fix: preserve systemd-resolved stub in DNS auto-detection#50
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0.4.1 introduced auto-detection of the host's DNS servers in linuxMacDNSServers(). When /etc/resolv.conf contained only the systemd-resolved stub (127.0.0.53), the code fell back to /run/systemd/resolve/resolv.conf to grab the raw upstream nameservers. That bypasses systemd-resolved entirely: queries go straight to the upstream over plain UDP/53, dropping DoT, DNSSEC, and split-DNS. On hosts where resolved is configured with DoT-only upstreams (Mullvad with DNSOverTLS=opportunistic is the reported case), the raw UDP path is unreachable and every lookup times out. Read /etc/resolv.conf verbatim instead. On the typical systemd-resolved host that returns 127.0.0.53, and queries flow through resolved as the user configured. As a safety net for minimal images that ship without /etc/resolv.conf, fall back to /run/systemd/resolve/stub-resolv.conf, which also points at the stub. The raw upstream file is never read. Fixes #47
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Summary
linuxMacDNSServers(): read/etc/resolv.confverbatim instead of swapping the stub out for the raw upstreams in/run/systemd/resolve/resolv.conf./run/systemd/resolve/stub-resolv.confonly when/etc/resolv.confis absent (minimal images). The raw upstream file is never read.sysdns_test.gocovering the 127.0.0.53 regression, multi-nameserver order, IPv6 bracketing, malformed lines, and missing files.Why
0.4.1 (#43) replaced 127.0.0.53 with the raw Mullvad upstream from
/run/systemd/resolve/resolv.conf, so greyproxy tried plain UDP/53 to an upstream that systemd-resolved had been reaching over DoT. On hosts configured withDNSOverTLS=opportunistic(matclab's setup in #47) the direct UDP path is unreachable and every lookup times out. Letting queries flow through 127.0.0.53 keeps DoT, DNSSEC, and split-DNS intact.The container concern the original code cited (127.x.x.x being container-local) doesn't apply: greyproxy runs on the host, and the sandboxed client connects to it via the host's loopback, so 127.0.0.53 is reachable.
Fixes #47
Test plan
go test ./cmd/greyproxy/ -run TestResolvConf -vgo build ./...go vet ./cmd/greyproxy/