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  • How does Linux blocking I/O actually work?

    - by tgguy
    In Linux, when you make a blocking i/o call like read or accept, what actually happens? My thoughts: the process get taken out of the run queue, put into a waiting or blocking state on some wait queue. Then when a tcp connection is made (for accept) or the hard drive is ready or something for a file read, a hardware interrupt is raised which lets those processes waiting to wake up and run (in the case of a file read, how does linux know what processes to awaken, as there could be lots of processes waiting on different files?). Or perhaps instead of hardware interrupts, the individual process itself polls to check availability. Not sure, help?

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  • Fighting Spam - What can I do as an: Email Administrator, Domain Owner, or User?

    - by Chris S
    This is a Canonical Question about Fighting Spam. Also related: How to stop people from using my domain to send spam? There are so many techniques and so much to know about fighting SPAM. What widely used techniques and technologies are available to Administrator, Domain Owners, and End Users to help keep the junk out of our inboxes? We're looking for an answer that covers different tech from various angles. The accepted answer should include a variety of technologies (eg SPF/SenderID, DomainKeys/DKIM, Graylisting, DNS RBLs, Reputation Services, Filtering Software [SpamAssassin, etc]); best practices (eg mail on Port 25 should never be allowed to relay, Port 587 should be used; etc), terminology (eg, Open Relay, Backscatter, MSA/MTA/MUA, Spam/Ham), and possibly other techniques.

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  • How do I find out what a Spam Custom Rule is?

    - by SoaperGEM
    We use a Barracuda Spam Filter at work, and we also provide a mass emailing program to some of clients that send out newsletters. Lately one of them's been composing his latest company newsletter and has been trying to send preview messages to himself, but they've actually been quarantined by Barracuda as potential spam, even though they aren't. I can see the breakdown of the spam scoring headers in Barracuda, but I'm not sure what certain rules mean. Here's the breakdown: pts rule name description ---- ---------------------- -------------------------------------------------- 0.00 FUZZY_CPILL BODY: Attempt to obfuscate words in spam 2.21 HTML_IMAGE_ONLY_24 BODY: HTML: images with 2000-2400 bytes of words 0.00 HTML_MESSAGE BODY: HTML included in message 0.50 BSF_SC0_SA_TO_FROM_ADDR_MATCH Sender Address Matches Recipient Address 1.00 BSF_SC0_SA392f Custom Rule SA392f What is "Custom Rule SA392f"? Where do I find descriptions of these custom rules? And what does "images with 2000-2400 bytes of words" mean? Is that referring to the file size of the image, or something about the attributes on the <img> tag?

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  • How to whitelist a domain while blocking forgeries using that domain?

    - by QuantumMechanic
    How do you deal with the case of: wanting to whitelist a domain so that emails from it won't get eaten, but not having emails forged to appear to be from that domain get bogusly whitelisted whitelist_from_recvd looks promising, but then you have to know at least the TLD of every host that could send you mail from that domain. Often RandomBigCompany.com will outsource email to one or more sending companies (like Constant Contact and the like) in addition to using servers that reverse-resolve to something in its own domain. But it looks like whitelist_from_recvd can only map to one sending server pattern so that would be problematic. Is there a way to say something like "if email is from domain X, subtract N points from the spam score"? The idea would be that if the mail is legit, that -N will all but guarantee it isn't considered spam. But if it is spam, hopefully all the other failed tests will render it spam even with the -N being included.

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  • Using jQuery with form to eliminate spam

    - by Thierry-Dimitri Roy
    I have put a form on a web page where the user can send us data. Unfortunately, the webmaster does get a lot of spam through this form and the valid submissions gets buried. I have used captcha to bypass this problem. But I think that everyone would agree that captcha is a big annoyance to users. I switched to another solution: now the URL of the submit form points to null: <form id="sendDataForm" action="/null" method="post"> ... </form> And I bypass the form submission using jQuery excellent form plugin: $('#sendDataForm').ajaxForm({ url: '/ajax-data/' }); Since then, no spam has reached the webmaster, and valid comments gets through. The only drawbacks is that users without javascript cannot send us the form. But since this is on top of a javascript web application, we can safely assume that these are not valid users. My question is: in a world where 99% of users has javascript enabled (and a mechanism for those user could be build that uses captcha), why is this solution not more used? What drawback am I not seeing?

