docs: document musl static build challenges (#42)

Adds documentation explaining why musl static builds are not currently provided
and the technical challenges encountered during implementation attempts.

The main issue appears to be related to statically linking libpcap with musl,
which resulted in linker errors. The ubuntu-22.04 pinning already solves the
GLIBC compatibility issue reported in #40.

This document serves as a reference for anyone interested in pursuing musl
builds in the future.

MUSL_BUILD.md

@@ -0,0 +1,66 @@
+# musl Static Build Challenges
+
+This document explains why RustNet currently does not provide musl static builds and the technical challenges encountered during implementation attempts.
+
+## Background
+
+musl is a lightweight C standard library designed for static linking. It would allow RustNet to produce fully static binaries that work on any Linux distribution regardless of GLIBC version.
+
+## Why We Attempted musl Builds
+
+GitHub issue #40 reported that pre-built packages required GLIBC 2.38/2.39, which wasn't available on PopOS 22.04 (GLIBC 2.35). musl builds would theoretically solve this by creating fully static binaries.
+
+## Challenges Encountered
+
+### libpcap Linking Issues
+
+The primary challenge appears to be related to **libpcap** static linking with musl:
+
+- Installing `libpcap-dev` in Ubuntu-based cross-rs containers provides glibc-linked libraries
+- Attempting to statically link these with musl resulted in linker errors
+- Errors included undefined references to pthread, math (exp), and dynamic loading functions (dladdr)
+
+It's unclear whether this is due to:
+- Fundamental glibc/musl incompatibility when statically linking
+- Missing library specifications in the linker flags
+- Issues with how cross-rs musl images are configured
+- Something specific to our build configuration
+
+### eBPF Complications
+
+We initially attempted to include eBPF support, which required vendoring libelf and zlib. This was abandoned to simplify the problem, but even without eBPF the libpcap linking issues persisted.
+
+## Current Solution
+
+**We solved the original issue by pinning builds to ubuntu-22.04** (GLIBC 2.35), which ensures compatibility with PopOS 22.04 and similar distributions.
+
+For users on older distributions, the `cargo install` workaround is documented:
+```bash
+cargo install rustnet-monitor
+sudo setcap cap_net_raw,cap_net_admin=eip ~/.cargo/bin/rustnet
+```
+
+## Potential Future Approaches
+
+If someone wants to tackle musl builds in the future, areas to investigate:
+
+1. **Building libpcap from source** targeting musl in the pre-build step
+2. **Using Alpine Linux-based images** which have native musl packages
+3. **Custom linker flags** to properly link required libraries
+4. **Alternative pure-Rust packet capture** libraries (if they exist)
+
+We're uncertain which approach would work best, or if there are other issues we haven't discovered yet.
+
+## Why We're Not Pursuing This Now
+
+- The ubuntu-22.04 solution already addresses the reported issue
+- The complexity-to-benefit ratio seems high
+- `cargo install` provides a universal fallback for edge cases
+- More investigation would be needed to understand the root causes
+
+If you have experience with musl static linking and want to contribute, we'd welcome the help!
+
+---
+
+*Last updated: 2025-10-09*
+*Status: Not currently pursuing due to linking complexity*