Mar 25, 2012

Handling signals in bash

Due to the nature of a bash script, it is really difficult which can lead to a segmentation fault unless the script runs a binary within which can cause it afterwards. But what is the best way in bash in order to handle a segmentation fault? By catching its associated signal, that is, SIGSEGV (number 11, invalid memory reference).

In Linux, we have a great tool to do it: trap. The following script sets a handler up by means of the trap command. Its first parameter is the function or code which manages the exception, and the second one, the name (SEGV) or value (11) of the signal. We can also add more signals to the same trap.

[root@localhost ~]# cat

function trap_segfault 
    echo "We have a segmenation fault!"

    # Reset handler
    trap - 11
    exit 127 

# Start handler
trap 'trap_segfault' 11
# Here code which can produce a segmentation fault, such as:
kill -SEGV $$ 1>&2

Apart from catching segmentation faults, we can use this technique so as to treat other signals inside a bash script. In order to take a look at all the signals provided by Linux, you might run 'trap -l' or 'kill -l'. But if you want to extend the information about them, you will have to resort to the own man.

[root@localhost ~]# man 7 signal
       Signal     Value     Action   Comment
       SIGHUP        1       Term    Hangup detected on controlling terminal
                                     or death of controlling process
       SIGINT        2       Term    Interrupt from keyboard
       SIGQUIT       3       Core    Quit from keyboard
       SIGILL        4       Core    Illegal Instruction
       SIGABRT       6       Core    Abort signal from abort(3)
       SIGFPE        8       Core    Floating point exception
       SIGKILL       9       Term    Kill signal

Next example handles SIGHUP to show the PID of the script and SIGINT to terminate it.

[root@localhost ~]# cat 

function trap_sigint
    echo "Script terminated"

    # Reset handlers
    trap - HUP INT
    exit 0 

# Start handlers
trap 'echo PID: $$\n' HUP
trap 'trap_sigint' INT 

while [ true ];
    sleep 1

[root@localhost ~]# ./ &
[1] 3046

[root@localhost ~]# kill -HUP 3046
[root@localhost ~]# PID: 3046

[root@localhost ~]# kill -INT 3046
[root@localhost ~]# Script terminated

And finally, also say that we can use this utility to leave out signals. For instance, the following order makes the script ignore SIGTERM.

trap : SIGTERM

Mar 16, 2012

Setting up BIND to secure DNS with DNSSEC (II)

Let's continue with the second part of the article titled Setting up BIND to secure DNS with DNSSEC. First up, we have to know that the default configuration of BIND on CentOS 6 allows to use directly DNSSEC.

[root@dns ~]# cat /etc/named.conf
    dnssec-enable yes;
    dnssec-validation yes;
    dnssec-lookaside auto;

To begin with, we are going to generate the ZSK (it takes care of signing the records of the zone file) and KSK (it takes care of signing the ZSK) keys. Each command turns out two key pair files, public (.key) and private (.private). Pay attention to the permissions of the private keys. As you can guess, only root has access to the files.

[root@dns ~]# cd /var/named/test.local/

[root@dns test.local]# dnssec-keygen test.local

[root@dns test.local]# dnssec-keygen -f KSK test.local

[root@dns test.local]# ls -l
total 20
-rw-r--r--. 1 root root  428 Mar 11 20:33 Ktest.local.+005+05775.key
-rw-------. 1 root root 1010 Mar 11 20:33 Ktest.local.+005+05775.private
-rw-r--r--. 1 root root  602 Mar 11 20:36 Ktest.local.+005+06859.key
-rw-------. 1 root root 1774 Mar 11 20:36 Ktest.local.+005+06859.private
-rw-r--r--. 1 root root  163 Mar 11 19:15

Next step is to add the public key files to the zone file. We will have a couple of extra DNSKEY records included inside the file.

[root@dns test.local]# cat *.key >>

[root@dns test.local]# cat
test.local. IN DNSKEY 256 3 5 AwEAAc1...
test.local. IN DNSKEY 257 3 5 AwEAAda...

At this moment, we are able to sign the zone. The last argument of the command is the zone file and through the "-o" option, we can indicate the zone origin. This order creates a new version of the zone file by adding NSEC and RRSIG records.

