science

Spam and Open Relay Blocking System

Spam and Open Relay Blocking System is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Spam and Open Relay Blocking System rather than just read about it. In short: SORBS ("Spam and Open Relay Blocking System") was a list of e-mail servers suspected of sending or relaying spam (a DNS Blackhole List). It had been augmented with complementary lists that include various other classes of hosts, allowing for customized email rejection by its users.

Key takeaways

Spam and Open Relay Blocking System belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Spam and Open Relay Blocking System to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Spam and Open Relay Blocking System from memory before moving on to harder problems.

Reference excerpt

SORBS ("Spam and Open Relay Blocking System") was a list of e-mail servers suspected of sending or relaying spam (a DNS Blackhole List). It had been augmented with complementary lists that include various other classes of hosts, allowing for customized email rejection by its users.

History The SORBS DNSbl project was created in November 2001. It was maintained as a private list until 6 January 2002 when the DNSbl was officially launched to the public. The list consisted of 78,000 proxy relays and rapidly grew to over 3,000,000 alleged compromised spam relays. In November 2009 SORBS was acquired by GFI Software, to enhance their mail filtering solutions. In July 2011 SORBS was re-sold to Proofpoint, Inc. On June 5, 2024 SORBS was shut down and no longer available.

DUHL SORBS added IP ranges that belong to dialup modem pools, dynamically allocated wireless, and DSL connections as well as DHCP LAN ranges by using reverse DNS PTR records, WHOIS records, and sometimes by submission from the Internet service providers (ISPs) themselves. This is called the DUHL or Dynamic User and Host List. SORBS did not automatically rescan DUHL listed hosts for updated rDNS so to remove an IP address from the DUHL the user or ISP had to request a delisting or rescan. If other blocks are scanned in the region of listings and the scan includes listed netspace, SORBS automatically removed the netspace marked as static. Matthew Sullivan of SORBS proposed in an Internet Draft that generic reverse DNS addresses include purposing tokens such as static or dynamic, abbreviations thereof, and more. That naming scheme would have allowed end users to classify IP addresses without the need to rely on third party lists, such as the SORBS DUHL. The Internet Draft has since expired. Generally it is considered more appropriate for ISPs to simply block outgoing traffic to port 25 if they wish to prevent users from sending email directly, rather than specifying it in the reverse DNS record for the IP. SORBS' dynamic IP list originally came from Dynablock but has been developed independently since Dynablock stopped updating in December 2003.

Spam traps IP addresses that send spam to SORBS spamtraps are added to their spam database automatically or manually. In order to prevent being blacklisted, major free email services such as Gmail, Yahoo, and Hotmail, as well as major ISPs now implement outgoing anti-spam countermeasures. Gmail, for example, continues to get listed and delisted because they refuse abuse reports. However, smaller networks may still be unwittingly blocked. Because spammers use viruses, malware, and rootkits to force compromised computers to send spam, SORBS listed the IP addresses of servers that the infected system used to send its spam. Because of this, larger ISPs and corporate networks have started blocking port 25 in order to prevent these compromised computers from being able to send email except through designated email servers.

Preemptive listings SORBS maintained a list of networks and addresses that it believed are assigned dynamically to end users/machines, it referred to this list as the DUHL (Dynamic User/Host List). The list includes wide networks of computers sharing the same IP address using network address translation which are also affected (If one computer behind the NAT is allowed to send spam, the whole network will be blacklisted if the NAT IP is ever blacklisted.) This is a common method of pre-emptive blocking as most legitimate mail servers are hosted in data centers designed and provisioned for such services, the legitimate mail servers that are affected by such listings are most commonly home hobbyists running their own mail servers. The Spamhaus Policy Block List (PBL) is another such pre-emptive list which does not just list dynamic hosts, but also blocks hosts it believes should not be sending email directly to third-party servers. SORBS also operated another list which is similar to the Spamhaus PBL called the NoServers list, which is wholly maintained by the network administrators of the respective networks and is therefore theoretically False Positive free.

Escalated listings SORBS has been accused of deliberately targeting innocent users through escalated listings. Its website describes the process as follows: "An escalated listing on the other hand is where a whole network of IP addresses is listed in SORBS and all hosts and IPs (whether assigned to a single customer or multiple) are listed and therefore blocked or result in spam folder issues. Why does SORBS create escalated listings? The simple answer is to stop spam. You ask, 'How does listing innocent IPs help stop spam?' Simple, some providers don’t care about spam." There have been many heated discussions on this practice as often it would appear that email users who are caught in this trap have no recourse, because the listing applies to a block of IP addresses, and they are unable to release their own IP address.

False positives Due to the automation of SORBS listings it was possible for the addresses of legitimate mail servers to be listed from time to time. Therefore, users of the SORBS Spam list in particular should consider carefully any such implications and may wish to use the service as part of a larger spam blocking system. The SORBS 'No Servers' list is reported to be wholly administered by the network administrators of the networks concerned therefore it should be false positive free.

Statistics SORBS produced and published daily statistics about its list to the otherwise defunct usenet newsgroup news.admin.net-abuse.bulletins (NANAB). As of 7 April 2021 statistics published showed the following listing totals:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Spam and Open Relay Blocking System

Start with the simplest possible case. Write down what Spam and Open Relay Blocking System claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Spam and Open Relay Blocking System before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Spam and Open Relay Blocking System ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Spam and Open Relay Blocking System

In research: Spam and Open Relay Blocking System appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Spam and Open Relay Blocking System in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Spam and Open Relay Blocking System is common in secondary-school and first-year university syllabi. It links to neighbouring topics Email, Spamming, so understanding it makes those chapters shorter.
In everyday life: Look for Spam and Open Relay Blocking System outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Spam and Open Relay Blocking System in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Spam and Open Relay Blocking System means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Spam and Open Relay Blocking System out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Spam and Open Relay Blocking System in simple terms?

Why does Spam and Open Relay Blocking System matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Spam and Open Relay Blocking System?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Spam and Open Relay Blocking System.