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Old 11th February 2020, 12:53 PM   #1
bruto
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Windows 10 DNS flush

Just thought I'd throw out there, that if your DNS searching has slowed way down, it seems that flushing that cache will help. The command, running as computer administrator, and only in "CMD" not in the power shell, is "ipconfig/flushdns" .

My computer had slowed down to an average of about ten seconds for every DNS resolution (including embedded links), and it's back to normal now.
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Old 11th February 2020, 05:54 PM   #2
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Actually, flushing the cache should generally make it take longer...unless DNS addresses for URLs you’re going to have changed in the last few hours.


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Old 11th February 2020, 07:22 PM   #3
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Originally Posted by Hellbound View Post
Actually, flushing the cache should generally make it take longer...unless DNS addresses for URLs you’re going to have changed in the last few hours.


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So I would have thought, and that's why I thought it worth mentioning. Apparently if the cache gets bloated, or maybe corrupted, all DNS resolutions become very slow. The best times were rarely better than five seconds, and often long enough just to time out.
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Old 11th February 2020, 07:27 PM   #4
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If you'd like something that will make that not really an issue, and are relatively computer savvy I posted a thread about a pihole. It'll change the way you DNS, including blocking ads.
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Old 11th February 2020, 08:05 PM   #5
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Did you manually set your DNS or are you using what your ISP comes back with?
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Old 11th February 2020, 09:48 PM   #6
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Originally Posted by Donal View Post
Did you manually set your DNS or are you using what your ISP comes back with?
I tried it both ways. Right now I'm back on the automatic DNS. I tried various others that were said to be faster, and none made a significant difference.

I'm about to go on a trip (sans computer) so won't be able to test different DNS servers, including pihole, for a while. But at this point at least the difference will likely be parts of a second, instead of parts of a minute.
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Old 11th February 2020, 10:14 PM   #7
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Originally Posted by bruto View Post
I tried it both ways. Right now I'm back on the automatic DNS. I tried various others that were said to be faster, and none made a significant difference.

I'm about to go on a trip (sans computer) so won't be able to test different DNS servers, including pihole, for a while. But at this point at least the difference will likely be parts of a second, instead of parts of a minute.
I switched from my ISP's DNS to Google DNS, not for speed, but because my ISP's DNS would send me to the ISP's crappy search engine/ad farm site anytime I tried to access a host it couldn't resolve. I don't think there's significant difference in speed, but, if I attempt to access a non-existent URL (which might due to a typo, or an obsolete link on another site), I'd prefer that it tell me it's a bad URL rather than send me to a junk site that I have no interest in visiting.
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Old 11th February 2020, 10:51 PM   #8
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Originally Posted by CORed View Post
I switched from my ISP's DNS to Google DNS, not for speed, but because my ISP's DNS would send me to the ISP's crappy search engine/ad farm site anytime I tried to access a host it couldn't resolve. I don't think there's significant difference in speed, but, if I attempt to access a non-existent URL (which might due to a typo, or an obsolete link on another site), I'd prefer that it tell me it's a bad URL rather than send me to a junk site that I have no interest in visiting.
As it happens, my ISP assigns the Google DNS, which is why I'm not worrying too much about changing it.
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Old 12th February 2020, 09:11 AM   #9
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Originally Posted by CORed View Post
I switched from my ISP's DNS to Google DNS, not for speed, but because my ISP's DNS would send me to the ISP's crappy search engine/ad farm site anytime I tried to access a host it couldn't resolve. I don't think there's significant difference in speed, but, if I attempt to access a non-existent URL (which might due to a typo, or an obsolete link on another site), I'd prefer that it tell me it's a bad URL rather than send me to a junk site that I have no interest in visiting.
Cloudflare is really good, free, and fast.

1.1.1.1 or 1.0.0.1
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Old 12th February 2020, 01:03 PM   #10
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I use cloudflare, it made a big difference on my home network
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Old 13th February 2020, 02:05 AM   #11
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By the weirdest co-incidence, I had to do this at work today.

