Month: November 2012

Categories
Admin Network Technologies

The IT Detective Agency: Internet Explorer cannot display https web page, part II

Intro
They say when it rains it pours. As a harassed It support specialist its tempting to lump all similar-looking problems that come across your desk at the same time in the same bucket. This case shows the shallowness of that way of thinking, for that’s exactly what happened in this case. It occurred exactly when this case was occurring in which a user had an issue displaying a secure web site. That other case was described here.

The details
I was doing some work for a company when they came to me with a problem one user at HQ was having accessing an https web site. Everything else worked fine. The same web page worked fine from other sites.

Since I had helped them set up their proxy services I was keen to prove that the proxy wasn’t at fault. The user removed the proxy settings – still the error occurred. The desktop support got involved. I suggested a whole battery of tests because this was a weird one.
– what if you access via proxy?
– what if you take that laptop and access it from a VPN connection?
– what if you access another site which uses the same certificate-issuer?
– what if you access the host, but using http?
– can you PING the server?
– can you telnet to that server on port 443?
– what if you access the webpage by IP address?
– what if accessed via Firefox?
– etc.
The error, by the way, was

Internet Explorer cannot display the webpage

What you can try:
Diagnose Connection Problems

The answers came back like this:
– what if you access via proxy? It works!
– what if you take that laptop and access it from a VPN connection? It works.
– what if you access another site which uses the same certificate-issuer? It works.
– what if you access the host, but using http? It works.
– can you PING the server? Yes.
– can you telnet to that server on port 443? Yes
– what if you access the webpage by IP address? It does not work.
– Firefox? I don’t remember the answer to this.

Most of that thinking behind those tests is pretty obvious. Why so many tests? The networking group is kind of crotchety and understaffed, so they really wanted to eliminate all other possibilities first. And at one point I thought it could have been a desktop issue. Why would the network allow some of these packets through but not others when it doesn’t run a firewall? The desktop did have A-V and local firewall, after all.

So I didn’t have proof or even a good idea. Time to take out the big guns. We ran a trace on the server with tcpdump while the error occurred. The server was busy so we had to be pretty specific:

> tcpdump -s 1580 -w /tmp/drjcap.cap -i any host 10.19.79.216 and port 443

What do we see? We see the initial handshake go through just fine. Then a client Hello, then a server Hello, followed by a Certificate sent from the server to the client. The Certificate packet kept getting re-sent because there was no ACK to it from the client. So it’s beginning to smell like a dropped packet somewhere. I was hot on the trail but I decided I needed even stronger proof. We arranged to do simultaneous traces on both PC and server to compare the two results.

On the PC we had to install Wireshark. Actually I think Microsoft also has a utility to do traces but I’ve never used it. What did we find? Proof that there was indeed a dropped packet. That server certificate? Never received by the PC (which is acting as the SSL client). But why?

I had noticed one other funny thing about the trace comparisons. The server hello packet left as a packet of length 1518 bytes, but arrived, apparently, as two packets, one of length 778 bytes, the other of 770 bytes, i.e., it was fragmented. That should be OK. I don’t think the don’t fragment flag was set (have to check this). But it got me to wondering. Because the server certificate packet was also on the large side – 1460 bytes.

Regardless, I had enough ammo to go to the networking group with, which I did. It was like, “Oh yeah, our Telecom provider (let’s call it “CU”) implemented GETVPN around that time.” And further discussion revealed suspected other problems related to this change with MTU, etc. In fact they dredged up this nice description of the pitfalls that await GET VPN implementations from the Cisco deployment guide:

