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====== Ubuntu - Ping - Check whether a host is alive with TCP ======

TCP can be used to check the host's presence without using an ICMP ping.

This can be done, using multiple tools,

We will see them one by one.

----

===== Using hping =====

**Hping** is a very big tool out which can be used to send custom TCP packets to remote host with desired flags to analyse the reply.

This can be a good testing tool that can be used against your firewall configuration.  

So now lets check the host's presence with the help of TCP and hping.

<code bash>
hping -S -p 80 192.168.0.103
</code>

result:

<code bash>
HPING 192.168.0.103 (eth0 192.168.0.103): S set, 40 headers + 0 data bytes
len=46 ip=192.168.0.103 ttl=64 DF id=0 sport=80 flags=SA seq=0 win=5840 rtt=0.8 ms
len=46 ip=192.168.0.103 ttl=64 DF id=0 sport=80 flags=SA seq=1 win=5840 rtt=1.5 ms
len=46 ip=192.168.0.103 ttl=64 DF id=0 sport=80 flags=SA seq=2 win=5840 rtt=2.2 ms
len=46 ip=192.168.0.103 ttl=64 DF id=0 sport=80 flags=SA seq=3 win=5840 rtt=1.2 ms
</code>

----

----

Let's have a look at the tcpdump output at the host with PING disabled.

<code bash>
tcpdump -i eth0 -s0 host 192.168.0.102
</code>

result:

<code bash>
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
03:13:11.708350 IP 192.168.0.102.instantia > 192.168.0.103.http: S 1718487905:1718487905(0) win 512
03:13:11.730329 IP 192.168.0.103.http > 192.168.0.102.instantia: S 1566252172:1566252172(0) ack 1718487906 win 5840 <mss 1460>
03:13:11.708587 IP 192.168.0.102.instantia > 192.168.0.103.http: R 1718487906:1718487906(0) win 0
03:13:12.709646 IP 192.168.0.102.nessus > 192.168.0.103.http: S 1845389890:1845389890(0) win 512
03:13:12.709815 IP 192.168.0.103.http > 192.168.0.102.nessus: S 1576213111:1576213111(0) ack 1845389891 win 5840 <mss 1460>
03:13:12.710429 IP 192.168.0.102.nessus > 192.168.0.103.http: R 1845389891:1845389891(0) win 0
</code>

If you observe the TCPDUMP output, you will be able to see that, a SYN request (shown with "S" flag in Tcpdump output) and a Reset request(shown with "R" flag in tcpdump output), both are send by our requesting host(The host from where we send the tcp ping with hping).

Hping is sending a reset packet just after the syn, as it does not need the connection to be fully established.

----

===== Using tcping =====

Another utility that can be used to check the presence of the host, when ping is disabled is tcping.

This utility can be easily installed if you have rpmforge repo enabled.

Lets see its usage.

<code bash>
tcping 192.168.0.103 80
</code>

result:

<code bash>
192.168.0.103 port 80 open.
</code>

----

===== Using nmap =====

The famous NMAP tool can also be used to check the host aliveness by using TCP.  NMAP does this by simply sending a TCP ACK packet to the host(For which a RST packet will be send back by the remote host).

  * **-PT** option in NMAP can be used for TCP PING.  An ACK packet is send to 80 port by default when -PT flag is used with NMAP.

<code bash>
nmap -PT 192.168.0.103
</code>

result:

<code bash>
Starting Nmap 4.85BETA5 ( http://nmap.org ) at 2013-01-15 14:18 IST
Interesting ports on 192.168.0.103:
Not shown: 995 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
80/tcp  open  http
111/tcp open  rpcbind
443/tcp open  https
MAC Address: 08:00:27:55:D1:CC (Cadmus Computer Systems)
Nmap done: 1 IP address (1 host up) scanned in 0.30 seconds
</code>

----

If you want to specify a particular port with -PT option for tcp ping, then you can do that as shown below.

<code bash>
nmap -PT443 192.168.0.103
</code>

result:

<code bash>
Starting Nmap 4.85BETA5 ( http://nmap.org ) at 2013-01-15 14:20 IST
Interesting ports on 192.168.0.103:
Not shown: 995 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
80/tcp  open  http
111/tcp open  rpcbind
443/tcp open  https
</code>