And... counter-intuitively, it really did speed everything up.

Things were going so badly that log-ins to some applications were timing out.

After flushing the cache, I'm back to instant log-ins.
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Old 13th February 2020, 07:09 AM   #12
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Originally Posted by Donal View Post
I use cloudflare, it made a big difference on my home network
Ever since I changed my home network to DNS over HTTPS it moves like lightning. I've really become a fan of the PiHole.
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Old 13th February 2020, 07:30 AM   #13
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Am I alone in thinking that this sounds like a bug in Windows ?

Why on earth should it be necessary for the end-user to resort to such shenanigans ? I don't know any other systems where this would be required.

In other words, Microsoft should fix their buggy software.


Sigh ...
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Old 13th February 2020, 07:49 AM   #14
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Its more the crappy consumer grade networking gear we get on top of the crappy consumer grade internet service.

I get "1 Gbps" service from Verizon and am lucky to see it break 700 Mbps. They even say in the fine print the max is really 800 Mbps
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Old 13th February 2020, 08:10 AM   #15
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Originally Posted by Donal View Post
Its more the crappy consumer grade networking gear we get on top of the crappy consumer grade internet service.

I get "1 Gbps" service from Verizon and am lucky to see it break 700 Mbps. They even say in the fine print the max is really 800 Mbps

I don't know what this is a response to, but what on earth does that have to do with DNS handling on Windows systems?
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Old 13th February 2020, 11:25 AM   #16
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I don't think it's odd to have to clear out DNS in any system. They have commands and the ability to do it in every flavor of Linux, Apple, and Windows.
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Old 13th February 2020, 11:35 AM   #17
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I’m trying to figure out why this might happen. I’ve worked on windows systems since WfW, and only had to flush the cache for things like changing IP addresses. Makes me wonder what is actually causing the problem.

Even then, the local DNS cache is transient; it should clear entries in it as they expire.

I’d suspect this is a symptom of some other issue. Try doing an ipconfig /displaying (needs admin) next time to see what’s in your cache; might give an idea of what’s going on.


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Old 13th February 2020, 11:38 AM   #18
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Originally Posted by Hellbound View Post
I’m trying to figure out why this might happen. I’ve worked on windows systems since WfW, and only had to flush the cache for things like changing IP addresses. Makes me wonder what is actually causing the problem.

Even then, the local DNS cache is transient; it should clear entries in it as they expire.

I’d suspect this is a symptom of some other issue. Try doing an ipconfig /displaying (needs admin) next time to see what’s in your cache; might give an idea of what’s going on.


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ipconfig /displaydns
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Old 13th February 2020, 12:17 PM   #19
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Originally Posted by plague311 View Post
ipconfig /displaydns


Doh! Thank you. Autocorrect is out to ruin me


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Old 13th February 2020, 01:13 PM   #20
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Originally Posted by Hellbound View Post
Doh! Thank you. Autocorrect is out to ruin me


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I saw the phone tag and figured LoL
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Old 13th February 2020, 11:42 PM   #21
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Originally Posted by Blue Bubble View Post
Am I alone in thinking that this sounds like a bug in Windows ?

Why on earth should it be necessary for the end-user to resort to such shenanigans ? I don't know any other systems where this would be required.

In other words, Microsoft should fix their buggy software.


Sigh ...
There's bugs in everything, just about. They will fix it eventually.
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Old 14th February 2020, 02:32 AM   #22
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Originally Posted by plague311 View Post
I don't think it's odd to have to clear out DNS in any system. They have commands and the ability to do it in every flavor of Linux, Apple, and Windows.
We clearly have very different expectations (note: I started programming 13 years before you were born, so that just might have something to do with my expectations ).

Since flushing the DNS cache requires "sudo" or "administrator" access, how do you think "vanilla" users are going to achieve that ? Hint: they won't.

