UPSTREAM PR #17878: server : run child server on localhost

[loci-dev]

Mirrored from ggml-org/llama.cpp#17878

When passing in --host 0.0.0.0, the child runs on host 0.0.0.0 and the router tries to access it at 0.0.0.0. I can't think of why the child should not always run on 127.0.0.1.

get_free_port() binds to INADDR_ANY, which should select a port that is available across all interfaces. This can be changed to INADDR_LOOPBACK if we ensure the child will only ever bind to 127.0.0.1. If not, then INADDR_ANY is a safe choice.

fixes #17862

[loci-review]

Explore the complete analysis inside the Version Insights

Performance Analysis Summary: PR #496

Overview

This PR implements a networking configuration fix for the llama.cpp server router, forcing child server instances to bind exclusively to localhost (127.0.0.1) rather than inheriting the router's host configuration. The changes span 2 files with 7 line additions and 2 deletions, modifying only server infrastructure code without touching inference or tokenization logic.

Performance Impact

No measurable performance impact detected. Power consumption analysis across all binaries shows changes below 0.001%:

• build.bin.libllama.so: 0.22 nJ reduction (194,204 nJ baseline)
• build.bin.llama-run: 1.48 nJ reduction (219,166 nJ baseline)
• build.bin.llama-cvector-generator: 0.95 nJ increase (249,477 nJ baseline)
• build.bin.llama-tts: 0.73 nJ increase (253,600 nJ baseline)
• All other binaries: 0.0% change

Inference Performance: No impact on tokens per second. The modified code paths (server-models.cpp, server-models.h) handle only HTTP routing, child process management, and network configuration. Core inference functions (llama_decode, llama_encode, llama_tokenize) remain unchanged. No modifications to model loading, tokenization, sampling, KV cache, or computational graph execution.

Code Changes

The PR adds a hostname field to server_model_meta structure and implements three key modifications:

1. Sets inst.meta.hostname = "127.0.0.1" during model instance initialization
2. Passes --host 127.0.0.1 explicitly to child server processes via command-line arguments
3. Updates proxy connection logic to use meta->hostname instead of base_params.hostname

These changes resolve a routing failure where child servers bound to 0.0.0.0 were unreachable by the router. The fix is purely correctness-focused with no computational overhead—localhost connections have identical latency characteristics to the previous configuration, and the additional command-line argument adds negligible parsing overhead.

Security improvement: Child servers are no longer exposed on external network interfaces, reducing attack surface while maintaining functional equivalence for local routing scenarios.