server: report HTTP handler panics to Sentry (closes #107)

DNSWATCHER_SENTRY_DSN was read but never used. This ports the Sentry
integration from gohttpserver with sentry-go v0.49.0. The server's start
hook calls sentry.Init when the DSN is set; a DSN Sentry cannot parse
fails the hook, so startup stops with the parse error. sentryhttp, with
Repanic, reports handler panics and passes them on to chi's Recoverer.
Shutdown sends queued reports once the HTTP server has stopped. The
client uses the older transport (DisableTelemetryBuffer): with the
default one, Flush can return before sending a report made just before
it. Client reports are off, so only panics are sent. The DSN is checked
at server start, not in config, so the config test is unchanged.
sentry-go raises several golang.org/x modules and moves go-spew and
go-difflib to untagged commits.

Model: opus-5-5

README.md

@@ -347,9 +347,9 @@ minute, failed logins included; beyond that it answers `429 Too Many Requests`
 without checking the password. A Prometheus server scraping every 15 seconds
 sends 4 a minute. IPv6 addresses in one /64 count as one client. When the
 request comes from a private or loopback address, such as a reverse proxy's,
-the client address is taken from the `X-Real-IP` or `X-Forwarded-For` header
-the proxy sets; a proxy that sets neither makes all its clients share one
-allowance.
+the client address is taken from the `X-Real-IP` header the proxy sets, or else
+from `X-Forwarded-For`, as the last address in it that is not private or
+loopback. A proxy that sets neither makes all its clients share one allowance.
 
 **`DNSWATCHER_DNS_INTERVAL` and `DNSWATCHER_TLS_INTERVAL`** take a positive
 duration: a number followed by a unit such as `s`, `m` or `h`, for example
@@ -359,6 +359,13 @@ either is set to anything else, including a bare number or a zero or negative
 duration, dnswatcher refuses to start with an error naming the variable and
 the value.
 
+**`DNSWATCHER_SENTRY_DSN` reports crashes in HTTP requests to Sentry.** When it
+is set, a panic in an HTTP request handler is sent to Sentry, and the request
+still gets a `500 Internal Server Error` answer. Nothing else is sent to Sentry:
+DNS, port and TLS problems are reported as notifications. A value Sentry cannot
+parse stops dnswatcher at startup. At shutdown, reports not yet sent are sent,
+waiting at most 2 seconds.
+
 ### Example `.env`
 
 ```sh
@@ -618,7 +625,8 @@ repository's `Dockerfile` and runs it. The app needs:
    abandoned, and the number abandoned is logged at warn level rather
    than dropped silently. Notifications generated after shutdown has
    begun are refused and logged, so a late burst cannot extend the
-   shutdown.
+   shutdown. A DNS lookup, port check or TLS check that shutdown cuts
+   short saves nothing and sends no notification.
 
 ---
 

TODO.md

@@ -19,6 +19,12 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 
 # Completed Steps
 
+- 2026-10-01: `DNSWATCHER_SENTRY_DSN` reports panics in HTTP handlers to Sentry,
+  and a DSN Sentry cannot parse stops startup (closes #107).
+- 2026-10-01: a port or TLS check that shutdown cuts short saves nothing and
+  sends no notification, as a cut-short DNS lookup already did (closes #185).
+- 2026-10-01: the client address from `X-Forwarded-For` is the last entry that
+  is not a trusted proxy, not the first, which the client sets (closes #181).
 - 2026-10-01: a nameserver that does not answer is saved as `error` with the
   reason, and NS failure and NS recovery are notified (closes #104).
