I was just looking around info about netcat and telnet, trying to understand a bit more. Well… in few words: no point to install telnet if you have netcat! 🙂 Netcat is perfect for scripting, ’cause it’s non-interactive, UDP/TCP capable, can be a listener as well… very powerful tool. Here some example.

How to check if your httpd is up and running:

~ $ nc -zv localhost 80
Connection to localhost 80 port [tcp/http] succeeded!

1 2	~ $ nc -zv localhost 80 Connection to localhost 80 port [tcp/http] succeeded!

…and it closes gracefully 😉

How to check port-range ports:

~ $ nc -zv localhost 20-25
nc: connect to localhost port 20 (tcp) failed: Connection refused
Connection to localhost 21 port [tcp/ftp] succeeded!
Connection to localhost 22 port [tcp/ssh] succeeded!
nc: connect to localhost port 23 (tcp) failed: Connection refused
nc: connect to localhost port 24 (tcp) failed: Connection refused
nc: connect to localhost port 25 (tcp) failed: Connection refused

~ $ nc -zv localhost 20-25

nc: connect to localhost port 20 (tcp) failed: Connection refused

Connection to localhost 21 port [tcp/ftp] succeeded!

Connection to localhost 22 port [tcp/ssh] succeeded!

nc: connect to localhost port 23 (tcp) failed: Connection refused

nc: connect to localhost port 24 (tcp) failed: Connection refused

nc: connect to localhost port 25 (tcp) failed: Connection refused

… or a list of ports:

$ nc -zv localhost 20 22 80 443
nc: connect to localhost port 20 (tcp) failed: Connection refused
Connection to localhost 22 port [tcp/ssh] succeeded!
Connection to localhost 80 port [tcp/http] succeeded!
Connection to localhost 443 port [tcp/https] succeeded!

$ nc -zv localhost 20 22 80 443

nc: connect to localhost port 20 (tcp) failed: Connection refused

Connection to localhost 22 port [tcp/ssh] succeeded!

Connection to localhost 80 port [tcp/http] succeeded!

Connection to localhost 443 port [tcp/https] succeeded!

NOTE: If you want to grep or play with the “output” of the command, you need to use 2>&1
For example:

Shell

nc -zv localhost 1-1024 2>&1 | grep succeeded

1

nc -zv localhost 1-1024 2>&1 | grep succeeded

How to check the service that’s running on that port:

(From man) Alternatively, it might be useful to know which server software is running, and which versions. This information is often contained within the greeting banners. In order to retrieve these, it is necessary to first make a connection, and then break the connection when the banner has been retrieved. This can be accomplished by specifying a small timeout with the -w flag, or perhaps by issuing a “QUIT” command to the server:

$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready

$ echo "QUIT" | nc host.example.com 20-30

SSH-1.99-OpenSSH_3.6.1p2

Protocol mismatch.

220 host.example.com IMS SMTP Receiver Version 0.84 Ready

In some cases, it’s handy to add -q 1 at the end, if nc hangs (I’ve noticed this in some cases) Like this:

$ echo "QUIT" | nc host.example.com 20-30 <strong>-q 1</strong>

1	$ echo "QUIT" \| nc host.example.com 20-30 <strong>-q 1</strong>

Or how to send/receive a file:

On the receiver side:

$ nc -l 1234 &gt; /tmp/file_to_receive

1	$ nc -l 1234 > /tmp/file_to_receive

On the sender side:

$ cat file_to_send | nc receiver_ip_or_fqdn 1234

1	$ cat file_to_send \| nc receiver_ip_or_fqdn 1234

$ nc receiver_ip_or_fqdn 1234 &lt; file_to_send

1	$ nc receiver_ip_or_fqdn 1234 < file_to_send

There are plenty of things that you can do. These are just simple examples… enjoy! 🙂

Linux: Debian stable (currently version 7)

Packages:

apt-get install install bind9 isc-dhcp-server

1	apt-get install install bind9 isc-dhcp-server

Create a key required for DHCP server to update the DNS zones:

