Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.DNSchanger is the generic detection for Trojans that change a system’s DNS settings without the users’ knowledge or consent. Once the systems are infected and their DNS settings modified, systems use foreign DNS servers set up by the threat actors. Infected systems that attempt to access specific sites are redirected to sites specified by threat actors.
Domain Name System (DNS), is known as an Internet standard for the assigning of Internet Protocol (IP) addresses to domain names. Simply put, DNS interprets human-friendly host names to PC-friendly IP addresses.
It is common for users to automatically use DNS servers operated by their ISPs. Users who prefer more secure, faster, and reliable DNS servers usually go with a third-party service as ISP-administered DNS servers can be slow and unreliable.
More information about DNS and DNS hijacks can be found in our blog post, DNS Hijacks: What to Look For.
DNS changers/hijackers are usually bundled with other malware, such as rootkits, as seen in TDSS. Fake antivirus (FakeAV) programs have also been used to spread DNS changer Trojans, as seen in Rove Digital.
Malwarebytes can remove Trojan.DNSChanger without further user interaction. A reboot is often required to flush the “poisoned” DNS cache. Malwarebytes will prompt you to do this if needed.