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• How do I check a reverse PTR record?

  - by Daisetsu

  I need to check a reverse PTR record to make sure that a script I have is sending emails which will actually received by my users and not incorrectly marked as spam. I understand that the ISP which owns the IP range has to set up the PTR record, but how do I check if it is already set up?

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• PTR and A record must match?

  - by somecallmemike

  RFC 1912 Section 2.1 states the following: Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain. If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a alias defined by a CNAME. It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data. This does not make any sense to me, should an ISP keep matching A records for every PTR record? It seems to me that it's only important if the IP address that the PTR record describes is hosting a service that is sensitive to DNS being mismatched (such as email hosting). In that case the forward zone would be configured under a domain name (examples follow the format 'zone - record'): domain.tld -> mail IN A 1.2.3.4 And the PTR record would be configured to match: 3.2.1.in-addr.arpa -> 4 IN PTR mail.domain.tld. Would there be any reason for the ISP to host a forward lookup for an IP address on their network like this?: ispdomain.tld -> broadband-ip-1 IN A 1.2.3.4

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• PTR Record Troubles

  - by Physikal

  I am having a hell of a time getting our PTR record right. Our current PTR zone looks like this: $ttl 38400 @ IN SOA ns1.domain.com. admin.domain.com. ( 1268669139 10800 3600 604800 38400 ) xxx.xxx.xxx.in-addr.arpa. IN NS ns2.domain.com. xxx.xxx.xxx.in-addr.arpa. IN NS ns1.domain.com. 97 IN PTR mail.domain.com. xxx.xxx.xxx.xxx.in-addr.arpa. IN PTR mail.domain.com. 97.96/28. IN PTR mail.domain.com For some reason the only thing that works is the 97.96/28. When this line is in there it actually says I have a PTR record when reporting from intodns.com. If I remove that line, it says I have no PTR. I have followed instructions from http://www.philchen.com/2007/04/04/configuring-reverse-dns and when I follow those instructions intodns.com says I have no PTR. When it does work with the line 97.96/28., the PTR kicks back as (from intodns.com) : 97.xxx.xxx.xxx.in-addr.arpa -> mail.domain.com.xxx.xxx.xxx.in-addr.arpa Which is, to my knowledge, an incorrect PTR. I want it to just kick back as mail.domain.com, without the xxx.xxx.xxx.in-addr.arpa extension. I have tried everything I can think of but I can't fix it. I can't help but think it's one of those things that is so stupid and simple I'm going to do the ol'facepalm. Any help is greatly appreciated. Thanks! In the event that the domain zone is needed, here it is: $ttl 38400 @ IN SOA domain.com. [email protected]. ( 1265221037 10800 3600 604800 38400 ) domain.com. IN A xxx.xxx.xxx.xxx www.domain.com. IN A xxx.xxx.xxx.xxx ftp.domain.com. IN A xxx.xxx.xxx.xxx m.domain.com. IN A xxx.xxx.xxx.xxx localhost.domain.com. IN A 127.0.0.1 webmail.domain.com. IN A xxx.xxx.xxx.xxx admin.domain.com. IN A xxx.xxx.xxx.xxx mail.domain.com. IN A xxx.xxx.xxx.xxx domain.com. IN MX 5 mail.domain.com. domain.com. IN TXT "v=spf1 a mx a:domain.com ip4:xxx.xxx.xxx.xxx ?all" domain.com. IN NS ns1 domain.com. IN NS ns2 ns1 IN A xxx.xxx.xxx.xxx ns2 IN A xxx.xxx.xxx.xxx Any double entries in different formats were part of my troubleshooting process.

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• How to setup PTR (Reverse DNS) at Godaddy DNS manager

  - by PokemonCraft

  My hosting provider already did PTR setup. Now i have to setup it at my DNS holder i guess. What am i supposed to do at my Godaddy account. thank you. my ip : 64.250.113.235 and you can check my ptr record from here http://mxkit.com/webmaster-tools/ptr-check result Ptr records for 64.250.113.235 are: Server: 10.0.80.11 Address: 10.0.80.11#53 Non-authoritative answer: 235.113.250.64.in-addr.arpa name = notification.pokemoncraft.com.