 - 2026-10-01: a `DNSWATCHER_DNS_INTERVAL` or `DNSWATCHER_TLS_INTERVAL` that is
@@ -97,8 +103,6 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 
 # Future Steps
 
-- `DNSWATCHER_SENTRY_DSN` does nothing:
-  https://git.eeqj.de/sneak/dnswatcher/issues/107
 - trial run of the finished image:
   https://git.eeqj.de/sneak/dnswatcher/issues/149
 - 1.0 readiness: run it with a real config and read the logs:

go.mod

@@ -4,6 +4,7 @@ go 1.25.5
 
 require (
 	github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8
+	github.com/getsentry/sentry-go v0.49.0
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/go-chi/cors v1.2.2
 	github.com/go-chi/httprate v0.16.0
@@ -13,20 +14,20 @@ require (
 	github.com/spf13/viper v1.21.0
 	github.com/stretchr/testify v1.11.1
 	go.uber.org/fx v1.24.0
-	golang.org/x/net v0.50.0
-	golang.org/x/sync v0.19.0
+	golang.org/x/net v0.56.0
+	golang.org/x/sync v0.21.0
 )
 
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
-	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.10 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
@@ -42,10 +43,10 @@ require (
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/mod v0.32.0 // indirect
-	golang.org/x/sys v0.41.0 // indirect
-	golang.org/x/text v0.34.0 // indirect
-	golang.org/x/tools v0.41.0 // indirect
+	golang.org/x/mod v0.37.0 // indirect
+	golang.org/x/sys v0.46.0 // indirect
+	golang.org/x/text v0.39.0 // indirect
+	golang.org/x/tools v0.47.0 // indirect
 	google.golang.org/protobuf v1.36.8 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

go.sum

@@ -4,18 +4,22 @@ github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
-github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
+github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHkI4W8=
 github.com/frankban/quicktest v1.14.6/go.mod h1:4ptaffx2x8+WTWXmUCuVU6aPUX1/Mz7zb5vbUoiM6w0=
 github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
 github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
+github.com/getsentry/sentry-go v0.49.0 h1:Ehejknu1l023Ub7QoRBVLAI7g3Jnhqku4oWx4B4Sh5s=
+github.com/getsentry/sentry-go v0.49.0/go.mod h1:nuMJAoCfe1u0Bts2ocyNI+TW8HT84vRMqwA5Qq/SKUI=
 github.com/go-chi/chi/v5 v5.2.5 h1:Eg4myHZBjyvJmAFjFvWgrqDTXFyOzjj7YIm3L3mu6Ug=
 github.com/go-chi/chi/v5 v5.2.5/go.mod h1:X7Gx4mteadT3eDOMTsXzmI4/rwUpOwBHLpAfupzFJP0=
 github.com/go-chi/cors v1.2.2 h1:Jmey33TE+b+rB7fT8MUy1u0I4L+NARQlK6LhzKPSyQE=
 github.com/go-chi/cors v1.2.2/go.mod h1:sSbTewc+6wYHBBCW7ytsFSn836hqM7JxpglAy2Vzc58=
 github.com/go-chi/httprate v0.16.0 h1:8V5DH9j6pSK6UQoBsTpvMyFxycqaKEIToyPKzHJjUa8=
 github.com/go-chi/httprate v0.16.0/go.mod h1:A8lo+qRhk+s9LiuP5saS7XCGDXRXMcrueq0NfIuCa/I=
+github.com/go-errors/errors v1.4.2 h1:J6MZopCL4uSllY1OfXM374weqZFFItUbrImctkmUxIA=
+github.com/go-errors/errors v1.4.2/go.mod h1:sIVyrIiJhuEF+Pj9Ebtd6P/rEYROXFi3BopGUQ5a5Og=
 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=
 github.com/go-viper/mapstructure/v2 v2.4.0/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
@@ -38,8 +42,12 @@ github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq
 github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/pelletier/go-toml/v2 v2.2.4 h1:mye9XuhQ6gvn5h28+VilKrrPoQVanw5PMw/TB0t5Ec4=
 github.com/pelletier/go-toml/v2 v2.2.4/go.mod h1:2gIqNv+qfxSVS7cM2xJQKtLSTLUE9V8t9Stt+h56mCY=
-github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/pingcap/errors v0.11.4 h1:lFuQV/oaUMGcD2tqt+01ROSmJs75VG1ToEOkZIZ4nE4=