/usr/sbin/rndc-confgen -a

1	/usr/sbin/rndc-confgen -a

This will create /etc/bind/rndc.key, whose contents will look something like this:

key "rndc-key" {
algorithm hmac-md5;
secret "+zZSeeetHWFdNwECit1Ktw==";
};

key "rndc-key" {

algorithm hmac-md5;

secret "+zZSeeetHWFdNwECit1Ktw==";

};

BIND configuration

Configuration files:

/etc/hosts

127.0.0.1 localhost
10.0.60.60 dns.lab.loc dns

# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

127.0.0.1 localhost

10.0.60.60 dns.lab.loc dns

# The following lines are desirable for IPv6 capable hosts

::1 localhost ip6-localhost ip6-loopback

ff02::1 ip6-allnodes

ff02::2 ip6-allrouters

/etc/bind/named.conf.local

// Do any local configuration here
// Consider adding the 1918 zones here, if they are not used in your organization
include "/etc/bind/zones.rfc1918";

include "/etc/bind/rndc.key";

zone "lab.loc" {
type master;
file "/etc/bind/db.lab.loc";
allow-update { key rndc-key; };
};

zone "60.0.10.in-addr.arpa" {
type master;
file "/etc/bind/db.10.0.60";
allow-update { key rndc-key; };
};

// Do any local configuration here

// Consider adding the 1918 zones here, if they are not used in your organization

include "/etc/bind/zones.rfc1918";

include "/etc/bind/rndc.key";

zone "lab.loc" {

type master;

file "/etc/bind/db.lab.loc";

allow-update { key rndc-key; };

};

zone "60.0.10.in-addr.arpa" {

type master;

file "/etc/bind/db.10.0.60";

allow-update { key rndc-key; };

};

/etc/bind/named.conf.options

(just to setup the external forwarders)

options {
directory "/var/cache/bind";

// If there is a firewall between you and nameservers you want
// to talk to, you may need to fix the firewall to allow multiple
// ports to talk. See http://www.kb.cert.org/vuls/id/800113

// If your ISP provided one or more IP addresses for stable
// nameservers, you probably want to use them as forwarders.
// Uncomment the following block, and insert the addresses replacing
// the all-0's placeholder.

forwarders {
<strong>208.67.222.222;208.67.220.220;8.8.8.8;8.8.4.4;</strong>
};

//========================================================================
// If BIND logs error messages about the root key being expired,
// you will need to update your keys. See https://www.isc.org/bind-keys
//========================================================================
dnssec-validation auto;

auth-nxdomain no; # conform to RFC1035

allow-query {
10.0.60/24;
127.0.0.1;
};
allow-transfer {
10.0.60/24;
127.0.0.1;
};

listen-on-v6 { any; };
};

options {

directory "/var/cache/bind";

// If there is a firewall between you and nameservers you want

// to talk to, you may need to fix the firewall to allow multiple

// ports to talk. See http://www.kb.cert.org/vuls/id/800113

// If your ISP provided one or more IP addresses for stable

// nameservers, you probably want to use them as forwarders.

// Uncomment the following block, and insert the addresses replacing

// the all-0's placeholder.

forwarders {

208.67.222.222;208.67.220.220;8.8.8.8;8.8.4.4;

};

//========================================================================

// If BIND logs error messages about the root key being expired,

// you will need to update your keys. See https://www.isc.org/bind-keys

//========================================================================

dnssec-validation auto;

auth-nxdomain no; # conform to RFC1035

allow-query {

10.0.60/24;

127.0.0.1;

};

allow-transfer {

10.0.60/24;

127.0.0.1;

};

listen-on-v6 { any; };

};

/etc/bind/db.lab.loc

$ORIGIN lab.loc.
$TTL 24h ;$TTL (DNS time-to-live setting) used for all RRs without explicit TTL value

