again? Can I reinstall Windows without a cdrom drive? I've deleted the upper and lower filters as was suggested in Google searches. Took the disk drive out and checked that it was installed correctly (no reason it shouldn't have been). Still: nothing works.

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• What is the difference between yum, apt-get, rpm, ./configure && make install

  - by Saif Bechan

  I am new to Linux and am running CentOs. When I want to update or install certain software I came across three ways. Sometimes it's: yum install program rpm -i program.rpm wget program.tar.gz unpack ./configure make make install That last one is a real pain, esp when you come from windows where a program install is usually one click and then a nice guide. Now can someone please explain to me: Why are there so many different ways to do this? Which one do you recommend to use and why? Are there any other ways for installing programs?

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• When HDD becomes full, how to create a symbolic link to the data store on another disk?

  - by Brij Raj Singh

  I have a Linux Ubuntu machine which has an X GB hard disk. There is folder, say, /opt/software/data. The disk /dev/sda1 is almost full and I have attached another disk at /dev/sda2 which is mounted at /hdd2. Is it possible for me to link the folders /opt/software/data with /hdd2/software/data so, that every file get stored in the /hdd2/software/data but may be referred from the /opt/software/data? I can't do a reinstall of the software that creates this data, to change the default location of storage.

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• Pass parameters to a script securely

  - by codeholic

  What is the best way to pass parameters to a forked script securely? E. g. passing parameters through command line operands is not secure, since someone who has an account on the host can run ps and see them. Unnamed pipe is quite secure, as far as I understand, isn't it? I mean, passing parameters to STDIN of the forked process. What about passing parameters in environment vars? Is it secure? What about passing parameters by other means I didn't mention?

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• File/folder Write/Delete wise, is my server secure?

  - by acidzombie24

  I wanted to know if someone got access to my server by using a nonroot account, how much damage can he do? After i su someuser I used this command to find all files and folders that are writeable. find / -writable >> list.txt Here is the result. Its most /dev/something and /proc/something and these /var/lock /var/run/mysqld/mysqld.sock /var/tmp /var/lib/php5 Is my system secure? /var/tmp makes sense but i am unsure why this user has write access to those folders. Should i change them? stat /var/lib/php5 gives me 1733 which is odd. Why write access? why no read? is this some kind of weird use of a temp file?

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• How do I prevent 'net ads join' from doing DDNS update?

  - by genehack

  I'm using 'net ads join' to add Linux servers to an AD domain. The servers are multi-homed, with a public IP on eth1 and a non-routable private background network on eth0 (in the 172.20 space, used for netboots and installs and stuff -- no routing to the Internet on that network). When I 'net ads join', it appears that a DDNS entry is getting created for the 172.20 interface. How can I prevent this from happening? (FWIW, my powers at the AD level are very limited -- I can join servers and delete server records but that's about it...)

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• My computer gets hosted when using 3G

  - by WeaklyTyped

  I recently activated 3G(UMTS) connection on my cellular network. I use Nokia S60 phone as a modem while connecting from Scientific Linux 6. While I am able to access net pretty well, the problem is my computer gets hosted. By using ip-address in a web-browser, anyone can access my localhost (running apache), this never happens with my wireline (ADSL) connection. Furthermore, even the Shell prompt changes from: [mach2@localhost ~]$ to [[email protected] ~]$ where XX.XX.XX.XX is the IP of the 3G connection. The very thought of putting my machine 'in the wild' is disconcerting. How can I rectify the situation without disabling services like apache, sshd and others?

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• ssh freezes when trying to connect to some hosts

  - by NS Gopikrishnan

  When I try to ssh to particular machine/s in a list, The SSH command happens to be freezing. I tried out setting ssh timeout. But then also it's freezes even after the timeout. In verbose mode : OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003 debug1: Reading configuration data /etc/ssh/ssh_config debug1: Applying options for * debug1: Connecting to x358.x.server.com [10.x.x.x] port 22. debug1: fd 3 clearing O_NONBLOCK debug1: Connection established. debug1: identity file /export/home/sqlrpt/.ssh/identity type -1 debug1: identity file /export/home/sqlrpt/.ssh/id_rsa type -1 debug1: identity file /export/home/sqlrpt/.ssh/id_dsa type 2 At this point it freezes. A work around I thought was to create a child process for each ssh calls and if the process doesn't respond after a timeout - Kill it. But are there any less complex ways, so that I can accommodate it in a shell script itself rather than going for a C/C++ program ?