3.8 Designing Around MTU Issues Because of additional IPsec overhead added to each packet, MTU related issues are very common in IPsec deployments, and MTU size becomes a very important design consideration. If MTU value is not carefully selected by either predefining the MTU value on the end hosts or by dynamically setting it using PMTU discovery, the network performance will be impacted because of fragmentation and reassembly. In the worst case, the user applications will not work because network devices might not be able to handle the large packets and are unable to fragment them because of the df-bit setting. Some of the scenarios which can adversely affect traffic in a GET VPN environment and applicable mitigation techniques are discussed below. LAN MTU of 1500 – WAN MTU 44xx (MPLS) In this scenario, even after adding the 50-60 byte overhead, MTU size is much less than the MTU of the WAN. The MTU does not affect GET VPN traffic in any shape or form. LAN MTU of 1500 – WAN MTU 1500 In this scenario, when IPsec overhead is added to the maximum packet size the LAN can handle (i.e. 1500 bytes) the resulting packet size becomes greater than the MTU of the WAN. The following techniques could help reduce the MTU size to a value that the WAN infrastructure can actually handle. Manually setting a lower MTU on the hosts By manually setting the host MTU to 1400 bytes, IP packets coming in on the LAN segment will always have 100 extra bytes for encryption overhead. This is the easiest solution to the MTU issues but is harder to deploy because the MTU needs to be tweaked on all the hosts. TCP Traffic Configure ip tcp adjst-mss 1360 on GM LAN interface. This command will ensure that resulting IP packet on the LAN segment is less than 1400 bytes thereby providing 100 bytes for any overhead. If the maximum MTU is lowered by other links in the core (e.g. some other type of tunneling such as GRE is used in the core), the adjust-mss value can be lowered further. This value only affects TCP traffic and has no bearing on the UDP traffic. 3.8.1.1 Host compliant with PMTU discovery For non-TCP traffic, for a 1500 packet with DF bit set, the GM drops the packet and send ICMP message back to sender notifying it to adjust the MTU. If sender and the application is PMTU compliant, this will result in a packet size which can successfully be handled by WAN. For example, if a GM receives a 1500 byte IP packet with the df-bit set and encryption overhead is 60 bytes, GM will notify the sender to reduce the MTU size to 1440 bytes. Sender will comply with the request and the resulting WAN packets will be exactly 1500 bytes

I don’t claim to understand all those scenarios, but it shouts pretty loudly. Watch out for problems with packets of size 1400 bytes or larger!

Why would they want GET VPN? To encrypt the HQ communication over the WAN. So the idea is laudable, but the execution lacking.

Case: understood but not yet closed! The fix is not yet in…

To be continued…

Categories
Admin Uncategorized Web Site Technologies

The IT Detective Agency: Internet Explorer cannot display https web page

Intro
It’s a weird thing when a site that’s always worked for you suddenly stops working. Such was the behaviour observed today by a friend of mine. He could no longer access an old Oracle Enterprise Manger web site, and just this one web site. All other web sites were fine. What’s up, he asked?

The details
Well we tried this and that, reloading pages, re-starting the PC, and tests to make sure the DNS resolution was occurring correctly. It was. We logged on to his PC as another user to try the access. This tests registry settings specific to his userid. I thought that would work, but it did not. I tried the web site from my PC – worked great! The people around him also worked great. Give up? Never. For him the error page popped up quickly, by the way. He didn’t have Firefox, but I was tempted to have him install it and try that. I was pretty sure it would have worked.

He did have putty. We used his putty to telnet access the server on the same port as the https listener – we could connect, though of course we couldn’t really do anything beyond that. So there was no firewall-type issue.

We tried other https sites from his browser – no problems with those.

I was hemming and hawing and muttering something about publisher certificate revocation when that prompted someone to recall that a related Microsoft setting adjustment had come out just last week. It requires that web sites have certificate lengths of at least 1024 bits. For discussion see this article. Could that be related to this problem he asks me? Could it? Could it? I quickly checked the key bit length of the server certificate on the OEM server. Yup. 512 bits. Then I checked the key length of another OEM server that he could still access. Yup. 1024 bits. It was a newer installation so that actually does make sense. This popular article about ciphers also mentions how to use openssl to find the key length (openssl s_client …).

Why could I and others do the access? Simple. We hadn’t (yet) received the patch. When we do, we won’t be able to access it either.

So whose fault is it, anyways? I kind of agree with Microsoft on this one. If you’re still running web sites with 512 bit-length keys, it’s time to change your certificates to something longer and more secure. After all on the Internet we’re required to have 2048 bit-length keys for almost two years now.

