Redo log files are written by redo log writter process. The files are written in Oracle proprietary format and cannot be read directly. However there are simple command for reading the logfile. I had a situation where I wanted to read the blocks of online redo log files. I was knowing the commands to read datafile blocks by giving the file number and block numbers, but it took me lot of time to actually search the command for redo log files.
Anyway after speding some time, I come to know some of the ways we can dump the content of redo log files.

We basically dump the output of redo log files in a trace and then read the trace file to understand the content. Below are some of the useful command.

The following ways of dumping a redo log file are covered

1. To dump records based in DBA (Data Block Address)
2. To dump records based on RBA (Redo Block Address)
3. To dump records based on SCN
4. To dump records based on time
5. Dump the file header information
6. Dump an entire log file

1. To dump records based on DBA  (Data Block Address)

Connect to database using sysdba and execute the below command
ALTER SYSTEM DUMP LOGFILE ‘filename’  DBA MIN (fileno) (blockno) DBA MAX (fileno) (blockno);

Example:

ALTER SYSTEM DUMP LOGFILE ‘u01/oracle/V7323/dbs/arch1_76.dbf’ DBA MIN 5 . 31125 DBA MAX 5 . 31150;
This will cause all the changes to the specified range of data blocks to be dumped to the trace file.  In the example given, all redo records for file #5, blocks 31125 thru 31150 are dumped.

2. To dump records based on RBA (Redo Block Address)

This will dump all redo records for the range of redo addresses specified for the given sequence number and block number.

Syntax:
ALTER SYSTEM DUMP LOGFILE ‘filename’ RBA MIN seqno blockno RBA MAX seqno blockno;

Example:
ALTER SYSTEM DUMP LOGFILE ‘u01/oracle/V7323/dbs/arch1_76.dbf’ RBA MIN 2050 13255 RBA MAX 2255 15555;

3. To dump records based on SCN

Using this option will cause redo records owning changes within the SCN range
specified to be dumped to the trace file.

ALTER SYSTEM DUMP LOGFILE ‘filename’ SCN MIN minscn SCN MAX maxscn;

Example:
ALTER SYSTEM DUMP LOGFILE ‘u01/oracle/V7323/dbs/arch1_76.dbf’ SCN MIN 103243  SCN MAX 103294;

4. To dump records based on time

Using this option will cause redo records created within the time range specified to be dumped to the trace file.
ALTER SYSTEM DUMP LOGFILE ‘filename’ TIME MIN value TIME MAX value;

Example:
ALTER SYSTEM DUMP LOGFILE ‘u01/oracle/V7323/dbs/arch1_76.dbf’ TIME MIN 299425687 TIME MAX 299458800;

Please Note: the time value is given in REDO DUMP TIME

5. Dump the file header information

This will dump file header information for every online redo log file.

alter session set events ‘immediate trace name redohdr level 10’;

6. Dump an entire log file:

ALTER SYSTEM DUMP LOGFILE ‘filename’;

Please note: Fully qualify the filename, and include the single quotes.

Example:
ALTER SYSTEM DUMP LOGFILE ‘u01/oracle/V7323/dbs/arch1_76.dbf’;

References:
http://yumianfeilong.com/2007/04/02/how-to-dump-redo-log-file-information/