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• is it possible in this case to get two IPv6 networks to communicate over the IPv4 Internet?

  - by user239167

  I have a setup of a laptop connected to a cellular IPv4 network (3G usb dongle) and PC connected to another IPv4/IPv6 network. Both have been configured to run as IPv6 routers and each has its IPv6 network. The laptop connects to the Internet via the 3G network with IPv4 address (private one and it is mostly changing) on one interface and has the other interface on AP mode providing its own private IPv6 network (2001:db8:444::/64). The PC connects to the Internet via the organization network with both public IPv4 and IPv6 addresses on one interface and has the other interface on AP mode providing its own private IPv6 network (2001:db8:222::/64). Both running Linux Ubuntu. Is it possible to get the two IPv6 network (2001:db8:222::/64 and 2001:db8:444::/64) to communicate over the IPv4 Internet? Is openvpn of any good in this case to get IPv6 traffic over IPv4-IPv4 tunnel? Thanks for helping in advance

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• Grub hangs intermittently on "Starting Up..."

  - by Griffo

  Hi all, I've had this problem for a while now. My linux server is set to wake-on-lan but occasionally it halts at Grubs "Starting Up..." and goes no further. This is not due to additional hardware being attached such as a flash drive or anything as I never plug anything into it. It may boot perfectly 40 times in a row and then hit this issue. Sometimes it gets the issue a couple of times in quick succession and doesn't happen for ages again. I'm not sure how to diagnose it since it doesn't seem to be reproducible. Any help much appreciated. Thanks

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• Disable passwd history feature with remember=0

  - by user1915177

  PAM version - pam-0.79 Is setting 0 allowed on "remember" option in /etc/pam.d/common-passwd file of pam.d module to disable passwd history feature? With "remember=0" in /etc/pam.d/common-passwd file, I am observing a memfault when running the passwd command as a USER. When browsed the source, the function in _set_ctrl in support.c file of pam_unix module handles wrong values of remember, but currently its not robust enough to handle 0, which is a wrong value. So the valid and only option to disable history feature, is to not include the "remember" option in /etc/pam.d/common-passwd file and not to set-up /etc/security/opasswd file? Could see in the following link mention of setting "remember" to 0 has no effect to remember value in "/etc/security/opasswd" file. =https://lists.fedorahosted.org/pipermail/linux-pam-commits/2011-June/000060.html

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• Manually forcing TCP connection to retry

  - by Vi.

  I have a TCP connection (SSH session to some computer for example) Network suddenly goes down and drops all packets (disconnected cable, out of range). TCP resends packets again and again, retrying with increasing delays. I see the problem and plug the cable back (or restore network somehow). TCP connection finally successfully resends some packet and continues. The problem is that I need to wait for a some timeout on point 5. I want to use my opened SSH session now and not wait for 5-10 seconds until it finds out that connection is working again. How to force all TCP connections to resend data without delays in GNU/Linux?

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• Munin "Available entropy" when using adress space layout randomization

  - by clawspoon

  Having just configured munin for statistics logging on my gentoo server (hardened profile), I am noticing that my "Available entropy" is consitently in the 200-300 range. This seems way to low, so I checked it manually using the command $ cat /proc/sys/kernel/random/entropy_avail 3544 Odd. Consistently very low values in Munin and practically filled up when checking manually. After thinking about the problem for a while I came to the conclusion that the problem is probably that I'm using Adress Space Layout Randomization which is using the entropy when running commands/programs. Since Munin runs a whole slew of programs all the entropy is used up, and Munin then measures how much entropy there is, resulting in the low values. Does anyone have any experience with this? How can this be avoided?

