(Serial) console flooded with kernel messages?

(If you want to ignore the explanations and see how to stop the Linux kernel from flooding the console with low importance messages, go straight to the bottom of the article, is the small bit at end with larger font.)

After connecting to the serial console on my Linksys WRT160NL router I was faced with the problem that the console was flooded with all sorts of messages such as:

DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.U DST=178.156.183.255 PROTO=UDP SPT=137 DPT=137 LEN=58
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
ACCEPT IN=br0 OUT=eth1 SRC=a.b.c.d DST=69.171.246.16 PROTO=TCP SPT=3651 DPT=443
DROP IN=eth1 OUT= SRC=X.Y.Z.U DST=178.156.183.255 PROTO=UDP SPT=137 DPT=137 LEN=58
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=178.156.183.146 DST=255.255.255.255 PROTO=UDP SPT=17500 DPT=17500 LEN=120
DROP IN=eth1 OUT= SRC=178.156.183.146 DST=178.156.183.255 PROTO=UDP SPT=17500 DPT=17500 LEN=120
DROP IN=eth1 OUT= SRC=178.156.183.146 DST=255.255.255.255 PROTO=UDP SPT=17500 DPT=17500 LEN=120
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=X.Y.Z.W DST=255.255.255.255 PROTO=UDP SPT=58488 DPT=2008 LEN=26
DROP IN=eth1 OUT= SRC=178.156.177.142 DST=255.255.255.255 PROTO=UDP SPT=17500 DPT=17500 LEN=153
DROP IN=eth1 OUT= SRC=178.156.177.142 DST=255.255.255.255 PROTO=UDP SPT=17500 DPT=17500 LEN=153

The serial console was working, but it was impossible to do anything practical in these conditions. I tried looking on the net for 'linux stop console flooding' and similar terms, but didn't get too far, except the fact the problem was the loglevel.

Here is the explanation of what this means (quote from Documentation/kernel-parameters.txt):

        loglevel=       All Kernel Messages with a loglevel smaller than the
                        console loglevel will be printed to the console. It can
                        also be changed with klogd or other programs. The
                        loglevels are defined as follows:

                        0 (KERN_EMERG)          system is unusable
                        1 (KERN_ALERT)          action must be taken immediately
                        2 (KERN_CRIT)           critical conditions
                        3 (KERN_ERR)            error conditions
                        4 (KERN_WARNING)        warning conditions
                        5 (KERN_NOTICE)         normal but significant condition
                        6 (KERN_INFO)           informational
                        7 (KERN_DEBUG)          debug-level messages

This was enough to go through my local git tree of the kernel in the Documentation directory and grep for loglevel. This brought me to this interesting bit from Documentation/sysctl/kernel.txt

==============================================================

printk:

The four values in printk denote: console_loglevel,
default_message_loglevel, minimum_console_loglevel and
default_console_loglevel respectively.

These values influence printk() behavior when printing or
logging error messages. See 'man 2 syslog' for more info on
the different loglevels.

- console_loglevel: messages with a higher priority than
  this will be printed to the console
- default_message_loglevel: messages without an explicit priority
  will be printed with this priority
- minimum_console_loglevel: minimum (highest) value to which
  console_loglevel can be set
- default_console_loglevel: default value for console_loglevel

==============================================================

So I ran 'cat /proc/sys/kernel/printk' and got (I managed to read it through the flood of messages from the firewall):

7       4       1       7

According to the explanations above, that meant that console_loglevel was too, so to fix it I ran:

echo '2 4 1 7' > /proc/sys/kernel/printk

And, behold, the serial console was usable.

(Rsnapshot) backup and security - I see problems

In my previous post I was asking for suggestions for backup solutions that would be open/free software, do backups over the network to a local HDD, be cross platform to allow Windows and Linux clients and not be too CPU/memory hungry (on the server).

