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docs: document environment variables #2417

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34 changes: 34 additions & 0 deletions src/lib.rs
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Expand Up @@ -35,6 +35,40 @@
)]
#![cfg_attr(feature = "document-features", doc = document_features::document_features!())]
//!
//! # Compile-time environment variables
//!
//! Compile-time environment variables are read from the build environment.
//!
//! - **`HERMIT_CAREFUL`** — Compiles the kernel with [`cargo careful`].
//!
//! [`cargo careful`]: https://github.com/RalfJung/cargo-careful
//!
//! # Run-time environment variables
//!
//! Run-time environment variables can be set via `env=KEY=VALUE` kernel args.
//!
//! ## Network environment variables
//!
//! - **`HERMIT_MTU`** — Sets the *maximum transmission unit* (MTU). Defaults to `1500`.
//! - **`HERMIT_MRG_RXBUF_SIZE`** — Sets the receive buffer size. Useful for testing receive buffer merging. Defaults to unset.
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@cagatay-y cagatay-y May 13, 2026

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We should probably clarify that the environment variable is only relevant for virtio-net devices and with a specific feature negotiated.

//! - **`HERMIT_IP`** — Sets the IPv4 address. Defaults to `10.0.5.3`.
//! - **`HERMIT_GATEWAY`** — Sets the gateway IPv4 address. Defaults to `10.0.5.1`.
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@cagatay-y cagatay-y May 13, 2026

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Since the DHCPv4 feature is enabled by default, it may be helpful to mention that the IP address set may be overwritten if addresses are successfully negotiated.

//! - **`HERMIT_MASK`** — Sets the network mask. Defaults to `255.255.255.0`.
//! - **`HERMIT_DNS1`** — Sets the first DNS server. Defaults to `9.9.9.9`.
//! - **`HERMIT_DNS2`** — Sets the second DNS server. Defaults to `1.1.1.1`.
//! - **`HERMIT_PCAP_PATH`** — Sets the packet capture file path. Defaults to `/root/`. See the `write-pcap-file` feature for details.
//!
//! ## Output environment variables
//!
//! - **`NO_COLOR`** — Prevents the addition of ANSI colors to the kernel output. Defaults to unset. For details, see [`NO_COLOR`].
//! - **`HERMIT_LOG_LEVEL_FILTER`** — Sets the lowest log level to print. Defaults to `info`.
//!
//! [`NO_COLOR`]: https://no-color.org/
//!
//! ## Deprecated environment variables
//!
//! - **`UHYVE_MOUNT`** — Sets the Uhyve mount point. Defaults to `/root`.
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@cagatay-y cagatay-y May 13, 2026

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Is there a preferred alternative for the deprecated variable? Might be useful to mention if there is one.

//!
//! [hermit-rs]: https://github.com/hermit-os/hermit-rs
//! [hermit-c]: https://github.com/hermit-os/hermit-c

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