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• Should I specify both INDEX and UNIQUE INDEX?

  - by Matt Huggins

  On one of my PostgreSQL tables, I have a set of two fields that will be defined as being unique in the table, but will also both be used together when selecting data. Given this, do I only need to define a UNIQUE INDEX, or should I specify an INDEX in addition to the UNIQUE INDEX? This? CREATE UNIQUE INDEX mytable_col1_col2_idx ON mytable (col1, col2); Or this? CREATE UNIQUE INDEX mytable_col1_col2_uidx ON mytable (col1, col2); CREATE INDEX mytable_col1_col2_idx ON mytable (col1, col2);

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• DNS PTR record when domain on shared IP address

  - by Marco Demaio

  Hello, I own a typical shared IP hosting plan and domain. I can modify the DNS of the domain from the control panel. The mailserver shares the same IP address, so my typical DNS config is: www.mydomain.com A -> IP mydomain.com A -> IP ftp.mydomain.com A -> IP mail.mydomain.com A -> IP mydomain.com MX(10) -> IP I read some Q&A on this site where they suggest to add PTR record mainly for mailserver. I would like to add PTR record to my domain, I have got two questions: 1) can PTR record be added even if the hosting/mailserver are on a shared IP address? Or do I need a dedicated IP. 2) How do I setup PTR record, I mean does it look like A record: mydomain.com (PTR) -> myip

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• Hash of unique value = unique hash?

  - by Nebs

  Theoretically does hashing a unique value yield a unique value? Let's say I have a DB table with 2 columns: id and code. id is an auto-incrementing int and code is a varchar. If I do ... $code = sha1($id); ... and then store $code into the same row as $id. Will my code column be unique as well? What about if I append the current time? eg: $code = sha1($id . time()); Thanks.

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• Unique Business Value vs. Unique IT

