V3.co.uk readers have reacted angrily to suggestions from security firm Symantec Hosted Services that a lack of knowledge and awareness about how to use Linux mail servers could be contributing to the disproportionately large number of Linux machines being exploited to send spam.
Symantec malware data analyst Mat Nisbet argued in a blog post last week that many companies running Linux to keep costs down "have not realised that leaving port 25 open to the internet also leaves them open to abuse".
Nisbet suggested that companies should "make sure that the systems are correctly set up to restrict access on port 25 to only authorised users" to limit the amount of spam being sent.
However, several V3.co.uk readers disagreed. One, known as uxp, said: "Port 25 is not the problem. It's like saying all XSS attacks can be solved by closing port 80."
Another, Eric Moreau, said: "How can you implement a mail server and not [leave] port 25 open?", while GC wrote: "If you block port 25 the whole email system will grind to a halt. Port 25 is the SMTP port used by email servers to exchange email."
However, Symantec Hosted Services senior analyst Paul Wood defended the firm's research and its advice.
"Some ISPs block clients from using TCP port 25 outbound as this is often how botnet-infected computers will transmit spam directly to other mail servers. Blocking it helps to minimise the amount of spam originating from their networks," he said.
"Some ISPs will re-enable this traffic if requested, such as for a particular corporate mail server. Some ISPs also enforce rate-limiting on this traffic to mitigate against large spam runs, triggering alerts when a large amount of traffic is detected."
Wood added that the blocking of port 25 means that the ISP's clients are obliged to use the ISP's 'smarthost' to route email traffic.
In this case, he said, "you would be expected to configure your mail server to send all outbound traffic via the smarthost", which will relay email only from IP addresses and authorised domains connected on the ISP's client-side networks.
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