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  • What sources do spammers use to get email addresses?

    - by Andrew Grimm
    From what sources do email spammers get their addresses? Wikipedia mentions the following: Harvesting email addresses from publicly available sources. This includes web pages (web crawling), usenet posts, mailing list archives, DNS and WHOIS records Guessing email addresses (directory harvest attack) Asking people for their emails for one purpose, such as jokes of the day, and selling the email addresses elsewhere Getting access to people's address books (which Quechup utilized) Scanning an infected computer for email addresses. Are there any other techniques used? Are any of the techniques above now obsolete?

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  • check if a domain is blacklisted / blocked

    - by Henry
    Some clients report to us that our site is not accessible through their internet connection. We suspect our site is wrongfully blocked by some security software/firewall/public blacklist. How can we verify that, other than trying them one by one? There are so many security software out there that it is not practical... Thx

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  • Can't seem to stop Postfix backscatter

    - by Ian
    I've just migrated to a Postfix system and can't seem to stop the backscatter messages to unknown addresses on the site. I have a file, validrcpt, that lists all the valid emails on the site - about eight of them. Yet when a message is sent to a non-existent address, instead of just dropping it, postfix is replying with a "Recipient address rejected: User unknown in virtual mailbox table" email. Do I have something set wrong? I've read http://www.postfix.org/BACKSCATTER_README.html but unless I'm caffeine deficient, I don't see what's happening and perhaps I'm just to used to my old qmail setup. Here's postconf -n: alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases append_dot_mydomain = no biff = no broken_sasl_auth_clients = yes config_directory = /etc/postfix content_filter = smtp-amavis:[127.0.0.1]:10024 home_mailbox = Maildir/ inet_interfaces = all inet_protocols = ipv4 local_recipient_maps = hash:/etc/postfix/validrcpt mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/dovecot.conf -m "${EXTENSION}" mailbox_size_limit = 0 mydestination = localhost myhostname = localhost mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 myorigin = /etc/mailname policy-spf_time_limit = 3600s readme_directory = no recipient_bcc_maps = hash:/etc/postfix/recipient_bcc recipient_delimiter = + relay_recipient_maps = hash:/etc/postfix/relay_recipients relayhost = smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache smtp_use_tls = yes smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu) smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination,check_policy_service unix:private/policy-spf,reject_rbl_client zen.spamhaus.org,reject_rbl_client bl.spamcop.net,reject_rbl_client cbl.abuseat.org,check_policy_service inet:127.0.0.1:10023 smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination smtpd_sasl_auth_enable = yes smtpd_sasl_authenticated_header = yes smtpd_sasl_local_domain = $myhostname smtpd_sasl_path = private/dovecot-auth smtpd_sasl_security_options = noanonymous smtpd_sasl_type = dovecot smtpd_sender_restrictions = reject_unknown_sender_domain smtpd_tls_auth_only = yes smtpd_tls_cert_file = /etc/dovecot/dovecot.pem smtpd_tls_key_file = /etc/dovecot/private/dovecot.pem smtpd_tls_mandatory_ciphers = medium smtpd_tls_mandatory_protocols = SSLv3, TLSv1 smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache smtpd_use_tls = yes tls_random_source = dev:/dev/urandom virtual_gid_maps = static:5000 virtual_mailbox_base = /home/vmail virtual_mailbox_domains = digitalhit.com virtual_mailbox_maps = hash:/etc/postfix/vmaps virtual_minimum_uid = 1000 virtual_uid_maps = static:5000

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  • How do you test a new email filtering system?

    - by Zoredache
    What method do you use to test or evaluate potential new email filtering systems before you set it up on your production network? I am particularly interested in methods that are appropriate for small/medium sized organizations with a single mail server without the resources to build a duplicate of their email system.

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  • How to block this URL pattern in Varnish VCL?

    - by iTech
    My website is getting badly hit by spambots and scrappers, I am using Cloudflare but the problem still remains there. The problem is spambots accessing non-existing urls causing a lot of load to my drupal backend which goes all the way and bootstraps db just to serve a 404 error doc. I cant simply dish out non-drupal 404's for all page not found errors, as I need to have drupal catch them. Since, varnish is in front it can check if the bot is acting nice and asking for valid url - if not it servers them a 404 or 403. These bots are causing errors using this pattern : http://www.megaleecher.net/http:/www.megaleecher.net/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_S/Using_iPhone_As_USB_Mass_Storage Now, pls. suggest a regex varnbisg VCL directive which catches this URL pattern and serves a 404 error from varnish, preventing it from reaching apache/drupal ?