  - by barry.perkins

  When the age of computing started, technology was new, exciting, full of potential and had a long way to grow. Vendor architectures were proprietary, and limited in function at first, growing in capability and complexity over time. There were few if any "standards", let alone "open standards" and the concepts of "open systems", and "open architectures" were far in the future. Companies employed intelligent, talented and creative people to implement the best possible solutions for their company. At first, those solutions were "unique" to each company. As time progressed, standards emerged, companies shared knowledge, business capability supplied by technology grew, and companies continued to expand their use of technology. Taking advantage of change required companies to struggle through periodic "revolutionary" change cycles, struggling through costly changes that were fraught with risk, resulted in solutions with an increasingly shorter half-life, and frequently required altering existing business processes and retraining employees and partner businesses. The pace of technological invention and implementation grew at an ever increasing rate, making the "revolutionary" approach based upon "proprietary" or "closed" architectures or technologies no longer viable. Concurrent with the advancement of technology, the rate of change in business increased, leading us to the incredibly fast paced, highly charged, and competitive global economy that we have today, where the most successful companies are companies that are good at implementing, leveraging and exploiting change. Fast forward to today, a world where dramatic changes in business and technology happen continually, a world where "evolutionary" change is crucial. Companies can no longer afford to build "unique IT", nor can they afford regular intervals of "revolutionary" change, with the associated costs and risks. Human ingenuity was once again up to the task, turning technology into a platform supporting business through evolutionary change, by employing "open": open standards; open systems; open architectures; and open solutions. Employing "open", enables companies to implement systems based upon technology, capability and standards that will evolve over time, providing a solid platform upon which a company can drive business needs, requirements, functions, and processes down into the technology, rather than exposing technology to the business, allowing companies to focus on providing "unique business value" rather than "unique IT". The big question! Does moving from "older" technology that no longer meets the needs of today's business, to new "open" technology require yet another "revolutionary change"? A "revolutionary" change with a short half-life, camouflaging reality with great marketing? The answer is "perhaps". With the endless options available to choose from, it is entirely possible to implement a solution that may work well today, but in 5 years time will become yet another albatross for the company to bear. Some solutions may look good today, solving a budget challenge by reducing cost, or solving a specific tactical challenge, but result in highly complex environments, that may be difficult to manage and maintain and limit the future potential of your business. Put differently, some solutions might push today's challenge into the future, resulting in a more complex and expensive solution. There is no such thing as a "1 size fits all" IT solution for business. If all companies implemented business solutions based upon technology that required, or forced the same business processes across all businesses in an industry, it would be extremely difficult to show competitive advantage through "unique business value". It would be equally difficult to "evolve" to meet or exceed business needs and keep up with today's rapid pace of change. How does one ensure that they do not jump from one trap directly into another? Or to put it positively, there are solutions available today that can address these challenges and issues. How does one ensure that the buying decision of today will serve the business well for years into the future? Intelligent & Informed decisions - "buying right" In a previous blog entry, we discussed the value of linking tactical to strategic The key is driving the focus to what is best for your business, handling today's tactical issues while also aligning with a roadmap/strategy that is tightly aligned with your strategic business objectives. When considering the plethora of possible options that provide various approaches to solving today's complex business problems, it is extremely important to ensure that vendors supplying those options, focus on what is best for your business, supplying sufficient information, providing adequate answers to questions, addressing challenges, issues, concerns and objections honestly and openly, and focus on supplying solutions that are tailored for, and deliver the most business value possible for your business. Here are a few questions to consider relative to the proposed options that should help ensure that today's solution doesn't become tomorrow's problem. Do the proposed solutions: Solve the problem(s) you are trying to address? Provide a solid foundation upon which to grow/enhance your business? Provide tactical gains that align with and enable your strategic business goals/objectives? Provide an infrastructure that can be leveraged with subsequent projects? Solve problems for the business overall, the lines of business, or just IT? Simplify your current environment Provide the basis for business: Efficiency Agility Clarity governance, risk, compliance real time business visibility and trend analysis Does your IT staff have the knowledge/experience to successfully manage the proposed systems once they are deployed in production? Done well, you will be presented with options tailored to your business, that enable you to drive the "unique business value" necessary to help your business stand out from others, creating a distinct competitive advantage, delivering what your customers need, when they need it, so you can attract new customers, new business, and grow top line revenue, all at a cost that provides a strong Return on Investment/Return on Assets. The net result is growth with managed cost providing significantly improved profit margin and shareholder value.

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• DHCP and DNS on none AD 2003 Server PTR is updating but no A records

  - by user29819

  I have a strange issue, I have a DHCP and DNS server running in a non AD environment, on Windows 2003 server. I setup DHCP to update DNS A and PTR records even if the client doesnt request it, but I only see PTR records updated, the A records are not created at all. The domain is "local" forward zone is called "local" and in the option 15 set to "local" (actual name) the PTR records are created with the right name (example: win64_ent.local), What am I missing here ?

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• TTL for PTR records on Windows 2003 Server DNS

  - by Kyle Brandt

  When I look at the TTL (Time to Live) for PTR records (Reverse Lookup Zone) on one of our Windows 2003 DNS servers I see some are at 15 minutes, others are at 20. They have "Delete this record when it becomes stale" checked. These PTR records are for workstations that get IPs from Windows DHCP, so I think that creates the PTR records dynamically? How is TTL for these records set?

