On 11/20/2017 05:54 PM, plug-requ...@pdxlinux.org wrote:
With the mail/phone issues I've had recently I want to check that I'm
doing things correctly. Two instances of not reaching web pages.
I can load (and ping)www.opendkim.org, but cannot load (or ping)
lists.opendkim.org. This means their mail list page is off-line. Yes?
No, not necessarily.
Yesterday and today I try to accesswww.verizonwireless.com but the
page
won't load. Neither can I ping that server. This means that their web site
is down. Yes?
Again, not necessarily.
More and more IT / Security groups are going to dropping the ICMP (ping)
packets for "security" reasons, and this affects several tools that have
long been used by the layperson and professional alike.
When the firewall receives an ICMP packet, ping and traceroute both will
show failure and/or lack of "up: state. If the attacker knows the device is
there by DNS resolution or IP address, they have a known target, and the
dropping of packets (IMO) is just obscuring things a little bit.
This is more of an ol' skool mentality that reflects a serious lack of a
deeper understanding of networking and of being a good & useful netizen.
In reality, we should actually see less and less of this over time as
the ICMP protocol suite is very useful and blocking it doesn't amount to
very much that's good and/or useful.
Other tools that you can try are telnet to the port for the service in
question, nmap to check for all open ports (potential for looking like an
attacker), and netcat (nc) to test for specific, or scan for all open,
ports.
A deeper way to search if things are connecting at all is the use of
netstat, Wireshark, or tcpdump in some cases.
I've been a network engineer for over a decade and I've learned to use
the simplest tool possible for a task. In this case tcping is a very
good simple and easy to use tool for anyone who's just wanting to test
connectivity to network host. Most implementations allow for a port
number to be specified. If you're unsure of the port number from the
Linux cli you can cat the /etc/services file to get a listing of udp &
tcp ports w. description that's updated by IANA.
e.g.
~$ cat /etc/services
# Network services, Internet style
#
# Note that it is presently the policy of IANA to assign a single
well-known port number for both TCP and UDP; hence, officially ports
have two entries
# even if the protocol doesn't support UDP operations.
#
# Updated from http://www.iana.org/assignments/port-numbers and other
sources like http://www.freebsd.org/cgi/cvsweb.cgi/src/etc/services .
# New ports will be added on request if they have been officially
assigned by IANA and used in the real-world or are needed by a debian
package.
# If you need a huge list of used numbers please install the nmap package.
tcpmux 1/tcp # TCP port service multiplexer
echo 7/tcp
echo 7/udp
discard 9/tcp sink null
discard 9/udp sink null
systat 11/tcp users
This particular implementation,
https://www.elifulkerson.com/projects/tcping.php , provides a lot of
HTTP mode options as well setting wait interval for a response,
calculating jitter (variance in delay) and prefer ipv4 or ipv6.
Network connectivity is going to likely become more problematic, and our means
of testing things will likely become more restricted as time passes.
Just my guess, but it's a trend I have been noticing.
I don't follow nor subscribe to this logic at all. I expect network
connectivity and ways to test to it to only get better as everyone and
their grandparents demand well performing, highly reliable internet
connectivity from all of their devices, everywhere. 5 years ago while
working at an ISP in downtown Portland I had to work on networking
problems from end-users of our ISP business partners who where
complaining about high ping times in their favorite online game. And I'm
talking about sub 150 ms round-trip ping times which is used as the
measuring stick for toll quality VoIP.
Also, consider that IPv6 provides improved QOS functionality, better
security and faster routing on an end to end connection basis. As
infrastructure gets re-designed and changed out I only expect network
connectivity to get less problematic with greater visibility into
problems provided by better tools and information.
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