2018-04-28

How to force OpenSSH to log in with a specific password or public key

This blog post explains how to force the OpenSSH client to log in with a specific password or public key. This is useful if some of the SSH client config files (/etc/ssh/ssh_config, /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2, ~/.ssh/config, ~/.ssh/known_hosts) or the ssh-agent are in a broken state, and you want to try whether login works independently of these client-side issues.

Run this command to log in, substituting the "${...}" values:

SSH_AUTH_SOCK= /usr/bin/ssh -F /dev/null \
    -o UserKnownHostsFile=/dev/null -o GlobalKnownHostsFile=/dev/null \
    -o StrictHostKeyChecking=no \
    -p "${PORT}" -i "${KEYFILE}" -- "${USERNAME}"@"${HOST}"

Usage notes:

  • To use the default port (22), drop the -p "${PORT}".
  • To use password login instead of public key login, drop the -i "${KEYFILE}".
  • If you don't know where your public key file is, try -i ~/.ssh/id_rsa
  • To use the same username as your local client username, drop the "${USERNAME}"@.

How it works:

  • SSH_AUTH_SOCK= disables the ssh-agent for this connection.
  • Spelling out /usr/bin/ssh makes sure that shell aliases, shell functions and strange directories in $PATH have no effect on which SSH client is used.
  • -o UserKnownHostsFile=/dev/null -o GlobalKnownHostsFile=/dev/null makes existing host keys in known_hosts files to be ignored, thus the connection will be established even if old or incorrect host keys are saved there. Please note that this also makes it impossible to detect a man-in-the-middle attack, so attackers may be able to steal your password if you use a password to log in; also attackers can steal the contents of your session (commands and their results).
  • -o StrictHostKeyChecking=no suppresses the prompt to add the host key to the known_hosts files.

2018-04-24

A quest to find a fast enclosure for multiple SATA 3.5" hard drives

This blog post documents the quest I'm undertaking to find a fast enclosure for multiple SATA 3.5" hard drives, supporting both USB 3 and eSATA, and the ability to read from both hard drives at the same time with at least 275 MB/s total speed. So far I haven't found a fast enough enclosure, so the quest is till ongoing. I'll keep updating the blog post with speed benchmark results.

The maximum sequential read speed my drives support are 112 MB/s and 170 MB/s. (There are much faster drives on the market, e.g. Seagate IronWolf NAS 10 TB can read 250 MB/s in the first 1 TB of the disk.)

I've decided not to order the IcyBox IB-RD3662U3S, because my online research indicates it would be too slow. It uses the chipset JMicron JMB 352 (produced in 2014), which doesn't support UASP (thus it's slow and it uses too much CPU) and maximum SATA speed is 3 Gbit/s.

I've ordered the StarTech S3520BU33ER instead, which uses the chipset JMicron JMS 562 (also produced in 2014), which supports UASP and maximum SATA speed is 6 Gbit/s. I'll run the benchmarks after it arrives.

I've also found OWC 0GB Mercury Elite Pro Dual RAID USB 3.1 / eSATA Enclosure Kit, which is potentially even faster. It supports USB 3.1, eSATA, UASP, and claims to be very fast: more than 400 MB/s over both USB and eSATA. It also uses the same chipset: JMicron JMS 562. It's avaialable from amazon.com and from the manufacturer's webshop (with expensive international delivery).

Depending on the computer it can be much faster to connect the 2 hard drives within separate single-drive enclosures, using separate USB 3 ports or using an unpowered hub. I'm not pursuing this option right now, because I have other uses for my USB ports, and I want low CPU usage (eSATA uses less than USB 3).

For a home media server, it may be cheaper to buy a NAS, e.g. QNAP TS-251+ with Ethernet and HDMI ports, DLNA with full HD video transcoding and other media server features, with maximum transfer speed of 224 MB/s. (Other kinds of QNAP NASes don't seem the be any faster.) However, with a NAS I wouldn't get the flexibility and configurability of stock Debian operating system running on a stock amd64 CPU with 4 GiB of RAM on this machine.