;SOA - Start of Authority. This is the record that states that this server is authoritative for the specified domain
;The SOA record lists the name server for the domain, and next the e-mail address of the administer of the domain
;(note that the @ has been replaced by a period).
@ IN SOA dns.lab.loc. root.lab.loc. (
2014032109 ; serial YYYYMMDDNN
10800 ; refresh (3 hours)
1800 ; retry (30 minutes)
604800 ; expire (1 week)
38400 ; minimum (10 hrs 40 min)
)
IN NS dns.lab.loc. ;Specifies the name server to use to look up a domain
; IN NS dns2.lab.loc. ;Specifies the name server to use to look up a domain
IN A 10.0.60.60 ; IP Address(es) of the DNS server(s)
; IN A 10.0.60.61 ; IP Address(es) of the DNS server(s)
IN MX 10 dns.lab.loc. ;Specifies mail server(s) for the domain

; HOSTS
dns IN A 10.0.60.60
;dns2 A 10.0.60.61

esxi01 IN A 10.0.60.71
esxi02 IN A 10.0.60.72
esxi03 IN A 10.0.60.73

freenas IN A 10.0.60.80

mail IN CNAME dns
dnsmaster IN CNAME dns
storage IN CNAME freenas

$ORIGIN lab.loc.

$TTL 24h ;$TTL (DNS time-to-live setting) used for all RRs without explicit TTL value

;SOA - Start of Authority. This is the record that states that this server is authoritative for the specified domain

;The SOA record lists the name server for the domain, and next the e-mail address of the administer of the domain

;(note that the @ has been replaced by a period).

@ IN SOA dns.lab.loc. root.lab.loc. (

2014032109 ; serial YYYYMMDDNN

10800 ; refresh (3 hours)

1800 ; retry (30 minutes)

604800 ; expire (1 week)

38400 ; minimum (10 hrs 40 min)

)

IN NS dns.lab.loc. ;Specifies the name server to use to look up a domain

; IN NS dns2.lab.loc. ;Specifies the name server to use to look up a domain

IN A 10.0.60.60 ; IP Address(es) of the DNS server(s)

; IN A 10.0.60.61 ; IP Address(es) of the DNS server(s)

IN MX 10 dns.lab.loc. ;Specifies mail server(s) for the domain

; HOSTS

dns IN A 10.0.60.60

;dns2 A 10.0.60.61

esxi01 IN A 10.0.60.71

esxi02 IN A 10.0.60.72

esxi03 IN A 10.0.60.73

freenas IN A 10.0.60.80

mail IN CNAME dns

dnsmaster IN CNAME dns

storage IN CNAME freenas

/etc/bind/db.10.0.60

; BIND reverse file for lab.loc
$ORIGIN 60.0.10.in-addr.arpa.
$TTL 24h
@ IN SOA dsn.lab.loc. root.lab.loc. (
2014032104 ; serial number YYMMDDNN
10800 ; Refresh (3 hours)
3600 ; Retry (1 hour)
604800 ; Expire (1 week)
38400 ; Min TTL (10 hours 40 minutes)
)
IN NS dns.lab.loc.
; IN NS dns2.lab.loc.

;LIST OF HOSTS (reverse)

60 IN PTR dns.lab.loc.

71 IN PTR esxi01.lab.loc.
72 IN PTR esxi02.lab.loc.
73 IN PTR esxi03.lab.loc.

80 IN PTR freenas.lab.loc.

; BIND reverse file for lab.loc

$ORIGIN 60.0.10.in-addr.arpa.

$TTL 24h

@ IN SOA dsn.lab.loc. root.lab.loc. (

2014032104 ; serial number YYMMDDNN

10800 ; Refresh (3 hours)

3600 ; Retry (1 hour)

604800 ; Expire (1 week)

38400 ; Min TTL (10 hours 40 minutes)

)

IN NS dns.lab.loc.

; IN NS dns2.lab.loc.

;LIST OF HOSTS (reverse)

60 IN PTR dns.lab.loc.

71 IN PTR esxi01.lab.loc.

72 IN PTR esxi02.lab.loc.

73 IN PTR esxi03.lab.loc.

80 IN PTR freenas.lab.loc.

DHCP configuration

Here there is just one file that has to be modified: dhcpd.conf

/etc/dhcp/dhcpd.conf

Here we need to enter the key in plain text.