[root@dns test.local]# dnssec-signzone -o test.local

[root@dns test.local]# ls -l *.signed
-rw-r--r--. 1 root root 3429 Mar 11 20:50

Now we only need to update the named.conf file in order to point at the new signed file. Remember to reload the service for the changes to take effect.

[root@dns test.local]# cat /etc/named.conf
zone "test.local" IN {
    type master;
    file "test.local/";

[root@dns ~]# /etc/init.d/named restart

I will finally write a third article so as to carry out some test about this.

Mar 12, 2012

Setting up BIND to secure DNS with DNSSEC (I)

DNSSEC stands for Domain Name System Security Extensions. From the acronym, we ought to figure out that basically, this extension takes care of adding security to the DNS protocol.

When the DNS protocol came out, like most of the initial protocols of Internet, it was not designed by taking into account the security. Over time, the DNS protocol has become one of the biggest risks related to security. We only have to remember the series of articles about ARP poisoning (I, II and III).

DNSSEC allows zones to be verified by signing them with a zone key. Thus, the information about a specific zone can be authenticated as coming from a nameserver which has signed it with a concrete private key, as long as the client has the nameserver's public key.

How does DNSSEC work? Signs all zone records with the same name and type by using a Zone Signing Key (ZSK). This process turns out a signature which is stored in a RRSIG (Resource Record Signature) record. In turn, the public part of the ZSK is a DNSKEY (DNS Public Key) record. Afterwards, ZSK will be signed by other different key known as KSK (Key Signing Key). KSK is also stored in a DNSKEY record.

So in this way, we will be able to use DNSSEC to check DNS secure results (from a signed zone validated correctly), insecure results (from an unsigned zone), and fake results (signature fails when is validated or zone is unsigned but parent says which should be signed).

Let's get started by configuring a simple zone called test.local on a CentOS 6.2 server. To begin with, we are going to install BIND (version 9.7.3) and define our zone in the configuration file. The type of zone will be master, that is, it designates the nameserver as authoritative for this zone.

[root@dns ~]# hostname

[root@dns ~]# yum install bind

[root@dns ~]# cat /etc/named.conf
options {
    listen-on port 53 { any; };
    allow-query     { localhost; localnets; };

zone "test.local" IN {
    type master;
    file "test.local/";

Then we have to create the zone file. As commented before, it will be a simple file with an unique address record specifying the IP of the server (

[root@dns ~]# mkdir /var/named/test.local

[root@dns ~]# cat /var/named/test.local/
; Zone name
$ORIGIN test.local.

; Default Time to Live (TTL)
$TTL    86400

; Start Of Authority resource record
; (serial-number time-to-refresh time-to-retry time-to-expire minimum-TTL)
@       IN SOA dns.test.local. admin.test.local. (1 6H 1H 1W 1D)

; NameServer record (announces the authoritative nameserver for a particular zone)
        IN NS  dns.test.local.

; Address record (specifies an IP address to assign to a name)
dns     IN A

So far, this is the normal behavior of a DNS server without authenticating its zones. If you restart the BIND service right away, you could use it as a normal DNS server. Next week, I will end up by setting the configuration for DNSSEC.

[root@dns ~]# /etc/init.d/named restart

Mar 4, 2012

Mounting file systems via Samba (CIFS) or NFS through automount (autofs) (II)

This is the second part of the article Mounting file systems via Samba (CIFS) or NFS through automount (autofs). In the first one, we started to prepare autofs in order to automatically mount a couple of remote directories by NFS and Samba.

Now let's carry on by supposing that centos02 exports the directory /nfs via NFS and centos03 exports the directory /samba via Samba. I am not going to note down how to configure NFS or Samba, because next week I intend to write an article related to this.

The following file shows the content of the auto.centos file. By means of this configuration file, we are indicating the NFS or Samba paths to the autofs daemon. When someone (or something) accesses the /mnt/tmp/nfs directory, this filesystem will be automatically imported from centos02 via NFS. The same occurs for the /mnt/tmp/samba directory imported from centos03 via Samba.

[root@centos01 ~]# cat /etc/auto.centos
nfs     -fstype=nfs                                        centos02:/nfs
samba   -fstype=cifs,username=samba_user,password=xxxxxx   ://centos03/samba

When the timeout has expired and there has been no activity on the directories during that time, the directories will also be automatically dismounted.

Finally, you just have to restart the autofs service.

[root@centos01 ~]# service autofs restart