2018-04-21

How to update the BIOS on a Lenovo T400 laptop

This blog post explains how to update the BIOS to version 3.24 (released on 2012-12-16, latest release as of 2018-04-21) on a Lenovo T400 laptop.

You will need a working and charged battery pack for the BIOS update, so install the battery pack first and start charging it.

If you are running Windows XP, Windows Vista or Windows 7 on the laptop, download the BIOS Update Utility from here (the choose 32-bit or the 64-bit version depending on your Windows type, or try both versions if you don't know), and run it, and you are done.

Otherwise, if you are able to burn a CD or DVD (either on the Lenovo T400 laptop or on another computer), and you have a working DVD reader in the Lenovo T400, then download the installer DVD .iso from here, burn it to a DVD, insert the DVD to the Lenovo T400, reboot the Lenovo T400, press the blue ThinkVantage button (near the top left corner of the keyboard), press F12 to select a boot device, select the DVD, boot from it.

Otherwise, if you have a USB pen drive of at least 34 MB in size whose contents can be overwritten, and you have a Linux system running (either on the Lenovo T400 laptop or on another computer), then connect the pen drive, figure out the device name using sudo fdisk -l (typically it will be /dev/sdb or /dev/sdc, but be extra careful, otherwise you will overwrite the contents of some other drive), run this command to download: wget https://download.lenovo.com/ibmdl/pub/pc/pccbbs/mobiles/7uuj49uc.iso; run this command to copy the bootable BIOS update utility to the pen drive: sudo dd if=7uuj49uc.iso of=/dev/sdB bs=49152 skip=1; sync (replacing /dev/sdB with the device of the pen drive). Insert the pen drive to one of the USB slots of the Lenovo T400, reboot the Lenovo T400, press the blue ThinkVantage button (near the top left corner of the keyboard), press F12 to select a boot device, select USB HDD, boot from it.

After booting into the BIOS update utility, follow the instructions to update the system software. (Don't reboot or turn off until asked.) The next reboot will take longer, the Lenovo logo will appear and disappear 3 times. After that you are done.

Now if you enter the BIOS setup at boot time (by pressing the blue ThinkVantage button), you will see version 3.24 (7UET94WW) 2012-10-17.

2018-04-09

How to change which characters are selected by double-clicking in xterm

Various terminal emulators on Linux (e.g. xterm, gnome-terminal, rxvt) have word selection: when you double-click a character, it selects the entire word containing the character. This blog post explains how to customize which characters are part of a word in xterm.

The various defaults are for ASCII characters (in addition to digits and the letters a-z and A-Z):

  • gnome-terminal: # % & + , - . / = ? @ \ _ ~
  • rxvt: ! # $ % + - . / : _
  • xterm default: _
  • xterm in Ubuntu: ! # % & + , - . / : = ? @ _ ~

It's possible to customize which characters are part of a word in xterm by specifying the charClass resource. The values :48 mean: consider these characters part of a word. Other numbers are character ranges, for example 43-47 mean the ASCII characters 43 (+), 44 (,), 45 (-), 46 (.) and 47 (/).

Here is how to trigger various default behaviors from the command-line:

  • gnome-terminal: xterm -xrm '*.VT100.charClass: 35:48,37:48,38:48,43-47:48,61:48,63-64:48,92:48,95:58,126:48'
  • rxvt: xterm -xrm '*.VT100.charClass: 33:48,35-37:48,43:48,45-47:48,58:48,95:58'
  • xterm default: xterm -xrm '*.VT100.charClass: 95:48'
  • xterm in Ubuntu: xterm -xrm '*.VT100.charClass: 33:48,35:48,37-38:48,43-47:48,58:48,61:48,63-64:48,95:48,126:48'

To save the setting permanently, add a line like this to your ~/.Xresources file (create it if it doesn't exist):

! Here is a pattern that is useful for double-clicking on a URL (default xterm in Ubuntu):
XTerm.VT100.charClass: 33:48,35:48,37-38:48,43-47:48,58:48,61:48,63-64:48,95:48,126:48

Make sure above that the line containing charClass doesn't start with !, because that would be a comment.

The change takes affect automatically the next time you log in. To make it take effect earlier (for all xterms you start), run: xrdb -merge ~/.Xresources