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• PTR Record for host in VLSM subnet

  - by paradroid

  I understand that this is the way a PTR record would be made on a Class A subnet (10.100.250.100 255.0.0.0) dnscmd /RecordAdd 10.in-addr.arpa. 100.250.100 PTR host.domain.tld To clarify the syntax, this is what it should be for a Class C subnet (192.168.1.100 255.255.255.0) dnscmd /RecordAdd 1.168.192.in-addr.arpa. 100 PTR host.domain.tld Is that right? Now how do I do this for a host with the IP address 172.31.111.210 on a 172.31.111.192/26 network? I'm not sure how to do this with a classless subnet mask.

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• Importance of PTR records on nameserver IP's?

  - by Aron Rotteveel

  I have not yet experienced any DNS failures due to a missing PTR record, but am wondering: Is there any added value in having a valid PTR record on a nameserver IP-address?

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• typedef boost::shared_ptr<MyJob> Ptr; or #define Ptr boost::shared_ptr

  - by danio

  I've just started wrking on a new codebase where each class contains a shared_ptr typedef (similar to this) like: typedef boost::shared_ptr<MyClass> Ptr; Is the only purpose to save typing boost::shared_ptr? If that is the case why not do #define Ptr boost::shared_ptr in one common header? Then you can do: Ptr<MyClass> myClass(new MyClass); which is no more typing than MyClass::Ptr myClass(new MyClass); and saves the Ptr definition in each class.

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• How important are PTR records for Email Servers?

  - by Kyle Brandt

  Does anyone know of any studies done to show how much email will be rejected if there is not ptr record for the SMTP banner name of an email server? Are reverse checks always done when enabled, or is it sometimes configured so if an spam program considers an email 'iffy', the reverse check is done?

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• Reverse DNS (PTR) for Email

  - by user3169495

  We are hosting our website (http://www.redappleapartments.com) with a hosting company in Sweden. And we are using godaddy as our email provider. So, our smtp servers are smtp.europe.secureserver.net mailstore1.europe.secureserver.net Our website sends plenty of emails to our customers and we are sending via godaddy (using SMTP authentication). Some of the emails are never delivered to the recipients. And sometimes we see such warning: The hostname in the SMTP greeting does not match the reverse DNS (PTR) Can somebody out there suggest how we can solve this problem?

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• PTR record not valid for all domains

  - by charnley

  We have an issue sending emails to certain domains, namely Time Warner and Cox. Last week, we decommissioned our Exchange 2003 server and now our Exchange 2010 server is doing all of the transport for our domain. We run our own authoritative name servers, so we are in charge of the DNS and have modified our PTR record to reflect the new server. All mailflow is working except for these 2 domains. When I telnet on port 25 to the mail servers for Cox and Time Warner I am receiving errors. For Cox the error is: 554... rejected - no rDNS And when I telnet to port 25 to the Time Warner mail server we get this: 554 5.7.1 - Connection refused. IP name lookup failed for x.x.x.x I have run through the outbound SMTP test on Microsoft Remote Connectivity Analyzer and get 100% completely successful results. MXToolbox comes up with all successful tests on SMTP as well, showing correct reverse banner check, and no blacklisting. DNSQueries.com shows a valid reverse DNS entry as well for us. Outbound emails to these 2 domains continue to sit in the queue. Any ideas or advice would be greatly appreciated. Thanks!

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• My Reverse DNS PTR record seems to be right, but I'm still getting bouncing email

  - by johnbr

  Hello, I have a service (statusme.com) where I let people know (for example) that their kid's soccer games are cancelled because of bad weather. We send out emails to the people who have registered. I have a second server as a backup, (vps.statusme.com) and I've set up the application to send some of the email through the second server. But I'm getting complaints from various recipient SMTP servers that the email is considered spam. So I did some investigating, and it appears that they think my reverse DNS record isn't correct. But when I look at it via various rDNS websites and instructions I found elsewhere on ServerFault, everything looks correct: jb$ host -t a vps.statusme.com 8.8.8.8 Using domain server: Name: 8.8.8.8 Address: 8.8.8.8#53 Aliases: vps.statusme.com has address 66.84.8.246 jb$ host -t ptr 246.8.84.66.in-addr.arpa 8.8.8.8 Using domain server: Name: 8.8.8.8 Address: 8.8.8.8#53 Aliases: 246.8.84.66.in-addr.arpa domain name pointer vps.statusme.com. I'm confused about what I'm doing wrong. Thanks for any suggestions.