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• Replacement for NIS/YP

  - by mdpc

  The company that I am working for is embarking on replacing the current locally developed NIS/YP structure with LDAP. We already have AD in house for the Windows stuff and would like to consider using an AD system. The AD people are quite restrictive and would not support extensive modifications. We have needs to have the replacement include the support the full capabilities of the NIS/YP suite include netgroups, login restrictions to specific servers for specific users or groups of users, consistent passwords between the *nix and Windows environment,etc. Our environment is a mixture of Linux (suse, RH, Debian), Sun, IBM, HP and MPRAS as well as a NETAPP. So whatever we use must be totally inclusive to all environment. We have looked at Likewise, but our management wants other alternatives to compare with. What other things should I be looking at and what is you assessment of the alternative? Thanks

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• Why does Exim puts emails on hold if there are frozen messages in the queue?

  - by user51932

  I've a CentOS with CPanel server working as a SMTP server, which currently uses 20 different hostnames and IP addresses to deliver email for an email newsletter service. However, it's extremely slow in sending emails. It's sending like 10 emails per minute, which I check by running the "exim -bpc" command. What could be affecting this? One thing I'm supposing, is that there are frozen messages in the queue, which are slowing down the sending until they're sent out, and are putting new messages on hold. What are the most common reasons a message can get frozen? Also, would it be more efficient to use 20 different small VPSs to send out email rather than use one large VPS with the 20 different hostnames and IPs in it?

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• using one disk as cache for others

  - by HugoRune

  Hi Given a PC with several hard drives: Is it possible to use one fast disk as a giant file cache? I.e. automatically copying frequently accessed data to that one disk, and transparently redirecting reads and writes to that disk, so that other drives would only have be accessed occassionally. (writes would have to be forwarded to the other disks after a while of course) Advantages: the other drives could be powered down most of the time; reducing power, heat, noise speed of the other drives would not matter much. cache disk could be solid state. How can I set such a system up? What OS supports these options? Is this possible at all using Windows or Linux?

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• how to setup a ssh acount with no terminal but port forwarding?

  - by admalledd

  I am trying to set up a new user account I can give to friends so they can SSH into my forward computer, and only allow forwarding of certain ports. I do not want my friends to have a shell, or be able to change what ports to where they are allowed to forward. example session: joe(friend) connects using PuTTY (that I have pre-set, he isn't good with computers) to example.com(my Internet facing computer) forwarding ports 8080,1990,25565 to him(with what ever end ports he wants, preferably they stay the same numbers) example ssh command to do similar (but he can still change the ports on my computer!) ssh -N [email protected] -p443 -L8080:192.168.1.2:8080 -L1990:127.0.0.1:1990 -L25565:127.0.0.1:25565 then, same story with other friend smith(same ports, same user even) except he is using linux, so cant use putty. is it possible to also leave default SSH functinality for all other users but this one? I found this when I was searching google, but alas, I did not quite understand what was being suggested, and I don't think they covered restricting port forwarding

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• Comparison of Unix shells

  - by Andy White

  Of the major Unix shells (bash, ksh, tcsh, zsh, others?), are there any compelling reasons to use one over another? Which is the most interactive/command-line friendly? Which is the most conducive/intuitive for writing scripts? Are there any major built-in features that one shell offers that others don't? Are any of these shells really good for one type of function, but not another? Or are they all pretty well-rounded/flexible? Is it just a matter of personal preference? I can make this community wiki if anyone prefers.

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• Which simple server virtualization solution to use?

  - by vvanscherpenseel

  For one part of our hosting platform we are currently using VMware Server 2 to create two virtual machines on one physical machine. One VM is used for hosting of small websites, the other VM is used as a staging environment. Both the host OS and guest OSes run CentOS Linux. Support for VMWare Server 2 has been discontinued and we are currently looking for a replacement. We only use basic functionality (we don't use snapshots, moving around VMs to different physical machines, or other 'advanced' functionality'). Just a box, with two VMs. We are looking for a virtualization solution that has long-term support, is stable and allows configuration/management from Mac OSx (I understood that Xen only has a Windows client). What would be the right solution for us?

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