+github.com/pingcap/errors v0.11.4/go.mod h1:Oi8TUi2kEtXXLMJk9l1cGmz20kV3TaQ0usTwv5KuLY8=
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
+github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 h1:Jamvg5psRIccs7FGNTlIRMkT8wgtp5eCXdBlqhYGL6U=
+github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/prometheus/client_golang v1.23.2 h1:Je96obch5RDVy3FDMndoUsjAhG5Edi49h0RJWRi/o0o=
 github.com/prometheus/client_golang v1.23.2/go.mod h1:Tb1a6LWHB3/SPIzCoaDXI4I8UHKeFTEQ1YCr+0Gyqmg=
 github.com/prometheus/client_model v0.6.2 h1:oBsgwpGs7iVziMvrGhE53c/GrLUsZdHnqNwqPLxwZyk=
@@ -48,8 +56,8 @@ github.com/prometheus/common v0.66.1 h1:h5E0h5/Y8niHc5DlaLlWLArTQI7tMrsfQjHV+d9Z
 github.com/prometheus/common v0.66.1/go.mod h1:gcaUsgf3KfRSwHY4dIMXLPV0K/Wg1oZ8+SbZk/HH/dA=
 github.com/prometheus/procfs v0.16.1 h1:hZ15bTNuirocR6u0JZ6BAHHmwS1p8B4P6MRqxtzMyRg=
 github.com/prometheus/procfs v0.16.1/go.mod h1:teAbpZRB1iIAJYREa1LsoWUXykVXA1KlTmWl8x/U+Is=
-github.com/rogpeppe/go-internal v1.10.0 h1:TMyTOH3F/DB16zRVcYyreMH6GnZZrwQVAoYjRBZyWFQ=
-github.com/rogpeppe/go-internal v1.10.0/go.mod h1:UQnix2H7Ngw/k4C5ijL5+65zddjncjaFoBhdsK/akog=
+github.com/rogpeppe/go-internal v1.14.1 h1:UQB4HGPB6osV0SQTLymcB4TgvyWu6ZyliaW0tI/otEQ=
+github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7so1lCWt35ZSgc=
 github.com/sagikazarmark/locafero v0.11.0 h1:1iurJgmM9G3PA/I+wWYIOw/5SyBtxapeHDcg+AAIFXc=
 github.com/sagikazarmark/locafero v0.11.0/go.mod h1:nVIGvgyzw595SUSUE6tvCp3YYTeHs15MvlmU87WwIik=
 github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 h1:+jumHNA0Wrelhe64i8F6HNlS8pkoyMv5sreGx2Ry5Rw=
@@ -84,18 +92,18 @@ go.yaml.in/yaml/v2 v2.4.2 h1:DzmwEr2rDGHl7lsFgAHxmNz/1NlQ7xLIrlN2h5d1eGI=
 go.yaml.in/yaml/v2 v2.4.2/go.mod h1:081UH+NErpNdqlCXm3TtEran0rJZGxAYx9hb/ELlsPU=
 go.yaml.in/yaml/v3 v3.0.4 h1:tfq32ie2Jv2UxXFdLJdh3jXuOzWiL1fo0bu/FbuKpbc=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
-golang.org/x/mod v0.32.0 h1:9F4d3PHLljb6x//jOyokMv3eX+YDeepZSEo3mFJy93c=
-golang.org/x/mod v0.32.0/go.mod h1:SgipZ/3h2Ci89DlEtEXWUk/HteuRin+HHhN+WbNhguU=
-golang.org/x/net v0.50.0 h1:ucWh9eiCGyDR3vtzso0WMQinm2Dnt8cFMuQa9K33J60=
-golang.org/x/net v0.50.0/go.mod h1:UgoSli3F/pBgdJBHCTc+tp3gmrU4XswgGRgtnwWTfyM=
-golang.org/x/sync v0.19.0 h1:vV+1eWNmZ5geRlYjzm2adRgW2/mcpevXNg50YZtPCE4=
-golang.org/x/sync v0.19.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
-golang.org/x/sys v0.41.0 h1:Ivj+2Cp/ylzLiEU89QhWblYnOE9zerudt9Ftecq2C6k=
-golang.org/x/sys v0.41.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
-golang.org/x/text v0.34.0 h1:oL/Qq0Kdaqxa1KbNeMKwQq0reLCCaFtqu2eNuSeNHbk=
-golang.org/x/text v0.34.0/go.mod h1:homfLqTYRFyVYemLBFl5GgL/DWEiH5wcsQ5gSh1yziA=
-golang.org/x/tools v0.41.0 h1:a9b8iMweWG+S0OBnlU36rzLp20z1Rp10w+IY2czHTQc=
-golang.org/x/tools v0.41.0/go.mod h1:XSY6eDqxVNiYgezAVqqCeihT4j1U2CCsqvH3WhQpnlg=
+golang.org/x/mod v0.37.0 h1:vF1DjpVEshcIqoEaauuHebaLk1O1forxjxBaVn884JQ=
+golang.org/x/mod v0.37.0/go.mod h1:m8S8VeM9r4dzDwjrKO0a1sZP3YjeMamRRlD+fmR2Q/0=
+golang.org/x/net v0.56.0 h1:Rw8j/hFzGvJUZwNBXnAtf5sVDVt+65SK2C7IxCxZt5o=
+golang.org/x/net v0.56.0/go.mod h1:D3Ku6r+V6JROoZK144D2XfMHFcMq/0zSfLelVTCFKec=
+golang.org/x/sync v0.21.0 h1:HLII4xRRTtCRkxYp4HNFF0Js/Og6q2i++KXbg0gHCwM=
+golang.org/x/sync v0.21.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
+golang.org/x/sys v0.46.0 h1:noSf2Fq6F8DBgS+LysIkx7rIExoNHJsxOAtPp4rthXw=