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• DKIM, SPF, PTR records are not working properly with my domain

  - by shihon

  I configured my server and well authenticate email system with DKIM key, SPF record and PTR records, when i start to sent out mails from phplist interface to my users ~50000, my domain is spammed by google. In headers, signed by and mailed by tag shows by my domain : appmail.co, I also test my domain via check mail provide by port25, report is: This message is an automatic response from Port25's authentication verifier service at verifier.port25.com. The service allows email senders to perform a simple check of various sender authentication mechanisms. It is provided free of charge, in the hope that it is useful to the email community. While it is not officially supported, we welcome any feedback you may have at . Thank you for using the verifier, The Port25 Solutions, Inc. team ========================================================== Summary of Results SPF check: pass DomainKeys check: neutral DKIM check: pass Sender-ID check: pass SpamAssassin check: ham ========================================================== Details: HELO hostname: app.appmail.co Source IP: 108.179.192.148 mail-from: [email protected] SPF check details: Result: pass ID(s) verified: [email protected] DNS record(s): appmail.co. SPF (no records) appmail.co. 14400 IN TXT "v=spf1 +a +mx +ip4:108.179.192.148 ?all" appmail.co. 14400 IN A 108.179.192.148 DomainKeys check details: Result: neutral (message not signed) ID(s) verified: [email protected] DNS record(s): DKIM check details: Result: pass (matches From: [email protected]) ID(s) verified: header.d=appmail.co Canonicalized Headers: content-type:multipart/alternative;'20'boundary=047d7b2eda75d8544d04c17b6841'0D''0A' to:[email protected]'0D''0A' from:shashank'20'sharma'20'<[email protected]>'0D''0A' subject:Test'0D''0A' message-id:<CADnDhbH9aDBk3Ho2-CrG7gwOoD6RNX0sFq4bWL64+kmo=9HjWg@mail.gmail.com>'0D''0A' date:Sat,'20'2'20'Jun'20'2012'20'16:44:50'20'+0530'0D''0A' mime-version:1.0'0D''0A' dkim-signature:v=1;'20'a=rsa-sha256;'20'q=dns/txt;'20'c=relaxed/relaxed;'20'd=appmail.co;'20's=default;'20'h=Content-Type:To:From:Subject:Message-ID:Date:MIME-Version;'20'bh=GS6uwlT+weKcrrLJ2I+cjBtWPq9nvhwRlNAJebOiQOk=;'20'b=; Canonicalized Body: --047d7b2eda75d8544d04c17b6841'0D''0A' Content-Type:'20'text/plain;'20'charset=UTF-8'0D''0A' '0D''0A' Hello'20'Senders'0D''0A' '0D''0A' --047d7b2eda75d8544d04c17b6841'0D''0A' Content-Type:'20'text/html;'20'charset=UTF-8'0D''0A' '0D''0A' Hello'20'Senders'0D''0A' '0D''0A' --047d7b2eda75d8544d04c17b6841--'0D''0A' DNS record(s): default._domainkey.appmail.co. 14400 IN TXT "v=DKIM1; k=rsa; p=MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhALGCOdMeZRxRHoatH7/KCvI1CKS0wOOsTAq0LLgPsOpMolifpVQDKOWT2zq/6LHVmDVjXLbnWO2d4ry/riy7ei66pLpnAV5ceIUSjBRusI8jcF9CZhPrh/OImsKVUb9ceQIDAQAB;" NOTE: DKIM checking has been performed based on the latest DKIM specs (RFC 4871 or draft-ietf-dkim-base-10) and verification may fail for older versions. If you are using Port25's PowerMTA, you need to use version 3.2r11 or later to get a compatible version of DKIM. Sender-ID check details: Result: pass ID(s) verified: [email protected] DNS