Problem is, it’s not so obvious how to change this key in OEM. It may be buried in a java keystore.

Case: almost closed!

Conclusion
With a little help from my friends I solved the case of the browser with the message Internet Explorer cannot display the webpage. Like all such problems it was quite puzzling for awhile, but once understood all the symptoms made sense and could be explained rationally.

Categories
Admin Consumer Tech

Useful Links – Technical Resources

Intro
From time-to-time I come across sites that really help out, and ones that true, might be found quickly in a Google search, but some offer capabilities you may not have even realized existed.

The links
So without further ado, here they are. They need some organizing yet!

– Geographic location finder for any IP address. There are zillions of them out there. Some are overly aggressive with advertizing so be wary. Here’s a really good one that shows your IP and perceived location without the ads: https://www.vpnmentor.com/tools/ipinfo/

although I normally use https://ipinfo.io/ .

Formula plotter (math) – very nice

https://www.desmos.com/calculator

Naming Convention

ISO 2-letter country codes

Everyone is pretty familiar with these two-letter codes. Theer is also a 3-letter convention as well. But what is much less well known is the UN way to designate almost every single location on the planet with a 3-letter code:

UNECE 3-letter place/city/town codes

Click on a country and it will show you all the 3-letter codes.

This is not the same as the IATA airport codes! It is more extensive.

Networking
Check your speed by downloading one of the large files from ftp://speedtest.tele2.net/ such as ftp://speedtest.tele2.net/50MB.zip .

Reputation links
– Reputation information for any mail server. Just plug in the IP. Will also show what blacklists the server is on and the reputation of others servers on that segment. I think it’s run by Cisco. http://www.senderbase.org/
– Reputation of any URL, sponsored by Bluecoat, makers of the free edition K9 Webfilter. http://sitereview.bluecoat.com/sitereview.jsp

Webmaster links
– archive.org will show what your site looked like in the past, sometimes even years in the past.
Now retired: – www.alexa.com, to see how unpopular your web site is. Retired by Amazon May 2022! Not sure if there is an equivalent…
– Say you see a photo on a web site and you want to know if it was used elsewhere, or if the site cropped it

– Open source information and knowledgeable discussion. http://www.softpanorama.org/

-Is this web site(s) up?

https://httpstatus.io/

Security

-0 Patch (I learned of this 12/2023) may be an option to get continued securtity coverage when your Windows 10 support runs out in October 2025. https://0patch.com/
-This is a very good article discussing the Windows 10 to Windows 11 upgrade options for those whose hardware doesn’t support Windows 11: https://www.msn.com/en-us/news/technology/when-windows-10-support-runs-out-you-have-5-options-but-only-2-are-worth-considering/ar-AA1lFA82?cvid=4d6aafa9d73448dbc024424b335d1451&ocid=winp2fptaskbarent&ei=15

-Good discussion of ongoing security matters, though too heavily weighted on Social networking. Articles daily with interesting reader comments. http://nakedsecurity.sophos.com/
– Web site malware checker. http://sitecheck.sucuri.net/scanner/. I’ve used it once. I guess it’s OK, but probably not nearly as thorough as commercial products. I guess it doesn’t look for vulnerabilities, just malware.
– urlquery.net, a service for analyzing and detecting web-based malware

IT
– most under-appreciated blog by an IT professional, networking concentration. http://drjohnstechtalk.com/blog/ Hey, I can toot my own horn!
– Screen-sharing application join.me
– Ookla’s Down Detector gives the status of hundreds of major web sites. https://downdetector.com/

Home User
– Best broadband speed test. https://fast.com/ It’s simple, but doesn’t shower you with aggressive ads.

– speedtest also looks good. https://speedtest.net/

Another better alternative to speedtest.net (again no aggressive ads) is https://speedtest.expereo.com/

I think nperf.com may be the best, however. https://nperf.com/

Prevent your neighbor’s WiFi SSIDs from being offered to you: https://www.howtogeek.com/331816/how-to-block-your-neighbors-wi-fi-network-from-appearing-on-windows/

– Permanent HTTP site – will never be https: http://neverssl.com/. It can be hard to find a site that doesn’t run SSL these days so this can be useful, e.g., when you want to sign on to guest Wifi such as hotels offer.

Find an Android phone: http://google.com/android/find

Programmers

Here’s a really good introduction to REST API by Microsoft: https://docs.microsoft.com/en-us/azure/architecture/best-practices/api-design

Computer and Linux Hobbyist
– Cheapest low-power Linux computer. http://www.raspberrypi.org/. At about $35 this looks really interesting for the hobbyist.

Towards a Responsible and Ethical Internet
– a search engine that doesn’t track! I just heard about this on Marketplace. Unfortunately it’s not as sophisticated as Google with type-aheads. I checked a few searches that should bring up some of my blog posts and – they did – so it must have some reach into obscure web sites duckduckgo.com

And another I’ve just learned about: https://www.startpage.com/

Security

Check out how many times yuor email address and other info about you have been leaked on the Internet. It’s enlightening and also depressing: https://haveibeenhacked.com/

A victim of Identity Theft?

Go to this checklist maintained by the Federal Trade Commission: https://IdentityTheft.gov/

Categories
Admin Internet Mail Linux Network Technologies

The IT Detective Agency: mail server went down with an old-school problem

Intro
I got a TXT from my monitoring system last night. I ignored it because I knew that someone was working on the firewall at that time. I’ve learned enough about human nature to know that it is easy to ignore the first alert. So I’ve actually programmed HP SiteScope alerts to send additional ones out after four hours of continuous errors. When I got the second one at 9 PM, I sprang into action knowing this was no false alarm!

The details
Thanks to a bank of still-green monitors I could pretty quickly rule out what wasn’t the matter. Other equipment on that subnet was fine, so the firewall/switch/router was not the issue. Then what the heck was it? And how badly was it impacting mail delivery?

This particular server has two network interfaces. Though the one interface was clearly unresponsive to SMTP, PING or any other protocol, I hadn’t yet investigated the other interface, which was more Internet-facing. I managed to find another Linux server on the outer network and tried to ping the outer interface. Yup. That worked. I tried a login. It took a whole long time to get through the ssh login, but then I got on and the server looked quite normal. I did a quick ifconfig – the inner interface listed up, had the right IP, looked completely normal. I tried some PINGs from it to its gateway and other devices on the inner network. Nothing doing. No PINGs were returned.

I happened to have access to the switch. I thought maybe someone had pulled out its cable. So I even checked the switch port. It showed connected and 1000 mbits, exactly like the other interface. So it was just too improbable that someone pulled out the cable and happened to plug another cable from another server into that same switch port. Not impossible, just highly improbable.

Then I did what all sysadmins do when encountering a funny error – I checked the messages file in /var/log/messages. At first I didn’t notice anything amiss, but upon closer inspection there was one line that was out-of-place from the usual:

Nov  8 16:49:42 drjmailgw kernel: [3018172.820223] do_IRQ: 1.221 No irq handler for vector (irq -1)

Buried amidst the usual biddings of cron was a kernel message with an IRQ complaint. What the? I haven’t worried about IRQ since loading Slackware from diskettes onto my PC in 1994! Could it be? I have multiple ways to test when the interface died – SiteScope monitoring, even the mail log itself (surely its log would look very different pre- and post-problem.) Yup.

That mysterious irq error coincides with when communication through that interface stopped working. Oh, for the record it’s SLES 11 SP1 running on HP server-class hardware.

What about my mail delivery? In a panic, realizing that sendmail would be happy as a clam through such an error, I shut down its service. I was afraid email could be piling up on this server, for hours, and I pride myself in delivering a faultless mail service that delivers in seconds, so that would be a big blow. With sendmail shut down I knew the backup server would handle all the mail seamlessly.

This morning, in the comfort of my office I pursued the answer to that question What was happening to my mail stream during this time? I knew outbound was not an issue (actually the act of writing this down makes me want to confirm that! I don’t like to have falsehoods in writing. Correct, I’ve now checked it and outbound was working.) But it was inbound that really worried me. Sendmail was listening on that interface after all, so I didn’t think of anything obvious that would have stopped inbound from being readily accepted then subsequently sat on.

But such was not the case! True, the sendmail listener was available and listening on that external interface, but, I dropped a hint above. Remember that my ssh login took a long while? That is classic behaviour when a server can’t communicate with its nameservers. It tries to do a reverse lookup on the ssh client’s source IP address. It tried the first nameserver, but it couldn’t communicate with it because it was on the internal network! Then it tried its next nameserver – also a no go for the same reason. I’ve seen the problem so often I wasn’t even worried when the login took a long time – a minute or so. I knew to wait it out and that I was getting in.

But in sendmail I had figured that certain communications should never take a long time. So a long time ago I had lowered some of the default timeouts. My mc file in the upstream server contains these lines:

...
dnl Do not use RFC1413 identd. p 762  It requires another whole in the F/W
define(`confTO_IDENT',`0s')dnl
dnl Set more reasonable timeouts for SMTP commands'
define(`confTO_INITIAL',`1m')dnl
define(`confTO_COMMAND',`5m')dnl
define(`confTO_HELO',`1m')dnl
define(`confTO_MAIL',`1m')dnl
define(`confTO_RCPT',`1m')dnl
define(`confTO_RSET',`1m')dnl
define(`confTO_MISC',`1m')dnl
define(`confTO_QUIT',`1m')dnl
...

I now think that in particular the HELO timeout (TO_HELO) of 60 seconds saved me! The upstream server reported in its mail log:

Timeout waiting for input from drjmailgw during client greeting

So it waited a minute, as drjmailgw tried to do a reverse lookup on its IP, unsuccessfully, before proceeding with the response to HELO, then went on to the secondary server as per the MX record in the mailertable. Whew!

More on that IRQ error
Let’s go back to that IRQ error. I got schooled by someone who knows these things better than I. He says the Intel chipset was limited insofar as there weren’t enough IRQs for all the devices people wanted to use. So engineers devised a way to share IRQs amongst multiple devices. Sort of like virtual IPs on one physical network interface. Anyways, on this server he suspects that something is wrong with the multipath driver which is loaded for the fiber channel host adapter card. In fact he noticed that the network interface flaked out several times previous to this error. Only it came back after some seconds. This is the server where we had a very high CPU when the SAN was being heavily used. The SAN vendor checked things out on their end and, of course, found nothing wrong with the SAN equipment. We actually switched from SAN to tmpfs after that. But we didn’t unload the multipath driver. Perhaps now we will.

Feb 22 Update
We haven’t seen the problem in over three weeks now. See my comments on what actions we took.

Conclusion
Persistence, patience and praeternatural practicality paid off in this perplexing puzzle!

Categories
Admin

Nmap: Swiss Army Knife of network utilities

Intro
I just wanted to put in a plug for nmap. It’s a very useful tool for any network specialist. I show a use case that came up today.

The details
While cleaning up DNS entries I came across a network segment that didn’t seem to have any active network devices, at least not after I cleaned up the old DNS entries for inactive devices.

So I wanted to see if I could tell the networking tech that this subnet is unused and could be allocated for some other purpose.

I remembered using nmap years ago, and that it was a powerful tool for this kind of thing. What I had in mind was to ping every IP on this segment to see if there were any undocumented hosts.

As it turns out I didn’t even have it installed, but it was very easy to get:

On SLES:
$ zypper install nmap

On CentOS:
$ yum install nmap

It doesn’t get easier than that!

A quick review of the man page showed that what I wanted was indeed possible. Here’s the syntax for a systematic PING sweep through a subnet:

$ nmap −sP 10.101.192.0/24

Starting Nmap 4.75 ( http://nmap.org ) at 2012-11-08 10:21 EST
Host 10.101.192.5 appears to be up.
Host 10.101.192.10 appears to be up.
Host 10.101.192.151 appears to be up.
Host 10.101.192.152 appears to be up.
Host 10.101.192.153 appears to be up.
Nmap done: 256 IP addresses (5 hosts up) scanned in 1.28 seconds

Now I know that subnet has rogue or at least undocumented hosts and is not unused!

The original usage for nmap, at least for me, was to fingerprint an unknown host:

$ nmap −A −T4 ossim.drj.com

Interesting ports on ossim.drj.com (10.22.235.19):
Not shown: 996 filtered ports
PORT    STATE  SERVICE  VERSION
22/tcp  open   ssh       (protocol 2.0)
80/tcp  open   http     Apache httpd
|_ HTML title: 302 Found
443/tcp open   ssl/http Apache httpd
|_ HTML title: Site doesn't have a title.
514/tcp closed shell
1 service unrecognized despite returning data. If you know the service/version, please submit the following fingerprint at http://www.insecure.org/cgi-bin/servicefp-submit.cgi :
SF-Port22-TCP:V=4.75%I=7%D=4/25%Time=51793070%P=x86_64-suse-linux-gnu%r(NU
SF:LL,29,"SSH-2\.0-OpenSSH_5\.5p1\x20Debian-6\+squeeze2\r\n");
Device type: WAP|general purpose|PBX
Running (JUST GUESSING) : Linux 2.6.X|2.4.X (93%), Vodavi embedded (85%)
Aggressive OS guesses: OpenWrt 7.09 (Linux 2.6.22) (93%), OpenWrt 0.9 - 7.09 (Linux 2.4.30 - 2.4.34) (92%), Linux 2.6.20.6 (89%), Linux 2.6.21 (Slackware 12.0) (88%), OpenWrt 7.09 (Linux 2.6.17 - 2.6.21) (88%), Linux 2.6.19 - 2.6.21 (88%), Linux 2.6.22 (Fedora 7) (88%), Vodavi XTS-IP PBX (85%), Linux 2.6.22 (85%)
No exact OS matches for host (test conditions non-ideal).
 
TRACEROUTE (using port 21/tcp)
HOP RTT    ADDRESS
1   0.87   10.202...
2   0.38   ...
3   0.57   ...
4   6.10   ...
5   114.64 ...
6   119.79 ...
7   103.43 ossim.drj.com (10.22.235.19)
 
OS and Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 35.02 seconds

Now that was kind of an unusual example in which nmap wasn’t too sure about the OS. Usually you get a positive ID of some sort. That’s a chatty server and I’m still not sure what it is.

Nmap can be used for nasty things and in an impolite way, network-wise. So be careful to tone it down. Target your hosts and protocols with care. It can guess what OS a host is running, what ports are open, all kinds of amazing stuff.

I checked PING and did not see a built-in capability to do a PING sweep, though it would have been easy enough to script it. That was my backup option.

Once I had to check on a single UDP port being open on port 80 for a webcast client called Kontiki (they call this protocol KDP). No other ports were open, necessitating the -PN switch.

Single UDP port check
$ nmap −PN −sU -p 80 29.239.11.4

Starting Nmap 4.75 ( http://nmap.org ) at 2013-07-23 13:59 EDT
Interesting ports on 29.239.11.4:
PORT   STATE         SERVICE
80/udp open|filtered http
 
Nmap done: 1 IP address (1 host up) scanned in 2.15 seconds

Three TCP ports checked
$ nmap ‐PN ‐sS ‐p 445,28080,28443 12.92.96.37

Results of that scan

Starting Nmap 5.51 ( http://nmap.org ) at 2017-04-13 09:18 EDT
Nmap scan report for 12.92.96.37
Host is up.
PORT      STATE    SERVICE
445/tcp   filtered microsoft-ds
28080/tcp filtered unknown
28443/tcp filtered unknown
 
Nmap done: 1 IP address (1 host up) scanned in 3.06 seconds

“filtered” means there were no reply packets to my SYN packets, usually a sign of an intervening firewall dropping packets. I’m not sure why it describes the host as “up” when actually it is down or behind a firewall. A state of closed indicates that a RST packet was received in reply, indicating that the port is closed on the host itself and it wasn’t a firewall that prevented the test from succeeding. the third possible state is open, which of couse means that it replied with a SYN-ACK to that probe on that port.

To fix the source port add a -g to the above command. E.g., some firewalls have trouble with permitting inbound UDP packets from port 53 so to test for that you throw in a -g 53 and try some random high destination port.