+golang.org/x/sys v0.46.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/text v0.39.0 h1:UbZz4pLOvn600D6Oh6GGEI6VAmndrEBLv8/6BEXzyus=
+golang.org/x/text v0.39.0/go.mod h1:3UwRclnC2g0TU9x8PZiyfOajCd1zaUNHF9cvqcQZ+ZM=
+golang.org/x/tools v0.47.0 h1:7Kn5x/d1svx/PzryTsqeoZN4TZwqeH5pGWjefhLi/1Q=
+golang.org/x/tools v0.47.0/go.mod h1:dFHnyTvFWY212G+h7ZY4Vsp/K3U4/7W9TyVaAul8uCA=
 google.golang.org/protobuf v1.36.8 h1:xHScyCOEuuwZEc6UtSOvPbAT4zRh0xcNRYekJwfqyMc=
 google.golang.org/protobuf v1.36.8/go.mod h1:fuxRtAxBytpl4zzqUh6/eyUujkJdNiuEkXntxiD/uRU=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

internal/middleware/export_test.go

@@ -1,6 +1,9 @@
 package middleware
 
-import "time"
+import (
+	"net/http"
+	"time"
+)
 
 // The /metrics rate limit, exported so the tests can count requests
 // against it.
@@ -8,3 +11,9 @@ const (
 	MetricsRequestLimit                = metricsRequestLimit
 	MetricsRequestWindow time.Duration = metricsRequestWindow
 )
+
+// RealIP is realIP, exported so the tests can check which address it
+// takes as the client's.
+func RealIP(r *http.Request) string {
+	return realIP(r)
+}

internal/middleware/middleware.go

@@ -209,6 +209,12 @@ func isTrustedProxy(ip net.IP) bool {
 
 // realIP extracts the client's real IP address from the request.
 // Proxy headers are only trusted from RFC1918/loopback addresses.
+//
+// Each proxy adds to the end of X-Forwarded-For the address it got the
+// request from, so the client can write every entry before the one the
+// first trusted proxy added. The client address is therefore the
+// rightmost entry that is not a trusted proxy, or the leftmost entry
+// when they all are.
 func realIP(r *http.Request) string {
 	addr := ipFromHostPort(r.RemoteAddr)
 	remoteIP := net.ParseIP(addr)
@@ -223,16 +229,26 @@ func realIP(r *http.Request) string {
 		return ip
 	}
 
-	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
-		if parts := strings.SplitN(
-			xff, ",", 2, //nolint:mnd
-		); len(parts) > 0 {
-			if ip := strings.TrimSpace(parts[0]); ip != "" {
-				return ip
-			}
+	// A proxy may add its entry as a header line of its own instead of
+	// appending to the line the client sent, so all lines form one list.
+	entries := strings.Split(
+		strings.Join(r.Header.Values("X-Forwarded-For"), ","), ",",
+	)
+	client := strings.TrimSpace(entries[0])
+
+	for i := len(entries) - 1; i > 0; i-- {
+		entry := strings.TrimSpace(entries[i])
+		if !isTrustedProxy(net.ParseIP(entry)) {
+			client = entry
+
+			break
 		}
 	}
 
+	if client != "" {
+		return client
+	}
+
 	return addr
 }
 

internal/middleware/middleware_test.go

@@ -342,9 +342,9 @@ func TestDashboardRendersWithSecurityHeaders(t *testing.T) {
 	}
 }
 
-// Addresses for the rate limit tests: a client connecting directly, a
-// trusted proxy, and a client behind that proxy as its X-Real-IP
-// header names it.
+// Addresses for the rate limit and realIP tests: a client connecting
+// directly, a trusted proxy, and a client behind that proxy as the
+// proxy's X-Real-IP or X-Forwarded-For header names it.
 const (
 	directClient  = "198.51.100.1:4000"
 	trustedProxy  = "10.0.0.1:4000"
@@ -482,3 +482,84 @@ func TestMetricsRateLimitKeysOnClientAddress(t *testing.T) {
 		})
 	}
 }
+
+// TestRealIP checks which address realIP takes as the client's. Each
+// element of forwardedFor is sent as an X-Forwarded-For header line of
+// its own, and 198.51.100.9 is always an entry the client wrote itself.