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  • Whitelisting website email so it is not rejected as spam

    - by Micah Burnett
    What are the processes I need to go through to make sure emails sent from my web server are not rejected as spam? This question is for legitimate site emails that members have requested like a daily newsletter which is generated and run in a nightly process, as well as confirmation emails. Some of the ideas I've heard are: Making sure the server sending the mail has reverse-dns lookup turned on. Manually submitting a whitelist request to major ISPs.

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  • SQL Server 2005 Sleeping SPID blocking another SPID...

    - by user173552
    I find many sleeping process my SQL Server database and looks like one of those sleeping SPIDs is blocking another process, and that process is getting suspended too... Could some one please explain this... 1.) How can a sleeping process block another process? 2.) I see many sleeping process...is this normal? Thanks

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  • Semi-blocking Transformations in SQL Server Integration Services SSIS

    In a SSIS data flow, there are multiple types of transformations. On one hand you have synchronous and asynchronous transformations, but on the other hand you have non-blocking, semi-blocking and fully-blocking components. In this tip, Koen Verbeeck takes a closer look on the performance impact of semi-blocking transformations in SSIS. Can 41,000 DBAs really be wrong? Join 41,000 other DBAs who are following the new series from the DBA Team: the 5 Worst Days in a DBA’s Life. Part 3, As Corrupt As It Gets, is out now – read it here.

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  • What determines which Javascript functions are blocking vs non-blocking?

    - by Sean
    I have been doing web-based Javascript (vanilla JS, jQuery, Backbone, etc.) for a few years now, and recently I've been doing some work with Node.js. It took me a while to get the hang of "non-blocking" programming, but I've now gotten used to using callbacks for IO operations and whatnot. I understand that Javascript is single-threaded by nature. I understand the concept of the Node "event queue". What I DON'T understand is what determines whether an individual javascript operation is "blocking" vs. "non-blocking". How do I know which operations I can depend on to produce an output synchronously for me to use in later code, and which ones I'll need to pass callbacks to so I can process the output after the initial operation has completed? Is there a list of Javascript functions somewhere that are asynchronous/non-blocking, and a list of ones that are synchronous/blocking? What is preventing my Javascript app from being one giant race condition? I know that operations that take a long time, like IO operations in Node and AJAX operations on the web, require them to be asynchronous and therefore use callbacks - but who is determining what qualifies as "a long time"? Is there some sort of trigger within these operations that removes them from the normal "event queue"? If not, what makes them different from simple operations like assigning values to variables or looping through arrays, which it seems we can depend on to finish in a synchronous manner? Perhaps I'm not even thinking of this correctly - hoping someone can set me straight. Thanks!

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  • Non-blocking ORM issues

    - by Nikolay Fominyh
    Once I had question on SO, and found that there are no non-blocking ORMs for my favorite framework. I mean ORM with callback support for asynchronous retrieval. The ORM would be supplied with a callback or some such to "activate" when data has been received. Otherwise ORM needs to be split of in a separate thread to guarantee UI responsiveness. I want to create one, but I have some questions that blocking me from starting development: What issues we can meet when developing ORM? Does word "non-blocking" before word "ORM" will dramatically increase complexity of ORM? Why there are not much non-blocking ORMs around? Update: It looks, that I have to improve my question. We have solutions that already allows us to receive data in non-blocking way. And I believe that not all companies that use such solutions - using raw SQL. We want to create more generic solution, that we can reuse in future projects. What difficulties we can meet?