Several people suggested rsnapshot, BackupPC, areca-backup, and rsync. Thank you for all your suggestions, you have been a tremendous help. I have decided to give rsnapshot a try since it was suggested to me that it would actually do what is supposed to do for Windows clients, too (which was initially my perceived show stopper for rsnapshot).

Still, when getting to the implementation, I was a little disappointed by the very permissive access that needs to be provided on the client machines, since the backup is initiated from the backup server. Even the so called more secure suggested solutions seem way too permissive for my taste, since losing the control over the backup system means basically giving total access to the data from all client machines, which is quite a big problem in my opinion.

The data-transfer mechanism employed by rsnapshot is simply

1. S ==(connects and reads all data)==> C
2. S stores data in the final storage area

Am I the only one seeing a problem with this idea? If the server can connect to all your client machines and read all areas as it pleases, even if you restrict it to some directories, the data is already compromised when the backup server is compromised (think .ssh private keys, files with wireless network passwords and so on; I won't say card information - you don't keep credit/debit card information on your computer, or at least not in plain text, do you?).

What I would consider a better alternative would be a server-initiated dialogue which goes a little like this (S is server, C is client, '=' represent connections via ssh):

1. S ---(requests backup initiation procedure)---> C
2. S waits for a defined period of time that C connects back to send (already encrypted) data; if it doesn't arrive, it aborts
3. S <===(sends encrypted data to be backed up)=== C
4. S <-(signals the completion of data transfer)-- C
5. S stores the data in the final storage area

This way, the server can allow access from the clients only in designated areas (even a chroot is possible) from designated clients, access can even be provided only after a port knocking procedure and only during the backup time frame (since the server initiates the negotiation, it can expect only then the knocks, but only then), so the server is quite well secured. The connection to the server can even be done through an unprivileged account, it can even be one account per client machine which can be limited to a scponly shell, if you care for that level of security.

On the other hand, the client information is secure since it can be encrypted directly on the client machine and sent only after encryption, the client machine can decide and control what it sends, while the backup server can only store what the client provides. Also, if the server is compromised, the clients' data and system aren't compromised at all, since the data is on the backup machine, but is encrypted with a key only known on the client (and a backup copy of it can be stored somewhere safe).

I am aware this approach can be problematic for permission (user/group preservation), but it doesn't happen if there is a local <-> remote user mapping or simply the numeric IDs are kept.

I am also aware this means smarter clients and might mean the Windows machine might not be able to implement this completely, but a little more security than "here is all my data" can still be achieved, can't it?

What do other people think? Am I insane or paranoid?

I think I can implement this type of protocol in some scripts (at least one for server and one for clients) and use the backup_script feature of rsnapshot to keep this clean and nice within rsnapshot.

What might prove problematic with this approach is that rsync spedup is lost (might be?) because the copy is done to a temporary directory which, I assume, is empty, so tough luck. Another problem seems to be that every time the backup is done, the client has to encrypt each of the files to backup, which seems to be a real performance penalty, especially if the data to be backed up is quite large.

Is there an encryption layer that does this automatically at file level in the same/similar manner that LUKS does for entire block devices? Having the right file names, but with scrambled/encrypted contents seems to be the ideal solution from this PoV.

Thanks for reading and possible suggestions you might point me to.

P.S.: I just thought of this, if there was an encryption layer implemented with fuse which is mounted in some directory on the client machine, the default rsnapshot mechanism could actually work, and this would mitigate the data accessibility issue and the performance issue since that file system could be contained within a chroot and the encryption/scrambling would be done transparently on the client, so no data is plainly accessible. Does anybody know such a FUSE implementation that does on-the-fly file encryption?

P.P.S.: EncFS does exactly what I want with its --reverse option which is exactly designed for this purpose:

Normally EncFS provides a plaintext view of data on demand. Normally it stores enciphered data and displays plaintext data. With --reverse it takes as source plaintext data and produces enciphered data on-demand. This can be useful for creating remote encrypted backups, where you do not wish to keep the local files unencrypted.

Great!