Опубликован: 06.08.2012 | Доступ: свободный | Студентов: 1189 / 25 | Оценка: 5.00 / 5.00 | Длительность: 53:41:00
Лекция 24:

Basic network access: clients

reget

Sooner or later, you will lose a connection in the middle of a transfer. According to Murphy's law, this will usually happen with a big file, and it will be shortly before the transfer is finished. You may be able to save the day with reget, which picks up the transfer where it left off. The scodeantics are the same as for get.

Unfortunately, not all versions of ftp have the reget command, and on many systems that do have the command, it doesn't work correctly. If you do decide to use it, you should first make a copy of the partially copied file, in case something goes wrong.

user

Normally, ftp attempts to log in using the user name of the user who started the ftp program. To make establishing connections easier, ftp checks for a file called .netrc when performing a login sequence. .netrc contains information on how to log in to specific systems. A typical .netrc might look like:

machine freebie  login grog  password foo
machine presto   login grog  password bar
machine bumble   login grog  password baz
machine wait     login grog  password zot
default          login ftp   password grog@example.org

Lines starting with the keyword machine specify login name (grog in this example) and password for each system. The last line is the important one: if the system is not mentioned by name, ftp attempts a login with user name ftp and password grog@example.org. Though this may be of use with systems you don't know, it causes a problem: if you want to connect to a machine without anonymous ftp, you will need to explicitly tell ftp not to attempt an auto-login. Do this with the -n option:

$ ftp -n ftp.remote.org

The .netrc file is a security risk: it contains all your passwords in readable form. Make sure it is secured so that only you can read or write it.

ftp is not overly clear about login failures. For example,

$ ftp ftp.tu-darmstadt.de
Connected to ftp.tu-darmstadt.de.
220 rs3.hrz.th-darmstadt.de FTP server (Version 4.1) ready.
331 Password required for grog.
530 Login incorrect.
Login failed.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp>

This error message is not very obvious: although you're not logged in, you still get the same prompt, and ftp produces enough verbiage that it's easy to oversee that the login attempt failed. To complete the login, use the user command:

ftp> user ftp
331 Guest login ok, send ident as password.
Password:                  password does not echo
230 Guest login ok, access restrictions apply.

sftp

sftp is yet another ssh-based program. It's designed to be as compatible as possible with ftp, so you use it in exactly the same manner. As with other ssh-related commands, you need to authenticate in an ssh-specific manner. In addition, it has an exec command, which allows you to run programs on the remote machine.

To use sftp, the remote machine must be able to run the sftp-server server. It is normally started from sshd. See page 454 for more details.

rsync

Frequently you want to keep identical copies of files on different machines. You can copy them, of course, but if there are only small changes in large files, this can be relatively inefficient. You can perform this task more efficiently with rsync, which is designed to keep identical copies of files on two different systems and to optimize network bandwidth while doing so. It's in the Ports Collection. Install in the normal manner:

# cd /usr/ports/net/rsync
# make install

By default, rsync uses ssh to perform the transfer, so you need to have ssh configured correctly. In particular, you should be using ssh-agent authentication.

You can use rsync like scp: the syntax is compatible up to a point. For example, you could copy a file from a remote system with:

$ rsync presto:/var/log/messages prestomessages

You don't need to install rsync just for that, of course: you can do exactly the same thing with scp. rsync has one advantage over scp, how ever, even in this case. The first time you copy the file, there's no difference. But files like /var/log/messages grow at the end, and the rest doesn't change. That's an ideal situation for rsync: it uses an algorithm that recognizes common parts of files (not necessarily at the beginning) and optimizes the transfer accordingly. The first time you run the program, you might see:

$ rsync -v /var/log/messages freebie:/var/tmp
messages
wrote 80342 bytes  read 36 bytes  53585.33 bytes/sec
total size is 80255  speedup is 1.00
$ rsync -v /var/log/messages freebie:/var/tmp
messages
wrote 535 bytes  read 726 bytes  840.67 bytes/sec
total size is 80255  speedup is 63.64

This example used the option -v to show details of what was transferred; otherwise you wouldn't see any output at all. The first time round, the entire file was copied, so there was no speed up. The second time, though, almost nothing needed to be copied, so the transfer was over 60 times as fast.

Copying directory hierarchies

rsync has a bewildering number of options for synchronizing directories. Consider the case where you maintain web pages locally, but your main web server is co-located somewhere else. After updating the local web pages, you can run a script to update the remote pages with commands like:

rsync -LHzav --exclude=RCS --exclude="*~" ~grog/public_html/* website:htdocs/grog
rsync -LHztpgov --exclude="*~" website:htdocs

The first rsync command synchronizes the local directory ~grog/public_html to the remote directory htdocs/grog on the system website. It includes all subdirectories with the exception of the RCS directories. The second command synchronizes the top level web directory only, and not the subdirectories, many of which shouldn't be maintained on the remote site. In each case, files ending in ~ are excluded (these are normally Emacs backup files), and in the second case the RCS subdirectories are also excluded. Let's look more carefully at all those options:

  • -L copies symbolic links (which the documentation refers to as "soft links") as separate files. If you don't include this option, symbolic links to files within the directory hierarchy will work, but links outside the hierarchy may be broken (depending on whether a file of that name exists on the destination system or not). In this example, a number of files are really located elsewhere, so it makes sense to copy them as files.
  • -H is pretty much the opposite of -L: by default, rsync doesn't check whether it has already copied a file, so if it finds another link to it, it will create a new file on the remote machine. -H tells it to keep track of links and simply create another link to any file it has already copied on the destination machine. This can only work if the two links have been copied by the same invocation of rsync.
  • The option -z tells rsync to compress data. This can significantly reduce traffic.
  • The option -a ("archive") is in fact a shorthand notation for a total of seven other options. We'll see some of them below. The others are:
    • -r: copy subdirectories recursively.
    • -l: create symbolic links where necessary. In this example, it's overruled by the -L option.
    • -D: copy device nodes (only for root).

    The other options are -p, -t, -g and -o. We don't want to copy subdirectories in the second example, so we state them explicitly. Together, they roughly correspond to the -p (preserve) option to some other copy programs.

  • The option -p tells rsync to set the permissions of the remote copy to be the same as those of the original file.
  • The option -t tells rsync to preserve the modification times of the original file on the remote copy.
  • The option -g tells rsync to set the group ownership of the remote copy to be the same as those of the original file.
  • The option -o tells rsync to set the ownership of the remote copy to be the same as those of the original file.
  • We've already seen the -v option: it gives information on what rsync is doing.

When copying directories with rsync, it's relatively easy to end up with the files in the wrong directory level: either they're in the parent directory, or in a subdirectory of the same name. Consider the following command to synchronize a mail folder to a laptop:

$ cd /home/grog
$ rsync -zHLav presto:/home/grog/Mail Mail

This would seem to duplicate the directory /home/grog/Mail on the remote system to a directory of the same name on the local system. In fact, it moves the contents of the host /home/grog/Mail to /home/grog/Mail/Mail on the local machine. To do what you expect, write:

$ rsync -zHLav presto:/home/grog/Mail .
Алексей Третьяков
Алексей Третьяков
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