record(s): appmail.co. SPF (no records) appmail.co. 14400 IN TXT "v=spf1 +a +mx +ip4:108.179.192.148 ?all" appmail.co. 14400 IN A 108.179.192.148 SpamAssassin check details: SpamAssassin v3.3.1 (2010-03-16) Result: ham (-0.1 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -0.0 T_RP_MATCHES_RCVD Envelope sender domain matches handover relay domain 0.0 HTML_MESSAGE BODY: HTML included in message -0.5 BAYES_05 BODY: Bayes spam probability is 1 to 5% [score: 0.0288] -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature 0.5 SINGLE_HEADER_1K A single header contains 1K-2K characters ========================================================== Original Email Return-Path: <[email protected]> Received: from app.appmail.co (108.179.192.148) by verifier.port25.com id hp7qqo11u9cc for <[email protected]>; Sat, 2 Jun 2012 07:14:52 -0400 (envelope-from <[email protected]>) Authentication-Results: verifier.port25.com; spf=pass [email protected] Authentication-Results: verifier.port25.com; domainkeys=neutral (message not signed) [email protected] Authentication-Results: verifier.port25.com; dkim=pass (matches From: [email protected]) header.d=appmail.co Authentication-Results: verifier.port25.com; sender-id=pass [email protected] DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=appmail.co; s=default; h=Content-Type:To:From:Subject:Message-ID:Date:MIME-Version; bh=GS6uwlT+weKcrrLJ2I+cjBtWPq9nvhwRlNAJebOiQOk=;b=pNw3UQNMoNyZ2Ujv8omHGodKVu/55S8YdBEsA5TbRciga/H7f+5noiKvo60vU6oXYyzVKeozFHDoOEMV6m5UTgkdBefogl+9cUIbt5CSrTWA97D7tGS97JblTDXApbZH; Received: from mail-pb0-f46.google.com ([209.85.160.46]:57831) by app.appmail.co with esmtpa (Exim 4.77) (envelope-from <[email protected]>) id 1SamIF-00055f-Om for [email protected]; Sat, 02 Jun 2012 16:44:51 +0530 Received: by pbbrp8 with SMTP id rp8so4165728pbb.5 for <[email protected]>; Sat, 02 Jun 2012 04:14:51 -0700 (PDT) MIME-Version: 1.0 Received: by 10.68.216.33 with SMTP id on1mr19414885pbc.105.1338635690988; Sat, 02 Jun 2012 04:14:50 -0700 (PDT) Received: by 10.143.66.13 with HTTP; Sat, 2 Jun 2012 04:14:50 -0700 (PDT) Date: Sat, 2 Jun 2012 16:44:50 +0530 Message-ID: <CADnDhbH9aDBk3Ho2-CrG7gwOoD6RNX0sFq4bWL64+kmo=9HjWg@mail.gmail.com> Subject: Test From: shashank sharma <[email protected]> To: [email protected] Content-Type: multipart/alternative; boundary=047d7b2eda75d8544d04c17b6841 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - app.appmail.co X-AntiAbuse: Original Domain - verifier.port25.com X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - appmail.co --047d7b2eda75d8544d04c17b6841 Content-Type: text/plain; charset=UTF-8 Hello Senders --047d7b2eda75d8544d04c17b6841 Content-Type: text/html; charset=UTF-8 Hello Senders --047d7b2eda75d8544d04c17b6841-- I also tried to send mail on yahoo , rediff but i get mails in spam. Please help me to sort out this issue

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• How to set two column unique in SQL.

  - by sxingfeng

  I am creating a table ,in the table two column is unique, I mean columnA and columnB do not have same value: such as : Table X A B 1 2(RIGHT,unique) 2 2(RIGHT, unique) 1 3(RIGHT, not unique) 2 3(RIGHT, not unique) 1 2 (WRONG, not unique) How to create such a table? many thanks!

