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(lemmy.zip)
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This could be a case of nodes running outdated software, the current standard protocols can handle any unknown configuration just fine. These are a source of a lot of holes and vulnerabilities that can pose a serious risk, they should be updated as soon as possible. If you have to keep outdated systems in your network e.g. running on legacy hardware, or make the ocasional connection, you'll have to monitor them and isolate them from your critical infrastructure, at least with a good firewall and a strict configuration when possible.