+func TestRealIP(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		remoteAddr   string
+		xRealIP      string
+		forwardedFor []string
+		want         string
+	}{
+		{
+			"untrusted peer, both headers ignored",
+			directClient, proxiedClient, []string{"198.51.100.9"},
+			"198.51.100.1",
+		},
+		{
+			"X-Real-IP from a trusted proxy wins",
+			trustedProxy, proxiedClient, []string{"203.0.113.8"},
+			proxiedClient,
+		},
+		{
+			"client's own entry, then the one the proxy added",
+			trustedProxy, "", []string{"198.51.100.9, 203.0.113.1"},
+			proxiedClient,
+		},
+		{
+			"several trusted proxies",
+			trustedProxy, "",
+			[]string{"198.51.100.9, 203.0.113.1, 10.0.0.3, 10.0.0.2"},
+			proxiedClient,
+		},
+		{
+			"proxy adds a header line of its own",
+			trustedProxy, "", []string{"198.51.100.9", proxiedClient},
+			proxiedClient,
+		},
+		{
+			"every entry a trusted proxy",
+			trustedProxy, "", []string{"10.0.0.3, 10.0.0.2"},
+			"10.0.0.3",
+		},
+		{
+			"empty where the client address belongs",
+			trustedProxy, "", []string{"203.0.113.1, , 10.0.0.2"},
+			"10.0.0.1",
+		},
+		{
+			"no headers from a trusted proxy",
+			trustedProxy, "", nil,
+			"10.0.0.1",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/", nil,
+			)
+			req.RemoteAddr = tt.remoteAddr
+
+			if tt.xRealIP != "" {
+				req.Header.Set("X-Real-IP", tt.xRealIP)
+			}
+
+			for _, line := range tt.forwardedFor {
+				req.Header.Add("X-Forwarded-For", line)
+			}
+
+			got := middleware.RealIP(req)
+			if got != tt.want {
+				t.Errorf("realIP = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}

internal/server/export_test.go

@@ -3,6 +3,8 @@ package server
 import (
 	"net/http"
 	"time"
+
+	"github.com/go-chi/chi/v5"
 )
 
 // RequestTimeout exports the handler execution budget applied by
@@ -22,3 +24,15 @@ func SetListenPort(s *Server, port int) {
 func HTTPServerOf(s *Server) *http.Server {
 	return s.httpServer
 }
+
+// EnableSentry runs the Sentry setup that the start hook runs, without
+// starting the HTTP server.
+func EnableSentry(s *Server) error {
+	return s.enableSentry()
+}
+
+// RouterOf returns the router SetupRoutes built, so a test can add a
+// route that panics.
+func RouterOf(s *Server) *chi.Mux {
+	return s.router
+}

internal/server/routes.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 	"time"
 
+	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/go-chi/chi/v5"
 	chimw "github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -25,6 +26,16 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(s.mw.Logging())
 	s.router.Use(chimw.Timeout(requestTimeout))
 
+	// Report panics in handlers to Sentry when DNSWATCHER_SENTRY_DSN is
+	// set. Repanic passes each panic on to chimw.Recoverer above, which
+	// still answers the request.
+	if s.sentryEnabled {
+		sentryHandler := sentryhttp.New(sentryhttp.Options{
+			Repanic: true,
+		})
+		s.router.Use(sentryHandler.Handle)
+	}
+
 	// Public, unauthenticated, read-only routes, the only ones
 	// REPO_POLICIES.md allows wildcard CORS on. CORS is middleware of
 	// this whole router, not of a Group, so that it also answers

internal/server/sentry_test.go

@@ -0,0 +1,190 @@
+package server_test
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/getsentry/sentry-go"
+	"github.com/spf13/viper"
+	"go.uber.org/fx"
+
+	"sneak.berlin/go/dnswatcher/internal/server"
+)
+
+// The tests below set env vars and touch the global state of viper and
+// of Sentry, so they cannot use t.Parallel.
+
+// standInDelay is how long the Sentry stand-in takes to answer. It
+// records a report only then, so a report that Shutdown did not wait
+// for has not been recorded yet when Shutdown returns.
+const standInDelay = 100 * time.Millisecond
+
+// sentryStandIn is a local HTTP server in place of Sentry's, so that
+// nothing a test reports leaves the host. It keeps the body of every
+// request it receives.
