The best open source alternative to OpenDNS is OpenNIC, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to OpenDNS and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to OpenDNS are DNSCrypt Protocol (Free, Open Source), NixNet DNS (Free, Open Source), AdGuard DNS (Free Personal, Open Source) and AhaDNS (Free, Open Source).
DNSCrypt is a protocol that authenticates communications between a DNS client and a DNS resolver. It prevents DNS spoofing. It uses cryptographic signatures to verify that responses originate from the chosen DNS resolver and haven't been tampered with.
AdGuard DNS is a foolproof way to block Internet ads that does not require installing any applications. It is easy to use and provides you with minimal necessary functions to block ads, counters, malicious websites, and adult content.