Lost in Technopolis

by John Wiegley

Script of the week: verify

Posted by John Wiegley on January 7, 2008 with labels:

This week’s script uses Leopard’s new xattr tool to store MD5 checksum information alongside any file you wish. Later, you can run the same script to ensure that the checksum has not changed.

For example, I run it on my Applications directory like this:

find /Applications -print0 | sudo xargs -0 verify

The first time it’s run, verify will print a lot of warnings about how none of the files currently have checksum information. But the second time you run it, it silently verifies that none of the files have changed. If there are changes, an error is printed to standard error, and an exit code of 1 is returned. It’s like a poor man’s Tripwire, but no external databases or configuration files are necessary, since it stores the checksum data in an extended attribute called “checksum”. This means that if you later move the file (even to another HFS+ volume), the verify script will still verify the current contents against the initial checksum.

If the file has legitimately changed, you must first delete the old checksum:

xattr -d checksum FILE

I recommend using this script on archival data, where changes are never expected. I created this script because I recently discovered some data corruption in a set of ISO files, and found to my dismay that ISO offers no built-in scheme to determine which files had been corrupted and which hadn’t, forcing me to throw out almost the entire lot. Now I use verify to ensure that from this point on, the ISO never changes.

On a security note: This scheme offers no security, unlike systems like Tripwire. A hacker who has permissions to change the file will also have permissions to reset the extended attribute containing the checksum.

Here’s the bash script:

#!/bin/bash

# verify, version 1.0
#   by John Wiegley 

if [[ -z "$1" ]]; then
    echo "usage: verify [OPTIONS] "
    echo "options:"
    echo "  -v     be verbose about verification/checksum setting"
    echo "  -s     only set checksums for files which don't have them"
    echo "  -f     force setting the checksum even if file doesn't match"
    exit 1
fi

ATTRNAME=checksum

verbose=false
setonly=false
force=false

while [[ "${1:0:1}" == "-" ]]; do
    if [[ "$1" == "-v" ]]; then
        verbose=true
        shift 1
    elif [[ "$1" == "-s" ]]; then
        setonly=true
        shift 1
    elif [[ "$1" == "-f" ]]; then
        force=true
        shift 1
    else
        break
    fi
done

error=false

for file in "$@"; do
    name=$(basename "$file")
    if [[ -f "$file" && "$name" != ".DS_Store" && \
          "$name" != ".localized" ]]; then
        CHKSUM=$(xattr -p $ATTRNAME "$file" 2> /dev/null)
        if [[ -z "$CHKSUM" ]]; then
            if [[ $verbose == true || $setonly == true ]]; then
                echo "Note: No existing checksum for $file, setting..."
            fi
            CURSUM=$(md5 -q "$file")
            xattr -w $ATTRNAME $CURSUM "$file"
            CHKSUM=$CURSUM
        elif [[ $setonly == false ]]; then
            CURSUM=$(md5 -q "$file")
            if [[ $CURSUM != $CHKSUM ]]; then
                echo "Error: Checksum mismatch for $file: $CURSUM != $CHKSUM" \
                    > /dev/stderr
                error=true
                if [[ $force == true ]]; then
                    xattr -w $ATTRNAME $CURSUM "$file"
                fi
            elif [[ $verbose == true ]]; then
                echo "Verified: $CHKSUM  $file"
            fi
        fi
    fi
done

[[ $error == true ]] && exit 1

exit 0

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