[martijnhoekstra]

Description

Hermit reads the gateway as part of ip=<ip>/<prefix>:<gw> (hermit-os/kernel src/env/ip_config.rs). The separate gateway=<gw> that urunc writes now is logged as an unparsable boot argument, and the guest gets no default route.

This changes CommandString to write one ip= argument, and adds hermit_rs_test.go: the address with and without a gateway, with application arguments after --, and with only application arguments.

Compatibility: Hermit main reads this form since hermit-os/kernel f2f05350. No Hermit version has read gateway= so far, so nothing is breaking. The latest released version, v0.13.2, (03-13-2026) doesn't take this format though - it takes -ip, -mask and -gateway. This PR changes nothing for that or any other currently released version.

Related issues

• Fixes hermit: the guest gets no default route (gateway= is not a Hermit boot argument) #1197

How was this tested?

• make test_unikernels passes. On the old code, the two cases with a gateway fail.
• The other unit test targets pass. test_unikontainers passes as a non-root user; as root, TestCopyFile and TestMoveFile fail, on 8355257 too.
• golangci-lint v2.15, the version CI uses, reports nothing new. make lint uses v2.9, which can't load a go1.27.3 module.
• The ctr e2e suite (go test ./tests/e2e -run TestE2E --ginkgo.focus=Ctr) with urunc built by make static: 13 of 14 specs pass, including Qemu-hermit-hello-world. Firecracker-unikraft-hello fails with the Unikraft guest's Assertion failure: ml->flags == mr->flags on Firecracker v1.17.0, the same on 8355257. Skipped because the test machine lacks devmapper, virtiofsd and Hyperlight: the two rumprun specs, Qemu-linux-read-file-virtiofs and Hyperlight-unikraft-hello.
• End to end in the setup from hermit: the guest gets no default route (gateway= is not a Hermit boot argument) #1197: urunc runs -append "ip=10.4.0.3/24:10.4.0.1", the guest logs Gateway: 10.4.0.1, and the routed client (192.168.77.1) gets its echo. The on-link client still does.

LLM usage

Claude Opus 5.5 (claude-opus-5-5) helped write the change, the test and this description, and ran the tests above. I reviewed the change and the results.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

make lint can't run here: its pinned golangci-lint v2.9 doesn't load a go1.27.3 module (on main too). golangci-lint v2.15, the version CI uses, reports no new issues. I filed #1201 -- it's two-liner fix to the makefile

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[cmainas]

Hello @martijnhoekstra ,

The PR looks good, let;s mark it ready so we can merge it.