Bottom line: I think it's ridiculous to expect end users to have to resort to such shenanigans. I have never once had to flush DNS caches (the only Windows systems I ever have to use are company-provided ones, and pretty well locked down - I do have administrator access though).
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Old 14th February 2020, 08:52 AM   #23
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Originally Posted by Blue Bubble View Post
We clearly have very different expectations (note: I started programming 13 years before you were born, so that just might have something to do with my expectations ).
Maybe. I would venture to say the average end user is probably significantly more knowledgeable than they were 13 years before I was born as well.

Originally Posted by Blue Bubble View Post
Since flushing the DNS cache requires "sudo" or "administrator" access, how do you think "vanilla" users are going to achieve that ? Hint: they won't.
Do a lot of end users use CLI? Admittedly, I don't use Linux as it's not something that's come into play for me. It's an option though, which means linux has had to do it at least a few times.

I'm way more prone to using the command because, by nature, I'm a network administrator or whatever you want to call us. I repeatedly make DNS changes that are easier to manage by flushing DNS's, though I run all my commands remotely and the end user doesn't even see it happening.

Originally Posted by Blue Bubble View Post
Bottom line: I think it's ridiculous to expect end users to have to resort to such shenanigans. I have never once had to flush DNS caches (the only Windows systems I ever have to use are company-provided ones, and pretty well locked down - I do have administrator access though).
I expect users of any device to know how to properly manage their machines. I'm not a mechanic, but I know I need oil changed. I'm not an HVAC guy, but I know my system needs to get maintained.

I don't think anyone expects the end user to resort to it. If they have a problem, take it to a professional. If they can fix it, they should.
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Old 14th February 2020, 09:10 AM   #24
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Originally Posted by Blue Bubble View Post
We clearly have very different expectations (note: I started programming 13 years before you were born, so that just might have something to do with my expectations ).

Since flushing the DNS cache requires "sudo" or "administrator" access, how do you think "vanilla" users are going to achieve that ? Hint: they won't.

Bottom line: I think it's ridiculous to expect end users to have to resort to such shenanigans. I have never once had to flush DNS caches (the only Windows systems I ever have to use are company-provided ones, and pretty well locked down - I do have administrator access though).
I agree that the need and the procedure are poorly implemented in Windows, so what else is new,right? Aside from everything else,as comments in this thread suggest, it's also counterintuitive, the kind of thing for which many users probably either pay some geek to fix completely, refresh windows, or just toss the thing. .

I'm far from a computer wizard, but did make sure I knew how to get admin. access, and even so it was difficult, because Windows has two different but similar command lines, and only the older "cmd" recognizes the required command.

I imagine, or at least hope, that the need for this is pretty rare, but certainly there seem to be plenty of people complaining of various Windows slowdowns, and it seems like poor design that the usual recommendation for many is to give it all up and reload Windows.

Apologies if this message is badly typed. I'm using a pretty nasty Android tablet while traveling. Co plain as one might about Windows, if you're used to it and DOS, Android is awful. I never know what button to push or what submerged u to go to to do anything.
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Old 14th February 2020, 12:30 PM   #25
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Windows actually has the old cmd processor and the PowerShell command line.

Both will recognize the command. PowerShell back-coverts to cmd if it’s not a PowerShell native command.

The PowerShell equivalents are:
Clear-DnsClientCache
Get-DnsClientCache

Neither requires admin access, either PowerShell or cmd.


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Old 14th February 2020, 12:57 PM   #26
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Originally Posted by Hellbound View Post
Windows actually has the old cmd processor and the PowerShell command line.

Both will recognize the command. PowerShell back-coverts to cmd if it’s not a PowerShell native command.

The PowerShell equivalents are:
Clear-DnsClientCache
Get-DnsClientCache

Neither requires admin access, either PowerShell or cmd.


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Not to doubt what you say, but when I asked powershell for "ipconfig" it just said no such thing existed and did not send me to cmd. Since none of the online references mentioned the powershell alternative it was as good as nonexistent unless I asked for help and figured it out. It was easier just to go to the command prompt. I've already forgotten whether I tried the command without logging in as admin, but in any case it wasn't as straightforward as it might have been.
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Old 14th February 2020, 01:06 PM   #27
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ipconfig works for me in powershell. Same output as from cmd.exe.
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Old 14th February 2020, 01:20 PM   #28
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Originally Posted by DallasDad View Post
ipconfig works for me in powershell. Same output as from cmd.exe.
How odd. I don't have a windows machine handy at the moment but I am pretty sure mine did not, as I assumed at first that I had either mistyped it or that it required admin mode, and I got the same response each time. I also tried just typing the command without parameters and asking for help. All I remember now is that in command and admin mode it worked the first time.

Fortunately, if you do enough searching someone somewhere has usually found a fix.
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Old 14th February 2020, 05:35 PM   #29
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Originally Posted by Hellbound View Post
I’m trying to figure out why this might happen. I’ve worked on windows systems since WfW, and only had to flush the cache for things like changing IP addresses. Makes me wonder what is actually causing the problem.

Even then, the local DNS cache is transient; it should clear entries in it as they expire.

I’d suspect this is a symptom of some other issue. Try doing an ipconfig /displaying (needs admin) next time to see what’s in your cache; might give an idea of what’s going on.


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It should, and when changes are made to our local network infrastructure, everything in the cache is set to expire in 10 minutes (or suchlike) so that old things don't persist and cause problems.

In my case there were a bunch of domain controllers that had been retired, and their entries had persisted in my local cache.

The symptoms of the error were quite weird, while logging in to the main application that I support, I'd often get "user doesn't exist, or account is locked" style errors.

I could see on the server logs that my client was not communicating with the server at all.

So I was sometimes being directed to non-existent domain controllers, but most of the time, not.

I had a good poke around on Friday to find out if a recent patch had caused the issue on my client machine, but couldn't find anyone talking about the problem on the net.

The general 'ipconfig /flushdns' conversation seems to be have been around for a long time.
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Old 14th February 2020, 05:36 PM   #30
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That’s odd, wonder if there’s a setting somewhere to disable cmd parsing in PowerShell? Have to look for it now.

I’ve always run cmd commands in PowerShell, even used them in scripts (have one running at work now using dsget, for example).


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Old 14th February 2020, 05:40 PM   #31
Hellbound
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Originally Posted by novaphile View Post
It should, and when changes are made to our local network infrastructure, everything in the cache is set to expire in 10 minutes (or suchlike) so that old things don't persist and cause problems.



In my case there were a bunch of domain controllers that had been retired, and their entries had persisted in my local cache.



The symptoms of the error were quite weird, while logging in to the main application that I support, I'd often get "user doesn't exist, or account is locked" style errors.



I could see on the server logs that my client was not communicating with the server at all.



So I was sometimes being directed to non-existent domain controllers, but most of the time, not.



I had a good poke around on Friday to find out if a recent patch had caused the issue on my client machine, but couldn't find anyone talking about the problem on the net.



The general 'ipconfig /flushdns' conversation seems to be have been around for a long time.


If it was domain controllers, then yeah, I can see that causing issues. In modern AD domains, the DNS is integrated with AD, so I could be issues with that. Especially considering both the computer account and the user account have to authenticate.

But yeah, cache issues should only happen when things change. With DCs I’d expect other issues just related to the fact that they’d changed, though (possibilities for certificate problems and similar there, too...options for error abound!)


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Old 14th February 2020, 06:29 PM   #32
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Of course I don't quite know what my problem was but wonder if the problem was not inherent in the cache itself but perhaps a corrupted file. I've never had the problem on other computers.
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Old 14th February 2020, 06:29 PM   #33
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Originally Posted by Hellbound View Post
If it was domain controllers, then yeah, I can see that causing issues. In modern AD domains, the DNS is integrated with AD, so I could be issues with that. Especially considering both the computer account and the user account have to authenticate.

But yeah, cache issues should only happen when things change. With DCs I’d expect other issues just related to the fact that they’d changed, though (possibilities for certificate problems and similar there, too...options for error abound!)


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Imagine, eight environments, each with about 30 servers, load balancers everywhere, multiple domains, inside separate forests.

Currently dealing with replacing all the virtual servers with new ones, W2008 and W2012 to W2016 (Note SQL server upgrades and application upgrades at the same time), which means that version of TLS has to be upgraded, which means that the middle ware has to be upgraded, and we're just talking about the windows world at this stage (forget about all the multi-agency stuff, and all the mainframe back ends).

Any day that no-one mentions firewalls or certificates is a good day for me.
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Old 14th February 2020, 07:03 PM   #34
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Please note, none of the stuff above is 'my work'.

My part is designing services to move information in and out of the databases via the server-side APIs.

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Old 15th February 2020, 03:13 AM   #35
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Originally Posted by plague311 View Post
I don't think it's odd to have to clear out DNS in any system. They have commands and the ability to do it in every flavor of Linux, Apple, and Windows.
It is odd to have to clear it to make DNS lookups faster. The whole point of the cache is to make DNS lookups faster by not having to query a DNS server across the network.

The only time I have ever felt the need to flush a DNS cache is when I’ve changed a DNS record and the copy in the cache hasn’t expired yet.
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Old 15th February 2020, 10:41 PM   #36
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There is a probably the equivalent of a memory leak in the cache. Caching is widely used in computer systems so it's a well understood software tool but this just sounds like a bug that will be fixed eventually.
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Old 17th February 2020, 01:10 AM   #37
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Originally Posted by a_unique_person View Post
Caching is widely used in computer systems so it's a well understood software tool
Well, there's this famous joke among computer folks:

There are 2 hard problems with computers: caching, naming things and off by one errors...

And then there's: http://letmegooglethat.com/?q=it%27s...+a+dns+problem
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Old 18th February 2020, 06:11 AM   #38
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Originally Posted by novaphile View Post
On my team, everyone twitches any time anyone mentions certificates...

Imagine, eight environments, each with about 30 servers, load balancers everywhere, multiple domains, inside separate forests.

Currently dealing with replacing all the virtual servers with new ones, W2008 and W2012 to W2016 (Note SQL server upgrades and application upgrades at the same time), which means that version of TLS has to be upgraded, which means that the middle ware has to be upgraded, and we're just talking about the windows world at this stage (forget about all the multi-agency stuff, and all the mainframe back ends).

Any day that no-one mentions firewalls or certificates is a good day for me.
A pal wrote this to monitor that websites were up and to validate certs etc. https://github.com/brabster/website-checker

Originally Posted by brabster
Where an https protocol is specified and a certificate is presented:

checks certificate is valid
checks certificate is trusted by the computer running the check
checks certificate is valid for at least a specified number of days
I wrote a friendlier front-end but it's locked away at my former employer.
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Last edited by Wudang; 18th February 2020 at 06:13 AM.
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Old 23rd February 2020, 05:30 AM   #39
jeremyp
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Originally Posted by lauwersw View Post
Well, there's this famous joke among computer folks:

There are 2 hard problems with computers: caching, naming things and off by one errors...
That's the trouble with C. In any reasonable language, it would go

There are 2 hard problems with computers: caching, naming things and
Code:
INDEX OUT OF RANGE ERROR. ABORT CORE DUMPED.
Quote:
Coincidentally, I spent the whole of last week trying to diagnose an intermittent problem with our internal time recording system. It turned out that one of its two DNS servers didn't know how to resolve the name of the active directory server.
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Old 24th February 2020, 02:52 PM   #40
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Originally Posted by jeremyp View Post
That's the trouble with C. In any reasonable language, it would go

There are 2 hard problems with computers: caching, naming things and
Code:
INDEX OUT OF RANGE ERROR. ABORT CORE DUMPED.
That's only roughly 50% of the off-by-one errors, and for the other 50% only those where it's an index into an array or other bounded entity. And any reasonable language doesn't abort on an out-of-bounds index.

BTW: In C++, you can have this behaviour with the standard containers:
Quote:
std::vector v(2);
v[2]; // does not check
v.at(2); // checks, throws an exception you can catch
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