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  • Non-blocking I/O using Servlet 3.1: Scalable applications using Java EE 7 (TOTD #188)

    - by arungupta
    Servlet 3.0 allowed asynchronous request processing but only traditional I/O was permitted. This can restrict scalability of your applications. In a typical application, ServletInputStream is read in a while loop. public class TestServlet extends HttpServlet {    protected void doGet(HttpServletRequest request, HttpServletResponse response)         throws IOException, ServletException {     ServletInputStream input = request.getInputStream();       byte[] b = new byte[1024];       int len = -1;       while ((len = input.read(b)) != -1) {          . . .        }   }} If the incoming data is blocking or streamed slower than the server can read then the server thread is waiting for that data. The same can happen if the data is written to ServletOutputStream. This is resolved in Servet 3.1 (JSR 340, to be released as part Java EE 7) by adding event listeners - ReadListener and WriteListener interfaces. These are then registered using ServletInputStream.setReadListener and ServletOutputStream.setWriteListener. The listeners have callback methods that are invoked when the content is available to be read or can be written without blocking. The updated doGet in our case will look like: AsyncContext context = request.startAsync();ServletInputStream input = request.getInputStream();input.setReadListener(new MyReadListener(input, context)); Invoking setXXXListener methods indicate that non-blocking I/O is used instead of the traditional I/O. At most one ReadListener can be registered on ServletIntputStream and similarly at most one WriteListener can be registered on ServletOutputStream. ServletInputStream.isReady and ServletInputStream.isFinished are new methods to check the status of non-blocking I/O read. ServletOutputStream.canWrite is a new method to check if data can be written without blocking.  MyReadListener implementation looks like: @Overridepublic void onDataAvailable() { try { StringBuilder sb = new StringBuilder(); int len = -1; byte b[] = new byte[1024]; while (input.isReady() && (len = input.read(b)) != -1) { String data = new String(b, 0, len); System.out.println("--> " + data); } } catch (IOException ex) { Logger.getLogger(MyReadListener.class.getName()).log(Level.SEVERE, null, ex); }}@Overridepublic void onAllDataRead() { System.out.println("onAllDataRead"); context.complete();}@Overridepublic void onError(Throwable t) { t.printStackTrace(); context.complete();} This implementation has three callbacks: onDataAvailable callback method is called whenever data can be read without blocking onAllDataRead callback method is invoked data for the current request is completely read. onError callback is invoked if there is an error processing the request. Notice, context.complete() is called in onAllDataRead and onError to signal the completion of data read. For now, the first chunk of available data need to be read in the doGet or service method of the Servlet. Rest of the data can be read in a non-blocking way using ReadListener after that. This is going to get cleaned up where all data read can happen in ReadListener only. The sample explained above can be downloaded from here and works with GlassFish 4.0 build 64 and onwards. The slides and a complete re-run of What's new in Servlet 3.1: An Overview session at JavaOne is available here. Here are some more references for you: Java EE 7 Specification Status Servlet Specification Project JSR Expert Group Discussion Archive Servlet 3.1 Javadocs

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  • A look at an example of anti-spam algorithm

    - by pragmaticCamel
    What is a good approach to an anti-spam algorithm for a website similar to reddit? Their anti-spam algorithm seems awfully broken (banning on words in the title and doing a horrible job for that matter). Considering a post spam because it has the word 'spam' in the title is really not a wise choice. Anyway, how can one approach such problem ? Are there any tools that help in such cases? Also, what are the /technical/ reasons behind reddit's choice not using reCAPTCHA on every post submission? It seems like a much better solution than what they have right now. Since reddit is basically a community-driven website why not give such power to the communities' trusted members?

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  • python blocking sockets, send returns immediately

    - by Mark
    Hi, I am writing a multithreaded socket application in Python using the socket module. the server listens for connections and when it gets one it spawns a thread for that socket. the server thread sends some data to the client. but the client is not yet ready to receive it. I thought this would have caused the server to wait until the client starts recv but instead returns immediately the client then calls recv which is blocking and no data is ever received. client socket constructor self.__clientSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) self.__clientSocket.connect((server, port)) server socket constructor self.servSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) self.servSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) #self.servSock.settimeout(None) self.servSock.setblocking(1) self.servSock.bind((self.addr,self.port)) self.servSock.listen(5) listening accept thread try: (cs, address) = self.servSock.accept() except socket.timeout: return threadName = '\r\nClient %s:%s\r\n' % (cs, address) print threadName clientSocketHandler = ClientSocket() clientSocketHandler.setClientSocket(cs) self.clients.newThread(self.clientFunc, {clientSocketHandler : "1"}, threadName).start() server and clients send/rec methods from inside ClientSocket receivedData = self.__clientSocket.recv(1024*1024) self.__clientSocket.send(s) any ideas why send() is returning straight away?

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