+type sentryStandIn struct {
+	server *httptest.Server
+	mu     sync.Mutex
+	bodies []string
+}
+
+func newSentryStandIn(t *testing.T) *sentryStandIn {
+	t.Helper()
+
+	standIn := &sentryStandIn{}
+	standIn.server = httptest.NewServer(http.HandlerFunc(
+		func(_ http.ResponseWriter, r *http.Request) {
+			body, err := io.ReadAll(r.Body)
+			if err != nil {
+				t.Errorf("reading request to the Sentry stand-in: %v", err)
+			}
+
+			time.Sleep(standInDelay)
+
+			standIn.mu.Lock()
+			defer standIn.mu.Unlock()
+
+			standIn.bodies = append(standIn.bodies, string(body))
+		},
+	))
+	t.Cleanup(standIn.server.Close)
+
+	return standIn
+}
+
+// dsn returns a DSN that points Sentry at the stand-in.
+func (s *sentryStandIn) dsn(t *testing.T) string {
+	t.Helper()
+
+	dsn, err := url.Parse(s.server.URL)
+	if err != nil {
+		t.Fatalf("parsing the stand-in URL: %v", err)
+	}
+
+	dsn.User = url.User("public-key")
+	dsn.Path = "/1"
+
+	return dsn.String()
+}
+
+// received reports whether a request to the stand-in contained text.
+func (s *sentryStandIn) received(text string) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for _, body := range s.bodies {
+		if strings.Contains(body, text) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func TestSentryUnsetDoesNothing(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_SENTRY_DSN", "")
+
+	srv := buildServer(t)
+
+	err := server.EnableSentry(srv)
+	if err != nil {
+		t.Fatalf("Sentry setup with no DSN: %v", err)
+	}
+
+	if sentry.CurrentHub().Client() != nil {
+		t.Error("Sentry was set up with no DSN configured")
+	}
+}
+
+// TestSentryReportsHandlerPanic checks that with a valid DSN a panic in
+// a handler is reported to Sentry, still reaches chimw.Recoverer, and
+// has been sent by the time Shutdown returns, and that nothing else is
+// sent to Sentry.
+func TestSentryReportsHandlerPanic(t *testing.T) {
+	standIn := newSentryStandIn(t)
+
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_SENTRY_DSN", standIn.dsn(t))
+
+	srv := buildServer(t)
+
+	err := server.EnableSentry(srv)
+	if err != nil {
+		t.Fatalf("Sentry setup with a valid DSN: %v", err)
+	}
+
+	// Sentry's client is global: close it so later tests find none.
+	t.Cleanup(func() {
+		sentry.CurrentHub().Client().Close()
+		sentry.CurrentHub().BindClient(nil)
+	})
+
+	const panicMessage = "handler panic in the Sentry test"
+
+	srv.SetupRoutes()
+	server.RouterOf(srv).Get(
+		"/panic",
+		func(http.ResponseWriter, *http.Request) {
+			panic(panicMessage)
+		},
+	)
+
+	// An ordinary request first: with client reports on, Sentry would
+	// add a count of its dropped transaction to the panic report.
+	serve(srv, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/.well-known/healthcheck", nil,
+	))
+
+	rec := serve(srv, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/panic", nil,
+	))
+	if rec.Code != http.StatusInternalServerError {
+		t.Errorf(
+			"status %d, want %d from the recoverer",
+			rec.Code, http.StatusInternalServerError,
+		)
+	}
+
+	err = srv.Shutdown(t.Context())
+	if err != nil {
+		t.Fatalf("Shutdown: %v", err)
+	}
+
+	if !standIn.received(panicMessage) {
+		t.Error("the panic had not been sent to Sentry when Shutdown returned")
+	}
+
+	if standIn.received("client_report") {
+		t.Error("Sentry was sent a client report, not only the panic")
+	}
+}
+
+func TestSentryInvalidDSNStopsStartup(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+	t.Setenv("DNSWATCHER_DATA_DIR", t.TempDir())
+	// Sentry cannot parse this: it has no public key before the host.
+	t.Setenv("DNSWATCHER_SENTRY_DSN", "https://sentry.test/1")
+
+	app := newServerApp(fx.Invoke(func(*server.Server) {}))
+
+	err := app.Start(t.Context())
+	if err == nil {
+		_ = app.Stop(t.Context())
+
+		t.Fatal("startup succeeded with an invalid DSN")
+	}
+
+	if !strings.Contains(err.Error(), "invalid DNSWATCHER_SENTRY_DSN") {
+		t.Errorf("startup error does not name the setting: %v", err)
+	}
+}

internal/server/server.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"time"
 
+	"github.com/getsentry/sentry-go"
 	"github.com/go-chi/chi/v5"
 	"go.uber.org/fx"
 
@@ -33,6 +34,10 @@ type Params struct {
 // shutdownTimeout is how long to wait for graceful shutdown.
 const shutdownTimeout = 30 * time.Second
 
+// sentryFlushTimeout is how long shutdown waits for Sentry to send the
+// error reports it still holds.