I needed to spoof another host’s IP address and send a simple PING (ICMP request) to diagnose what was going wrong with the reply. Here’s how I did that:

$ nmap −PE −e eth0 −S 10.42.48.1 10.1.145.10

But then I realized what I really needed to do to emulate the problem is to send a single TCP SYN packet to port 8081, without the accompanying ICMP probes that nmap is wont to throw in there first. Here’s how I built up that probe:

$ nmap −PN −sS −p 8081 −−max-retries 0 −e eth0 −S 10.42.48.1 10.1.145.10

Check if a web server is running
$ nmap −PN −p T:80,443 drjohnstechtalk.com
This will check both ports 80 and 443. It doesn’t execute any HTTP protocol. It’s just a quick and dirty test.

don’t have nmap but have something like netcat instead? A good tcp port check with netcat is
netcat -vzw5 <host> <port>. Here’s an actual example.

$ netcat ‐vzw5 drjohnstechtalk.com 443

DNS mismatch
drjohnstechtalk.com [50.17.188.196] 443 (https) open

Conclusion
Nmap is a great network tool that every IT network tech should be familiar with.

References and related
A more capable and complicated packet generation tool is scapy. I describe it in this blog post.

A simpler network for Windows (simpler than nmap for Windows) is PortQry. It was created by Microsoft.

Categories
Admin Network Technologies

Firewall is a significant drag on download speeds

Intro
This post might be a restatement of the obvious to some, but I thought it was noteworthy enough to measure and mention this affect. I was twiddling my thumbs during a long sftp upload when I began to notice these transfers I was doing went really quickly between some servers, and not so much between others. How to control for all variables except the ones I wanted to vary? How to measure things in such a way that an overworked network technician with vested interests in saying the status quo is “good enough” will listen to you? These are things I wrestled with.

The details

To be continued…

Categories
Admin Network Technologies

DIY Home Power Monitoring Solution – Perfect for Sandy

Intro
My recent experience losing power thanks to Sandy has gotten me thinking. How can I know when my power’s on? Or what if it gets shut off again, which by the way actually happened to me? I realized that I have all the pieces in place already and merely need to take advantage of the infrastructure already out there.

The details
I started with:

  • a working smartphone
  • an enterprise monitoring system
  • a SoHo router in my home

And that’s pretty much it!

The smartphone doesn’t really matter, as long as it receives emails. I guess a plain old cellphone would work just as well – the messages can be sent as text messages.

For enterprise monitoring I like HP SiteScope because it’s more economical than hard-core systems. I wrote a little about it in a previous post. Nagios is also commonly used and it’s open source, meaning free. Avoid Zabbix at all costs. Editor’s note: OK. I’ve changed my mind about Zabbix some seven years later. It’s still confusing as heck, but now I’m using it and I have to admit it is powerful. See this write-up.

A good SoHo router is Juniper SSG5. It extends the enterprise LAN into the home. You can carve up a Juniper router like that and provide a Home network, Work network, Home wireless and Work wireless. It’s great!

The last requirement is the key. The SoHo router at my house is always on, and so the enterprise LAN is always available, as long as I have power. Get it? I defined a simple PING monitor in SiteScope to ping my SoHo router’s WAN interface, which has a static private IP address on the enterprise LAN. If I can’t ping it, I’ve lost power, and use my monitoring system to send an email alert to my smartphone. When, or in the case of Sandy’s interminable outage, if, power ever comes back on, I send another alert letting me know that as well. If you’re not using SiteScope make sure you send several PINGs. A PING can be lost here or there for various reasons. siteScope sends out five PINGs at a regular interval, as a guideline.

Alternative
Of course if I had a business-class DSL or cable modem service with a dedicated IP I could have just PINGed that, but I don’t. With regaulr consumer grade service your IP can and will change from time to time, and using a dynamic DNS protocol (like dyndns) to mask that problem is a bit tricky.

Yes, if you call the power company they offer to call you back when your power is restored, but I like my monitor better. It tells me when things go off as well as on.

Conclusion
This is my necessity-is-the-mother-of-invention moment. Thank you, Sandy!