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rdd-copy

NAME

rdd-copy - copy a file, even if read errors occur

SYNOPSIS

rdd-copy [OPTION] src [dst]

rdd-copy -C [CLIENT OPTION] src [host:]dst

rdd-copy -S [SERVER OPTION]

DESCRIPTION

Rdd-copy is a file and device copying utility that includes features that are useful in a forensic environment. In particular, rdd-copy can compute cryptographic hashes over the data it copies, is robust with respect to read errors, and can copy data across a network.

Rdd-copy is best understood as a program that consists of a reader stage and one or more processing stages. The reader stage reads input data in a robust way. It will retry failed reads. If a read error persists, the reader stage substitutes zero bytes for the input bytes that it fails to read. The resulting bytes are passed to all subsequent processing stages.

The processing stages are enabled through command-line options. The current stages are: checksumming (Adler32 and CRC32), hashing (MD5 and SHA1), file output, network output, and statistics.

Rdd-copy can be run in local mode, in client mode, and in server mode. The mode is indicated by the first command-line argument.

Copying data across a network requires two rdd-copy processes: a client process that reads the data from disk and transmits it across the network, and a server process that reads the data from the network and writes it to a file or device.

LOCAL MODE

In local mode, rdd-copy copies source file src to destination file dst, handling read errors according to the options. If dst is not specified, the data in src will be read and optionally hashed, but it will not be written. To write to standard output, specify - as dst.

Rdd-copy will optionally compute an MD5 or a SHA1 hash value over the input bytes and the zero bytes it substitutes for blocks it cannot read. These hash values should be interpreted with care (see below).

Rdd-copy does NOT guarantee that the bytes it reads are the same bytes that are stored on the input medium. It simply takes what read(2) returns. Any hash values (see options) are computed over the bytes that read(2) returns or, if read(2) fails, over zero-valued fill bytes.

Rdd-copy does NOT guarantee that the bytes that it reads into memory (or the zero-valued bytes that it substitutes when a read error occurs) will be written to the output file correctly. If you wish to verify the correspondence between what rdd-copy saw and what got written to disk, you will have to recompute the MD5 and/or SHA1 hash values over the output file and compare them with the hash values reported by rdd-copy. This is a useful verification step, but beware that even this step cannot guarantee perfect correspondence with the data stored on the source medium.

The best end-to-end test is probably to read back the output file and compare each output byte to the corresponding input byte, unless that input byte was part of a block for which rdd-copy reported a read error.

Rdd-copy does NOT recover from persisting write errors. Rdd-copy was designed to handle unfriendly source media only. If you get write errors, you should replace your target medium.

READ ERRORS

In local mode and in client mode, rdd-copy reads from disk. Rdd-copy assumes that the source disk may be faulty and tries to be robust with respect to disk-read errors. In server mode, rdd-copy reads from the network and makes no attempt to survive read errors. The explanation below applies only to read errors that occur in local mode and in client mode.

When a read error occurs, rdd-copy reduces the block size to the minimum block size (see --min-block-size) and resets the read pointer to the location at which it started the read that failed.

Next, rdd-copy tries to read a series of minimum-sized blocks (see --min-block-size). When such a read fails, it is retried a user-specified number of times (see --nretry). If the read failure persists, rdd-copy normally will skip a minimum-sized block of input data and will write a minimum-sized block of zero bytes to the destination file. These zero bytes are also passed to all other rdd-copy processing stages (checksumming, hashing, and statistics).

Any persistent read failure counts toward the maximum number of read errors that the user will tolerate (see --max-read-err). If this maximum is reached, rdd-copy will exit immediately. By default, however, an infinite number of read errors is allowed.

After a read failure, rdd-copy continues to use the minimum block size to read data until it has read block-size bytes of data without errors. (block-size is the user-specified block size, see --block-size.) Only then will rdd-copy increase its block size again, doubling the size at each successful read, until it reaches the default block size.

CLIENT MODE

In client mode, rdd-copy operates as in local mode, except that the data will not be copied to a file, but will be written to a TCP connection to an rdd-copy server process.

In client mode, a destination file, dst, on a destination host must be specified. If no host is specified, localhost will be used.

SERVER MODE

In server mode, rdd-copy accepts one TCP connection from an rdd-copy client. The server process must be started before the client process. In server mode, rdd-copy will read data from a TCP connection and write it to a target file. For now, the target file must always be specified by the client. The main reason for this decision is to keep open the option of having inetd(8) or xinetd(8) start an rdd-copy server process.

OUTPUT

Informative messages, error messages, and statistics are all written to stderr.

OPTIONS

-C, --client

Run rdd-copy in client mode. If you use this option, it must come first.

-S, --server

Run rdd-copy in server mode. If you use this option, it must come first.

-p, --port <portnum>

Modes: client, server.

Specifies the port number <portnum> at which the server listens for an incoming connection. The default port is 4832.

-?, --help

Modes: all.

Print a usage message that includes this list of options.

-V, --version

Modes: all.

Print version information and exit

-v, --verbose

Modes: all.

Be verbose.

-q, --quiet

Modes: all.

Do not pose interactive questions.

-l, --log-file <logfile>

Modes: all.

Log all messages except progress messages to <logfile>.

-f, --force

Modes: local, server.

Force existing files to be overwritten. The default behavior is to bail out when the output file already exists.

-b, --block-size <size>

Modes: local, client.

Specify the default block size; <size> must be a power of two. While no read errors occur, rdd-copy will read and write blocks of <size> bytes.

-m, --min-block-size <size>

Modes: local, client.

