A naive try would be something like this: $ nc -l 8082 | nc remote_host 80 Yes, it does forward the request from local port 8082 to remote_host:80 , but the response is dumped to stdout , not routed back to the client as expected. Using a named pipe makes it work: $ mkfifo backpipe $ nc -l 8082 0<backpipe | nc ... Use tee to get a glimpse of the response through the pipe (I wasn't able to find a way to dump the request): $ nc -k -l 8082 0<backpipe | nc localhost 80 | tee... The GNU netcat has a different syntax than the stock nc . It also supports different switches. To listen to port 1234: $ netcat -l -p 1234...

Use IO redirection to swap stdout and stderr : $ prog 3>&1 >&2 2>&3 3>&-

Nmap port scan against host 10.2.1.3 , with TCP SYN scan ( -sS ), OS detection ( -O , capital O, not zero) and verbose output ( -v ): nmap -v -sS -O 10.2.1.3 If ping is blocked, add switch -PN : nmap -v -sS -O -PN 10.2.1.3 Shortcut option -A for version detection, Nmap Scripting Engine with the default set of scripts, remote OS detection, and traceroute (equivalent to: -sV -sC -O --traceroute ): nmap -A 10.2.1.3

This iptables rule redirects all port 80 traffic from subnet 192.168.0.0/24 to 192.168.0.1 running HTTP proxy (say squid): /usr/sbin/iptables -A PREROUTING -s 192.168.0.0/25...

With standalone broadcast receiver AndroidManifest.xml : <?xml version="1.0" encoding="utf-8"?> <manifes... Java code: package com.android.networkreceiver; import... Register listener inside Activity programmatically AndroidManifest.xml : <?xml version="1.0" encoding="utf-8"?> <manifes... Java code: package com.android.networklistener; import... Register/unregister broadcast receiver in onResume / onPause works because the connectivity change broadcast is sticky . I intentionally used two registerReceiver calls in onResume to demonstrate this - onReceive will be called once for each registration.

I got this error while enabling/disabling wifi on an Android device: W/ActivityManager( 2735): Unable to launch app com... The receiver was registered for network connection change: <receiver android:name="NetworkReceiver"> ... This was fixed by just rebooting the device!

There are two sides to an IP socket: the local side and the remote side: The getpeername function retrieves the peer address of the specified socket. #include <sys/socket.h> int getpeername(int soc... Example: struct sockaddr_in addr; socklen_t socklen = si... The getsockname function retrieves the locally-bound name of the specified socket. #include <sys/socket.h> int getsockname(int soc... Example: struct sockaddr_in addr; socklen_t socklen = si...

Client socket usually does not call bind . But I've seen code that does and it was puzzling to me what bind does to a client socket. Therefore, this little test program. It retrieves a web url and displays info about the socket. You can optionally give a bind host name/ip and port and see what it does. Here are my test results: $ ./client www.google.com Local addr: 172.16.0.... This is the code: #include <stdio.h> #include <stdlib.h> #incl...

This procedure works for xl2tpd . Edit L2TP configuration file /etc/xl2tpd/xl2tpd.conf : ; ; This is a minimal sample xl2tpd configurati... Edit PPP options file for L2TP /etc/ppp/options.xl2tpd.client : ipcp-accept-local ipcp-accept-remote refuse-... Edit PPP authentication file /etc/ppp/chap-secrets : # Secrets for authentication using CHAP # serve... Start xl2tpd : # /etc/init.d/xl2tpd start Connect to the server: # echo "c vpnserver" >/var/run/xl2tpd/l2tp-control Tail /var/log/messages , you should see something like: Sep 28 11:05:46 sta101894 pppd[26059]: pppd 2.4.5 ... ifconfig should show that ppp0 is added: # ifconfig ppp0 ppp0 Link encap:Point-to-P... To tear down the vpn: # echo "d vpnserver" >/var/run/xl2tpd/l2tp-control If authentication fails, try to set "require authentication" to no on both server and client and restart xl2tpd on both sides: # /etc/init.d/xl2tpd restart # echo "c vpnserve......

This procedure works for xl2tpd . Edit L2TP configuration file /etc/xl2tpd/xl2tpd.conf : ; Sample l2tpd.conf ; [global] ; listen... Edit PPP options file for L2TP /etc/ppp/options.xl2tpd : ipcp-accept-local ipcp-accept-remote ms-dns ... Edit PPP authentication file /etc/ppp/chap-secrets : # Secrets for authentication using CHAP # serve... Start the server: # /etc/init.d/xl2tpd start Monitor the log: # less /var/log/messages