+const sentryFlushTimeout = 2 * time.Second
+
 // Socket-level timeouts for the HTTP server.
 //
 // These bound time spent on the connection itself and are a distinct
@@ -82,14 +87,15 @@ const (
 
 // Server is the HTTP server.
 type Server struct {
-	startupTime time.Time
-	port        int
-	log         *slog.Logger
-	router      *chi.Mux
-	httpServer  *http.Server
-	params      Params
-	mw          *middleware.Middleware
-	handlers    *handlers.Handlers
+	startupTime   time.Time
+	port          int
+	sentryEnabled bool
+	log           *slog.Logger
+	router        *chi.Mux
+	httpServer    *http.Server
+	params        Params
+	mw            *middleware.Middleware
+	handlers      *handlers.Handlers
 }
 
 // New creates a new Server instance.
@@ -108,6 +114,12 @@ func New(
 	lifecycle.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
 			srv.startupTime = time.Now()
+
+			err := srv.enableSentry()
+			if err != nil {
+				return err
+			}
+
 			go srv.Run()
 
 			return nil
@@ -152,8 +164,11 @@ func (s *Server) Run() {
 	}
 }
 
-// Shutdown gracefully shuts down the server.
+// Shutdown gracefully shuts down the server, then sends the error
+// reports Sentry still holds.
 func (s *Server) Shutdown(ctx context.Context) error {
+	defer s.flushSentry()
+
 	if s.httpServer == nil {
 		return nil
 	}
@@ -184,3 +199,45 @@ func (s *Server) ServeHTTP(
 ) {
 	s.router.ServeHTTP(writer, request)
 }
+
+// enableSentry turns on Sentry error reporting when
+// DNSWATCHER_SENTRY_DSN is set, and does nothing when it is not. A DSN
+// that Sentry cannot parse is an error, so that startup stops instead
+// of running without the error reporting the operator asked for.
+func (s *Server) enableSentry() error {
+	if s.params.Config.SentryDSN == "" {
+		return nil
+	}
+
+	err := sentry.Init(sentry.ClientOptions{
+		Dsn:     s.params.Config.SentryDSN,
+		Release: s.params.Globals.Appname + "-" + s.params.Globals.Version,
+		// Use the transport that queues each report as it is made. With
+		// the default one, Flush can return before sending a report made
+		// just before it, such as one from the last request at shutdown.
+		DisableTelemetryBuffer: true,
+		// Send panic reports only, not Sentry's counts of what it dropped,
+		// such as the transaction it starts for every request.
+		DisableClientReports: true,
+	})
+	if err != nil {
+		return fmt.Errorf("invalid DNSWATCHER_SENTRY_DSN: %w", err)
+	}
+
+	s.log.Info("sentry error reporting activated")
+	s.sentryEnabled = true
+
+	return nil
+}
+
+// flushSentry sends the error reports Sentry still holds, waiting at
+// most sentryFlushTimeout.
+func (s *Server) flushSentry() {
+	if !s.sentryEnabled {
+		return
+	}
+
+	if !sentry.Flush(sentryFlushTimeout) {
+		s.log.Warn("sentry flush timed out; some error reports were not sent")
+	}
+}

internal/server/server_test.go

@@ -17,18 +17,13 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/state"
 )
 
-// buildServer wires a *server.Server exactly as cmd/dnswatcher does,
-// minus the watcher/resolver subtree that would touch live DNS. fx
-// builds the object graph but the lifecycle is never started, so no
-// OnStart hook runs and nothing listens or resolves. The caller must
-// first configure viper (config.New reads it), which is also why the
-// caller cannot run in parallel.
-func buildServer(t *testing.T) *server.Server {
-	t.Helper()
-
-	var srv *server.Server
-
-	app := fx.New(
+// newServerApp builds an fx app holding a *server.Server wired exactly
+// as cmd/dnswatcher wires it, minus the watcher/resolver subtree that
+// would touch live DNS, plus the given option. config.New reads viper,
+// so the caller must first configure it, which is also why the caller
+// cannot run in parallel.
