Introduction to rcfg-sim

rcfg-sim is a high-density SSH simulator for network devices. One Linux host stands up 50,000+ concurrent SSH listeners, each speaking real SSH and presenting a realistic device personality (Cisco IOS or Ciena 6500 TL1) that streams a generated running-config on demand.

It is purpose-built to load-test network automation and network configuration management (NCM) tooling at the scale real fleets actually reach — without buying hardware or running a datacentre of full-system emulators.

The problem it solves

Production network fleets routinely span hundreds to tens of thousands of devices. The failures that matter at that scale rarely reproduce in a small lab:

• Scheduler fairness when thousands of backups fire in the same window
• Burst handling and connection-pool exhaustion
• Diff-engine and storage behaviour against multi-megabyte configs
• Retry storms and timeout cascades under partial failure
• The real CPU / memory / file-descriptor envelope of a collector at scale

You cannot answer these with unit tests or a 50-device lab. The usual alternatives are expensive: roughly $2M of physical hardware, or full-system VM emulation that melts a datacentre. rcfg-sim gives you production-shaped load from a single commodity host (a 12 vCPU / 48 GB box comfortably drives the full 50k).

What it is

• A pair of small, dependency-light Go binaries: a config generator and an SSH server. See Architecture & concepts.
• A real SSH server, not a recording or a mock — clients negotiate a genuine SSH transport and read live bytes.
• A deterministic config generator that produces realistic Cisco IOS configs from ~30 KB to ~128 MB, byte-identical for a given seed.
• Observable and abusable on purpose: Prometheus metrics, fault injection, and tunable response latency.

What it is not

• Not a full Cisco emulator. It answers a bounded set of show commands convincingly; it does not implement IOS.
• Not a network topology or data-plane simulator. There is no routing, no forwarding, no link state. It is not a replacement for GNS3 or EVE-NG.
• Not a config-mutation engine. Devices serve configs; they do not accept config changes.

Who it is for

Anyone building or operating SSH-based network tooling: NCM platforms, backup/collection systems, automation frameworks, and the CI rigs that test them. It is maintained by the rConfig team and works with any SSH-based system — see Using rcfg-sim with rConfig.

Ready to try it? Head to the Quickstart.