It has been a while since I wrote a blog post. I have been super busy and do have things to write about.
First about this blog: I have migrated from WordPress hosted blog to a self-hosted blog running on Serendipity
My other (paywalled) blog is on Medium
You will see some cross-posting between the two platforms.
When traveling while working we often find ourselves using networks that have certain restrictions. For example, many free WiFi hotspots will block things like ICMP echo requests (ping) which makes things a bit difficult when you are trying to figure out problems with connectivity, where packet loss is occurring, and what is reachable and what is not. Enter hping, a TCP/IP utility that can do far more than the name suggests.
First an example using standard ping (ICMP):
ping -c1 http://www.google.com
PING http://www.google.com (184.108.40.206) 56(84) bytes of data.
— http://www.google.com ping statistics —
1 packets transmitted, 0 received, 100% packet loss, time 0ms
As we can see in the above output, we never got a reply. Is the network down in between? Is there a firewall that’s blocking our requests? We don’t know.. So let’s take a look at using hping to send a TCP packet, instead of ICMP, to port 80 on a well known website:
sudo hping -c 1 -S -p 80 http://www.google.com
HPING http://www.google.com (wlp2s0 220.127.116.11): S set, 40 headers + 0 data bytes
len=44 ip=18.104.22.168 ttl=127 id=13972 sport=80 flags=SA seq=0 win=11680 rtt=51.0 ms
— http://www.google.com hping statistic —
1 packets tramitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 51.0/51.0/51.0 ms
Ah-ha! We got a reply! So TCP connections to port 80 at least are not blocked. We can work with that (hint this is where having your own custom, configurable OpenVPN server comes in handy: configure it to answer on port 80 and you are back to work).
Breakdown of the options used: as with the standard ping tool, the “-c” flag tells it we only want to send N count requests. In this example we are only sending one (1) request. Without this command line flag, hping (and ping) will ping non-stop until you stop it with ctrl-c. The next flag is “-S” which sets the SYN flag as by default it will not be set by hping. The “-p” flag tells hping we want to send our request to a specific port, in the above example we use port 80 which is the standard non-SSL webserver port. The last option is the hostname or IP address we want to interact with (www.google.com in our example.)
I am going to try something new and use a terminal utility named asciinema to record the examples above. Please let me know what you think: Watch a video
of the above example made with asciinema!