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connect to a http server by using telnet or openssl s_client (in case of tls-only server)


# plaintext
telnet example.org 80

# on embedded systems
busybox telnet example.org 80

# tls
openssl s_client -connect example.org:443



GET / HTTP/1.1
host: example.org

HTTP/1.1 200 OK
Cache-Control: max-age=604800
Content-Type: text/html
Date: Tue, 11 Jul 2017 14:31:52 GMT
Etag: "*********+ident"
Expires: Tue, 18 Jul 2017 14:31:52 GMT
Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT
Server: ECS (iad/182A)
Vary: Accept-Encoding
X-Cache: HIT
Content-Length: 1270


link layer discovery protocol

tcpdump -nnvi eth0 -s 1500 -XX -c 1 'ether proto 0x88cc'

cisco discovery protocol

tcpdump -nnvi eth0 -s 1500 -c 1 'ether[20:2] == 0x2000'

scanning lldp and cdp

tcpdump -nnvi eth0 -s 1500 -c 1 '(ether[20:2] = 0x2000 or ether[12:2] = 0x88cc)'


connect to a whois server by using telnet. Response of whois requests is not standardized!


# telnet whois.denic.de 43
Connected to whois.denic.de.
Escape character is '^]'.
-T dn denic.de
% Copyright (c) 2010 by DENIC
% Version: 2.0
% Restricted rights.
% Terms and Conditions of Use
% The data in this record is provided by DENIC for informational purposes only.
% DENIC does not guarantee its accuracy and cannot, under any circumstances,
% be held liable in case the stored information would prove to be wrong,
% incomplete or not accurate in any sense.
% All the domain data that is visible in the whois service is protected by law.
% It is not permitted to use it for any purpose other than technical or
% administrative requirements associated with the operation of the Internet.
% It is explicitly forbidden to extract, copy and/or use or re-utilise in any
% form and by any means (electronically or not) the whole or a quantitatively
% or qualitatively substantial part of the contents of the whois database
% without prior and explicit written permission by DENIC.
% It is prohibited, in particular, to use it for transmission of unsolicited
% and/or commercial and/or advertising by phone, fax, e-mail or for any similar
% purposes.
% By maintaining the connection you assure that you have a legitimate interest
% in the data and that you will only use it for the stated purposes. You are
% aware that DENIC maintains the right to initiate legal proceedings against
% you in the event of any breach of this assurance and to bar you from using
% its whois service.
% The DENIC whois service on port 43 never discloses any information concerning
% the domain holder/administrative contact. Information concerning the domain
% holder/administrative contact can be obtained through use of our web-based
% whois service available at the DENIC website:
% http://www.denic.de/en/domains/whois-service/web-whois.html

Domain: denic.de
Nserver: ns1.denic.de 2001:668:1f:11:0:0:0:106
Nserver: ns2.denic.de
Nserver: ns3.denic.de
Dnskey: 257 3 8 AwEAAb/xrM2MD+xm84YNYby6TxkMaC6PtzF2bB9WBB7ux7iqzhViob4GKvQ6L7CkXjyAxfKbTzrdvXoAPpsAPW4pkThReDAVp3QxvUKrkBM8/uWRF3wpaUoPsAHm1dbcL9aiW3lqlLMZjDEwDfU6lxLcPg9d14fq4dc44FvPx6aYcymkgJoYvR6P1wECpxqlEAR2K1cvMtqCqvVESBQV/EUtWiALNuwR2PbhwtBWJd+e8BdFI7OLkit4uYYux6Yu35uyGQ==
Status: connect
Changed: 2017-07-17T17:04:03+02:00


Connection closed by foreign host.