To quickly log in to remote hosts without a password, use ssh-keygen
to generate an SSH key pair, and ssh-copy-id to install the key
pair on the remote system.
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Can't log in with ssh?  Or is ssh making you give a password where you
thought you wouldn't have to? Use ssh -v to see verbose error messages.
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Set the RSYNC_RSH variable to "ssh" to encrypt all your rsync traffic.
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If you want to search for a command you previously typed in the bash shell, 
type CTRL-R, then a few letters of what you want to search for.
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Need a copy of your old web site and you don't have a shell account
on the server any more?  Use wget with the --mirror option.
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You can't include web documents across domains with SSI, but with
an Apache ProxyPass directive you can to map part of one site
into another.
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If you don't want all the filesystems to undergo a regular fsck at
the same time, use tune2fs -c to set the maximum mount counts to
numbers with no common factors (such as 17, 19, 20, 21).
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You can keep /usr mounted read-only, except when installing and
upgrading software. Only read-write mounts count toward the mount count
for fsck, and your average time to boot decreases. And /usr won't get
fscked if the machine crashes and you have to reboot it.
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Is your mail server an open relay?  Use rlytest to check that it's not.
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Comment out anything you don't use or don't understand in /etc/inetd.conf.
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Need a secondary DNS server?  It is customary for Linux user group members
to give each other secondary DNS.  Ask at a user group meeting.
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If you can telnet to your computer and don't know how to turn telnet off,
unplug your computer now.
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IP Masquerading not working?  Before you rebuild the kernel, make sure you
have IP Forwarding turned on: cat /proc/sys/net/ipv4/ip_forward.  If
it's 0, do this to turn it on: echo 1 > /proc/sys/net/ipv4/ip_forward
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If someone's script is going haywire and making too many connections to
your system, just do a route add -host [hostname] to keep the offending
box from shutting yours down entirely.
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If you want to remove a file called "-rf" just type "rm -- -rf".  The
"--" tells rm that  everything after -- is a file name, not an option.
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Convert all those Control-Ms in Macintosh text files with:
tr '\r' '\n' < mac_text_file.txt
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To debug a CGI script, run it from the command line, or print information
to stdout and read it in the web server error log.