# DHCPD
ddns-updates on;
ddns-update-style interim;
update-static-leases on;
authoritative;
key rndc-key { algorithm hmac-md5; secret +zZSeeetHWFdNwECit1Ktw==;}
allow unknown-clients;
use-host-decl-names on;
default-lease-time 1814400; #21 days
max-lease-time 1814400; #21 days
log-facility local7;

# lab.loc DNS zones
zone lab.loc. {
primary localhost; # This server is the primary DNS server for the zone
key rndc-key; # Use the key we defined earlier for dynamic updates
}
zone 60.0.10.in-addr.arpa. {
primary localhost; # This server is the primary DNS server for the zone
key rndc-key; # Use the key we defined earlier for dynamic updates
}

# lab.loc LAN scope
subnet 10.0.60.0 netmask 255.255.255.0 {
range 10.0.60.100 10.0.60.200;
option subnet-mask 255.255.255.0;
option routers 10.0.60.2;
option domain-name-servers 10.0.60.60;
option domain-name "lab.loc";
ddns-domainname "lab.loc.";
ddns-rev-domainname "in-addr.arpa.";
}

# lab.loc STATIC assigned group
group {
host freenas.lab.loc {
hardware ethernet 00:0c:29:18:af:b4;
fixed-address 10.0.60.80;
ddns-hostname "freenas";
}
host esxi01.lab.loc {
hardware ethernet 00:0c:29:d4:14:ce;
fixed-address 10.0.60.71;
ddns-hostname "esxi01";
}
host esxi02.lab.loc {
hardware ethernet 00:0c:29:2c:30:fd;
fixed-address 10.0.60.72;
ddns-hostname "esxi02";
}
host esxi03.lab.loc {
hardware ethernet 00:0c:29:46:90:fd;
fixed-address 10.0.60.73;
ddns-hostname "esxi03";
}
}

# DHCPD

ddns-updates on;

ddns-update-style interim;

update-static-leases on;

authoritative;

key rndc-key { algorithm hmac-md5; secret +zZSeeetHWFdNwECit1Ktw==;}

allow unknown-clients;

use-host-decl-names on;

default-lease-time 1814400; #21 days

max-lease-time 1814400; #21 days

log-facility local7;

# lab.loc DNS zones

zone lab.loc. {

primary localhost; # This server is the primary DNS server for the zone

key rndc-key; # Use the key we defined earlier for dynamic updates

}

zone 60.0.10.in-addr.arpa. {

primary localhost; # This server is the primary DNS server for the zone

key rndc-key; # Use the key we defined earlier for dynamic updates

}

# lab.loc LAN scope

subnet 10.0.60.0 netmask 255.255.255.0 {

range 10.0.60.100 10.0.60.200;

option subnet-mask 255.255.255.0;

option routers 10.0.60.2;

option domain-name-servers 10.0.60.60;

option domain-name "lab.loc";

ddns-domainname "lab.loc.";

ddns-rev-domainname "in-addr.arpa.";

}

# lab.loc STATIC assigned group

group {

host freenas.lab.loc {

hardware ethernet 00:0c:29:18:af:b4;

fixed-address 10.0.60.80;

ddns-hostname "freenas";

}

host esxi01.lab.loc {

hardware ethernet 00:0c:29:d4:14:ce;

fixed-address 10.0.60.71;

ddns-hostname "esxi01";

}

host esxi02.lab.loc {

hardware ethernet 00:0c:29:2c:30:fd;

fixed-address 10.0.60.72;

ddns-hostname "esxi02";

}

host esxi03.lab.loc {

hardware ethernet 00:0c:29:46:90:fd;

fixed-address 10.0.60.73;

ddns-hostname "esxi03";

}

Once everything is configured, just restart bind and dhcp:

/etc/init.d/bind9 restart &amp;&amp; /etc/init.d/isc-dhcp-server restart

1	/etc/init.d/bind9 restart && /etc/init.d/isc-dhcp-server restart

Sources:

https://www.centos.org/docs/4/html/rhel-rg-en-4/s1-bind-zone.html

My notepad

Chris' IT notes…

Tag Archives: networking

Netcat – such a powerful ‘cat’!