In message <20030819081510.GH17972@apb.cequrux.com>, Alan Barrett writes:
>*BSD disklabels have magic numbers that should be easy to find.  I am
>sure I've seen references to a program that searches a disk for labels,
>but I can't remember where.

It's been posted to this list before.

Since it's only 3K:

/*
 * Copyright (c) 1997 David Brownlee (abs&#64;mono.org). Licence at end of file.
 * Attempt to find ffs filesystems in the event of a lost disklabel.
 *
 * This program itself will not modify any data on a disk, but using
 * disklabel and fsck _can_ cause loss of data - you have been warned!
 *
 * Compile with 'cc -Wall -O -o ffs_find ffs_find.c'.
 *
 * To use, run 'ffs_find /dev/rXXNc' (eg rsd0c, rsd1c, or rwd0c etc), note the
 * possible filesystems, disklabel appropriately and try to fsck.
 * (May want to use /dev/rXXNd on i386).
 *
 * It reports possible filesystems by checking for two 'superblock like'
 * blocks fs.fs_sblkno blocks apart.
 *
 * Caveats:
 *	It can find false positives. (It should check more rigorously).
 *	It assumes 512 byte blocks.
 *	It assumes the first & second superblocks of a filesystem are intact,
 *	and at offsets of fs.fs_sblkno and fs.fs_sblkno*2 respectively.
 *	The 'trick' with sbplus is not particularly palatable, 
 *
 * Comments/suggestions welcome!
 */

#include <stdio.h>
#include <string.h>
#include <sys/param.h>
#include <ufs/ufs/dinode.h>
#include <ufs/ffs/fs.h>

#define BLOCKSIZE	512
#define	SB_SIZE	(((sizeof(struct fs)+BLOCKSIZE-1)/BLOCKSIZE)*BLOCKSIZE)

typedef struct
    {
    struct fs	data;
    char	pad[BLOCKSIZE-1];
    }fsplus;

int main(int argc,char **argv)
    {
    FILE	*fds;
    fsplus	sblock;
    int		blockno = 0;
    int size = 0;
    int last = 0;

    if( argc!=2 )
	{
	fputs("Usage: ffs_find special\n",stderr);
	exit(3);
	}
    if( (fds=fopen(argv[1],"r"))==0 )
	{
	perror(argv[1]);
	exit(3);
	}
    if( fread(&sblock.data,SB_SIZE,1,fds)==0 )
	{
	fputs("Unable to read start of partition.\n",stderr);
	exit(3);
	}
    last=0;
    do	{
	if( sblock.data.fs_magic == FS_MAGIC )
	    {
	    printf("%d\n",blockno);
	    fflush(stdout);
	    if( last && last==blockno-sblock.data.fs_sblkno )
		{
		size=sblock.data.fs_size*(sblock.data.fs_fsize/BLOCKSIZE);
		printf("Possible filesystem at offset %u, size %u (%uMB).\n",
					    last-sblock.data.fs_sblkno,size,
					    size/(1024*1024/BLOCKSIZE));
		/*
		 * Do not skip to end of possible filesystem, as may be
		 * false positive
		 */
		}
	    last=blockno;
	    }
	memmove(&sblock.data,(char *)&sblock.data+BLOCKSIZE,
						sizeof(sblock.data)-BLOCKSIZE);
	++blockno;
	}while(fread((char *)&sblock.data+SB_SIZE-BLOCKSIZE,BLOCKSIZE,1,fds));
    return(0);
    }

/*
 * Copyright (c) 1997 David Brownlee (abs&#64;mono.org).  All rights reserved.   
 * 
 * This program is free software.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */