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DNS hijacking, DNS poisoning, or DNS redirection is the practice of subverting the resolution of Domain Name System (DNS) queries. [1] This can be achieved by malware that overrides a computer's TCP/IP configuration to point at a rogue DNS server under the control of an attacker, or through modifying the behaviour of a trusted DNS server so that it does not comply with internet standards.
If you see an image challenge question when sending mail, you should just be able to complete the challenge then send mail. If, however, you correctly answer the image challenge and still can't send mail, it could be a sign of a more serious issue with your account, which may require additional steps to secure your account.
The Internet Software Consortium produced a version of the BIND DNS software that can be configured to filter out wildcard DNS records from specific domains. Various developers have produced software patches for BIND and for djbdns. Other DNS server programs have followed suit, providing the ability to ignore wildcard DNS records as configured.
Domain Name System blocking, or DNS blocking / filtering, is a strategy for making it difficult for users to locate specific domains or websites on the Internet. It was first introduced in 1997 as a means to block spam email from known malicious IP addresses .
dnsmasq caches DNS records, reducing the load on upstream nameservers and improving performance, and can be configured to automatically pick up the addresses of its upstream servers. dnsmasq accepts DNS queries and either answers them from a small, local cache or forwards them to a real, recursive DNS server.
In combination with poorly configured DNS (with no reverse lookup zone) using RFC 1918 addressing, reverse DNS updates using this authentication scheme are forwarded en masse to the root DNS servers and thus increase the traffic to root DNS servers. There is an anycast group which deals with this traffic to take it away from the root DNS servers.
For instance, Firefox usually keeps 20 domain entries cached for 60 seconds. This may be configured via the network.dnsCacheEntries and network.dnsCacheExpiration configuration variables. Flushing the system's DNS cache may also help, which can be achieved e.g. in Linux with sudo service dns-clean start or in Windows with ipconfig /flushdns.
Round-robin DNS is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol service hosts, e.g., Web server, FTP servers, by managing the Domain Name System's (DNS) responses to address requests from client computers according to an appropriate statistical model.