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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘Communications Development’ Category

Determine actual message size limit when you only get “552 5.3.4 Message size exceeds fixed limit”

Posted by jpluimers on 2020/03/26

Often when you send large emails the only  reply you get is a non-descriptive message like 552 5.3.4 Message size exceeds fixed limit from the SMTP server without an indication what the limit actually is.

Most SMTP servers however implement extensions in the EHLO greeting that returns a SIZE mail parameter. You can query it by hand using this:

telnet aspmx.l.google.com smtp
Trying 108.177.119.27...
Connected to aspmx.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP 32si3005669edb.510 - gsmtp
EHLO example.org
250-mx.google.com at your service, [80.100.143.119]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
QUIT
221 2.0.0 closing connection 32si3005669edb.510 - gsmtp
Connection closed by foreign host.

There you can see the maximum message size at the time of writing is 157286400 bytes which is about 150 megabytes.

There is a nice Python script showing how to obtain it at [WayBack] Getting Information from EHLO | Erle Robotics Python Networking Gitbook Free (note this one does send an email, so you might want to trim the example if you just want to see the size).

More background reading:

Trimming down the Python script so it queries message size for each mail server of a domain

This turns out to be a tad more complex, because DNS functionality isn’t part of core Python, and the rdata part of DNS records ends with a dot, which might not be usable with the SMTP library.

References for me when trimming down:

–jeroen

Posted in Communications Development, Conference Topics, Conferences, Development, Event, Internet protocol suite, Power User, SMTP | Leave a Comment »

Linux – How to Securely Copy Files Using SCP examples

Posted by jpluimers on 2020/03/16

I love short and to the point examples. The list of permutations for scp is at [WayBack] Linux – How to Securely Copy Files Using SCP examples.

–jeroen

Read the rest of this entry »

Posted in *nix, *nix-tools, Development, Internet protocol suite, Power User, SSH, TCP | Leave a Comment »

ssh – Why OpenSSH deprecated DSA keys – Information Security Stack Exchange

Posted by jpluimers on 2020/03/10

In a lot of ssh-keygen related posts, you still see DSA being mentioned, though that has been deprecated and later removed from OpenSSH.

I wondered why, so I did some digging.

TL;DR: it’s complicated:

  • different standards mandating eventually conflicting parameters,
  • extending the parameters would require protocol extension,
  • a logjam vulnerability for certain combinations of parameters and finally
  • better algorithms having become available.

Some of the related topics cannot be archived in the WayBack machine or refuse being archived at Archive.is, so here is a list of partially archived relevant links:

–jeroen

Posted in Communications Development, Development, Internet protocol suite, Power User, Security, SSH, TCP | Leave a Comment »

Redirect Detective – A Free Tool To Trace Where Redirects End Up

Posted by jpluimers on 2020/02/27

Cool tool, but be aware it does aJavaScript:xmlhttpPost from , so your data can be logged [WayBackRedirect Detective – A Free Tool To Trace Where Redirects End Up.

For http://www.xs4all.nl/~jp, it shows these redirects (where in bold I’ve listed the changes):

Oh and it runs this call: JavaScript:xmlhttpPost("/linkdetect.px")?word=www.xs4all.nl%2F%7Ejp from [WayBackredirectdetective.com/ajax.js.

Note that this shows more redirects than the plain http ones, so wget from [WayBack] wget – How do I display all URLs in a redirect chain? – Unix & Linux Stack Exchange shows this:

$ wget http://www.xs4all.nl/~jp 2>&1 | grep Location:
Location: https://www.xs4all.nl/~jp [following]
Location: https://jp.home.xs4all.nl/ [following]

–jeroen

Posted in *nix, *nix-tools, Communications Development, Development, HTTP, Internet protocol suite, Power User, TCP, wget | Leave a Comment »

Links to alternatives for Delphi REST clients

Posted by jpluimers on 2019/11/13

A few good links to a few alternatives for the built in Delphi REST client [WayBack] Hi looking for good and complete tutorial/course how to create RESTFul services with Delphi. Or if anyone use Delphi for creating RESTFul services feedb… – Gordan Paunovic – Google+

–jeroen

Posted in Communications Development, Delphi, Development, HTTP, Internet protocol suite, JSON, REST, Software Development, TCP | Leave a Comment »

 
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