A connection proxy is used to direct network traffic between systems or act as an intermediary for network communications. Many tools exist that enable traffic redirection through proxies or port redirection, including HTRAN, ZXProxy, and ZXPortMap. 
The definition of a proxy can also be expanded out to encompass trust relationships between networks in peer-to-peer, mesh, or trusted connections between networks consisting of hosts or systems that regularly communicate with each other.
The network may be within a single organization or across organizations with trust relationships. Adversaries could use these types of relationships to manage command and control communications, to reduce the number of simultaneous outbound network connections, to provide resiliency in the face of connection loss, or to ride over existing trusted communications paths between victims to avoid suspicion.
APT28 used other victims as proxies to relay command traffic, for instance using a compromised Georgian military email server as a hop point to NATO victims. The group has also used a tool that acts as a proxy to allow C2 even if the victim is behind a router. APT28 has also used a machine to relay and obscure communications between CHOPSTICK and their server.
Cobalt Strike can be configured to have commands relayed over a peer-to-peer network of infected hosts. This can be used to limit the number of egress points, or provide access to a host without direct internet access.
Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific C2 protocol used by a particular adversary or tool, and will likely be different across various malware families and versions. Adversaries will likely change tool C2 signatures over time or construct protocols in such a way as to avoid detection by common defensive tools. 
Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious. Network activities disassociated from user-driven actions from processes that normally require user direction are suspicious.
Analyze network data for uncommon data flows (e.g., a client sending significantly more data than it receives from a server or between clients that should not or often do not communicate with one another). Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious. Analyze packet contents to detect communications that do not follow the expected protocol behavior for the port that is being used. 
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