+func newServerApp(option fx.Option) *fx.App {
+	return fx.New(
 		fx.NopLogger,
 		fx.Provide(
 			globals.New,
@@ -41,8 +36,18 @@ func buildServer(t *testing.T) *server.Server {
 			handlers.New,
 			server.New,
 		),
-		fx.Populate(&srv),
+		option,
 	)
+}
+
+// buildServer builds the server without starting the app's lifecycle,
+// so no OnStart hook runs and nothing listens or resolves.
+func buildServer(t *testing.T) *server.Server {
+	t.Helper()
+
+	var srv *server.Server
+
+	app := newServerApp(fx.Populate(&srv))
 
 	err := app.Err()
 	if err != nil {

internal/watcher/cancelled_test.go

@@ -0,0 +1,81 @@
+package watcher_test
+
+import (
+	"context"
+	"log/slog"
+	"reflect"
+	"testing"
+
+	"sneak.berlin/go/dnswatcher/internal/portcheck"
+	"sneak.berlin/go/dnswatcher/internal/resolver"
+	"sneak.berlin/go/dnswatcher/internal/state"
+	"sneak.berlin/go/dnswatcher/internal/tlscheck"
+	"sneak.berlin/go/dnswatcher/internal/watcher"
+)
+
+// TestCancelledCheckSavesNothing runs a check with its context already
+// cancelled, which is how the rest of a check runs once shutdown cuts it
+// short. The real resolver drops the DNS lookup without sending a query,
+// and the real port and TLS checkers fail without connecting. The port
+// and certificate state the last check saved must stay as it was, and
+// nothing may be notified.
+func TestCancelledCheckSavesNothing(t *testing.T) {
+	t.Parallel()
+
+	cfg := defaultTestConfig(t)
+	cfg.Hostnames = []string{host}
+
+	// newTestWatcher's watcher has stand-in checkers. This one, on the
+	// same state and notifier, has the real ones.
+	_, deps := newTestWatcher(t, cfg)
+	w := watcher.NewForTest(
+		cfg,
+		deps.state,
+		resolver.NewFromLogger(slog.Default()),
+		portcheck.NewStandalone(),
+		tlscheck.NewStandalone(),
+		deps.notifier,
+	)
+
+	// The last check found host at a local address, with both ports
+	// open and a good certificate.
+	const localIP = "127.0.0.1"
+
+	deps.state.SetHostnameState(host, hostnameState(
+		map[string]map[string][]string{nsA: {"A": {localIP}}},
+	))
+
+	ports := map[string]*state.PortState{
+		localIP + ":80":  {Open: true, Hostnames: []string{host}},
+		localIP + ":443": {Open: true, Hostnames: []string{host}},
+	}
+	for key, ps := range ports {
+		deps.state.SetPortState(key, ps)
+	}
+
+	certKey := localIP + ":443:" + host
+	cert := &state.CertificateState{CommonName: host, Status: "ok"}
+	deps.state.SetCertificateState(certKey, cert)
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	w.RunOnce(ctx)
+
+	for key, want := range ports {
+		got, _ := deps.state.GetPortState(key)
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("port %s saved as %+v, want %+v", key, got, want)
+		}
+	}
+
+	got, _ := deps.state.GetCertificateState(certKey)
+	if !reflect.DeepEqual(got, cert) {
+		t.Errorf("certificate saved as %+v, want %+v", got, cert)
+	}
+
+	notifications := deps.notifier.getNotifications()
+	if len(notifications) != 0 {
+		t.Errorf("sent %v, want no notifications", notifications)
+	}
+}

internal/watcher/watcher.go

@@ -658,6 +658,13 @@ func (w *Watcher) checkSinglePort(
 	hostnames []string,
 ) {
 	result, err := w.portCheck.CheckPort(ctx, ip, port)
+
+	// A check the context cut short says nothing about the port, so it
+	// is neither saved nor notified.
+	if ctx.Err() != nil {
+		return
+	}
+
 	if err != nil {
 		w.log.Error(
 			"port check failed",
@@ -733,6 +740,13 @@ func (w *Watcher) checkTLSCert(
 	hostname string,
 ) {
 	cert, err := w.tlsCheck.CheckCertificate(ctx, ip, hostname)
+
+	// A check the context cut short says nothing about the certificate,
+	// so it is neither saved nor notified.
+	if ctx.Err() != nil {
+		return
+	}
+
 	certKey := fmt.Sprintf("%s:%d:%s", ip, tlsPort, hostname)
 	now := time.Now().UTC()
 	prev, hasPrev := w.state.GetCertificateState(certKey)