Host Interface Detection
The agent detects the network interfaces on its host and reports them to the server. They appear in the Network interfaces section of the host page. This page explains how and when that detection happens, and what to do when an interface you expect is missing.
How interfaces are detected
Section titled “How interfaces are detected”The agent scans for the host’s interfaces once, when it starts up and connects to the server, and reports the usable ones. That list is then cached for the lifetime of the agent process, so it does not change until the agent restarts.
An interface is reported only when all of the following hold:
- It is up. A down interface is never reported.
- It has at least one IP address (IPv4 or IPv6). An interface with no address assigned is skipped, even when it is up.
- It is a real host interface. The loopback interface and Docker’s own interfaces (the default Docker bridge and the virtual interfaces Docker creates for containers) are filtered out. These are virtual or internal interfaces, not physical host uplinks.
The interface carrying the host’s default route is flagged as the default one.
When the agent itself runs inside a container it cannot read the host’s interfaces directly, so it briefly runs a helper with host networking to enumerate them. That helper needs to be able to start on the host.
A NIC is missing from the list
Section titled “A NIC is missing from the list”If an interface you expect is absent from the host page, check it in this order:
- Is it up? Bring the interface up on the host, then restart the agent so it scans again.
- Does it have an IP address? Assign at least one address to it, then restart the agent. This is a common surprise with a NIC that is kept up but never given an address: with no IP at all it is filtered out and never appears. Give it any address (even one on an unrelated subnet) and it will be reported.
- Is it a Docker or virtual interface? The loopback, the Docker bridge, and per-container virtual interfaces are excluded on purpose. Choose a physical host NIC instead.
- Did you change it after the agent started? The list is captured once at startup and cached. Any interface you bring up, assign an address to, or add later is only picked up after the agent restarts.