Specify the minimum read size; <size> must be a power of two. When a persistent read error occurs, at least this many bytes of data will be skipped and replaced with zero bytes in the destination file.

-n, --nretry <count>

Modes: local, client.

Retry failed reads up to <count> times. In many cases, using a large retry value makes little sense, because the operating system's device driver will not indicate a failed read until it has, itself, retried the read several times.

-o, --offset <size>

Modes: local, client.

Skip <size> bytes from the start of the input file before reading any data. The bytes that are skipped will not be included in any hash computation and will not be written to the output file.

-c, --count <size>

Modes: local, client.

Read at most <size> input bytes or read until end-of-file.

-z, --compress

Modes: client.

Compress network data.

-s, --split <size>

Modes: local, server.

If necessary, create multiple output files, none of which will be larger than <size> bytes. Each output file will have a name that consists of a sequence number followed by a dash and the name specified on the command line.

-r, --raw

Modes: local, client.

Access the device using the raw device. The data will not travel through the buffer cache.

-P, --progress <sec>

Modes: all.

Report progress (bytes read and percentage of data covered) every <sec> seconds.

-M, --max-read-err <count>

Modes: local, client.

Give up after <count> read errors.

--md5

Modes: all.

Compute an MD5 hash value over all data that was read without errors and over the zero-filled blocks that are used to replace bad blocks.

--sha, --sha1

Modes: all.

Compute a SHA1 hash value over all data that was read without errors and over the zero-filled blocks that are used to replace bad blocks.

--checksum, --adler32 <file>

Modes: all.

Compute an Adler32 checksum value over blocks of data produced by the reader stage. The last block to be checksummed may be smaller than the the block size that is used. All checksum values are written to <file>.

--checksum-block-size, --adler32-block-size <size>

Modes: all.

Compute Adler32 checksum values over data blocks with a size of <size> bytes. Only the last data block to be checksummed may be smaller than <size>. The default block size is 32 Kbyte.

--crc32 <file>

Modes: all.

Compute a CRC32 checksum value over blocks of data produced by the reader stage. The last block to be checksummed may be smaller than the the block size that is used. All checksum values are written to <file>.

--crc32-block-size <size>

Modes: all.

Compute CRC32 checksum values over data blocks with a size of <size> bytes. Only the last data block to be checksummed may be smaller than <size>. The default block size is 32 Kbyte.

-H, --histogram <file>

Modes: all.

Compute a histogram over each block of data produced by the reader stage. The histogramming block size can be set by the user (see --hist-block-size). For each block, write a single text line of statistics to <file>.

-h, --hist-block-size <size>

Modes: all.

Set the histogramming block size to <size> bytes. The default block size is 256 Kbyte.

--block-md5 <file>

Modes: all.

Compute the MD5 hash value over blocks of data produced by the reader stage. The last block to be hashed may be smaller than the block size. All MD5 values are written to text file <file>. Each line in this file contains a block number, followed by a space, followed by the hash value of the corresponding block.

--block-md5-size <size>

Modes: all.

Sets the block size of the block-wise MD5 computation. The default block size is 4 Kbyte.

A <size> argument may be followed by one of the following multiplicative suffixes: c 1, w 2, b 512, k 1024, M 1,048,576, and G 1,073,741,824.

EXAMPLES

rdd-copy --md5 /dev/hda1

Compute and print the MD5 hash value over /dev/hda1. On Linux, /dev/hda1 denotes the first partition of the primary master disk.

rdd-copy -b 16k -m 512 -l rdd-log.txt /dev/fd0 f.img

Create an image of a floppy disk (/dev/fd0). Copy 16 Kbyte at a time, but use blocks as small as a single sector (512 bytes) when read errors occur. Write all log messages to the file rdd-log.txt.

On the server: rdd-copy -S --sha1

On the client: rdd-copy -C --sha1 /dev/hdb snake:/images/disk.img

Copy the primary slave disk to host snake and store the data in file /images/disk.img. The client host computes a SHA1 hash over the data it reads from the disk; the server host computes a SHA1 hash over the data it receives from the network.

rdd-copy --count 512 /dev/hda mbr.img

Copy the master boot record (MBR) from the primary master disk to file mbr.img.

SEE ALSO

rdd-verify(1), raw(8)

NOTES

If you encounter read errors, do examine /var/log/messages (or the equivalent file on your operating system). It may contain useful device driver error messages.

On Linux (kernel 2.4 and lower) rdd-copy and other programs that read from a block device may yield an I/O error when they reach the end of the device, even if there's nothing wrong with the device. To the best of my knowledge, this is a Linux problem rather than an rdd-copy problem; the same problem occurs with GNU dd-copy and other programs. The problem is described in the following document: http://www.cftt.nist.gov/Notes_on_dd_and_Odd_Sized_Disks4.doc. The problem has apparently been solved in the Linux 2.6 kernel.

If you use rdd-copy to access a device, consider using the raw device (see raw(8)). This way, your data will not travel through the buffer cache.

BUGS

Server-side errors are not reported back to the client. Users must watch the server's output.

REPORTING BUGS

Report bugs to <rdd@holmes.nl>.

ACKNOWLEDGEMENTS

Many thanks to all who reported bugs and successes, and who suggested improvements. You know who you are.

COPYRIGHT

Copyright © 2002-2003 Netherlands Forensic Institute
This software comes with NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

HISTORY

Up to version 1.2-7a rdd-copy (then called rdd) used a different error recovery strategy. With the new strategy, users can no longer set the recovery threshold, so the --recovery-len option has been retired.