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• How to add PTR record for a /16 IP block in BIND using $GENERATE directive?

  - by yegle

  I'm trying to reverse map a block of IP using PTR record to some special name so their usage can be easily reflected by a simple nslookup. For example, here's a nslookup result: # nslookup 172.17.201.101 Server: 10.253.33.1 Address: 10.253.33.1#53 101.201.17.172.in-addr.arpa name = for.internal.use.only. And I learned that I can add PTR record for a /24 block by using $GENERATE directive $GENERATE 0-254 $.201.17.172 PTR for.internal.use.only. So here's the question: Am I doing right exposing infomation of IP address by adding PTR record? Any better idea? If the question above is YES, then how to add PTR record for a /16 IP range? I know I can write 255 lines of $GENTERATE directive but any better solution?

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• unique constraint (w/o Trigger) on "one-to-many" relation

  - by elgcom

  To illustrate the problem, I make an example: A tag_bundle consists of one or more than one tags. A unique tag combination can map to a unique tag_bundle, vice versa. tag_bundle tag tag_bundle_relation +---------------+ +--------+ +---------------+--------+ | tag_bundle_id | | tag_id | | tag_bundle_id | tag_id | +---------------+ +--------+ +---------------+--------+ | 1 | | 100 | | 1 | 100 | +---------------+ +--------+ +---------------+--------+ | 101 | | 1 | 101 | +--------+ +---------------+--------+ There can't be another tag_bundle having the combination from tag 100 and tag 101. How can I ensure such unique constraint when executing SQL "concurrently"!! that is, to prevent concurrently adding two bundles with the same tag combination Adding a simple unique constraint on any table does not work, Is there any solution other than Trigger or explicit lock. I come to only this simple way: make tag combination into string, and let it be unique. tag_bundle (unique on tags) tag tag_bundle_relation +---------------+--------+ +--------+ +---------------+--------+ | tag_bundle_id | tags | | tag_id | | tag_bundle_id | tag_id | +---------------+--------+ +--------+ +---------------+--------+ | 1 | 100,101| | 100 | | 1 | 100 | +---------------+--------+ +--------+ +---------------+--------+ | 101 | | 1 | 101 | +--------+ +---------------+--------+ but it seems not a good way :(

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• Creating a short unique string for each unique long string

  - by king.net

  I'm trying to create a url shortener system in c# and asp.net mvc. I know about hashtable and I know how to create a redirect system etc. The problem is indexing long urls in database. Some urls may have up to 4000 character length, and it seems it is a bad idea to index this kind of strings. The question is: How can I create a unique short string for each url? for example MD5 can help me? Is MD5 really unique for each string? NOTE: I see that Gravatar uses MD5 for emails, so if each email address is unique, then its MD5 hashed value is unique. Is it right? Can I use same solution for urls?

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• mysql unique-constraint

  - by Cypher

  I have two tables -- Variables (id, name) and Variable_Entries (id, var_id, value). I want each variable to have a unique set of entries. If I make the value entry unique then a different variable won't be able to have that same value which is not right. Is there some way to make the value column unique for identical var_id's?

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• UNIQUE Constraints in SQL (MS-SQL)

  - by rockbala

  Why are UNIQUE Constraints needed in database ? Can you provide any examples ? Primary Key is UNIQUE by default... Understandable as they are referred in other tables as Foreign keys... relation is needed to connect them for rdbms platform... but why would one refer to other columns as UNIQUE, what is benefit of doing so ?)

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• UNIQUE Constraints in SQL (SQL Server)

  - by rockbala

  Why are UNIQUE Constraints needed in database ? Can you provide any examples ? Primary Key is UNIQUE by default... Understandable as they are referred in other tables as Foreign keys... relation is needed to connect them for rdbms platform... but why would one refer to other columns as UNIQUE, what is benefit of doing so ?)

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