Client address is the right-most untrusted X-Forwarded-For entry (closes #144)

From a peer in TRUSTED_PROXIES, clientIP took the left-most
X-Forwarded-For entry, which the client writes itself behind a proxy
that appends to the header, so a client could choose the address the
rate limit counts and the request log records. It now walks the entries
from the right past trusted proxies and takes the first that is not
one. An entry written as an address with a port is read as that
address; with only trusted entries, or one that is not an address at
all, it uses the peer. Every X-Forwarded-For header line is read.
X-Real-IP is read only when there is no X-Forwarded-For entry.
README.md Settings says which entry is taken.

Model: opus-5-5

README.md

@@ -215,11 +215,12 @@ has its forwarded headers ignored, and the direct peer is logged and
 rate-limited instead. From a direct peer inside the set, the client address is
 the right-most `X-Forwarded-For` entry that is not in the set: each proxy
 appends the address it took the request from, so the entries further left were
-written by the client and are not used. If the search from the right finds only
+written by the client and are not used. An entry written as an address with a
-addresses in the set, or meets an entry that is not an IP address, the direct
+port, such as `198.51.100.7:5000` or `[2001:db8::1]:443`, is read as that
-peer is used. `X-Real-IP` is read only when `X-Forwarded-For` is missing or
+address. If the search from the right finds only addresses in the set, or meets
-empty. The container image does not use this default; see
+an entry that is not an address at all, such as `unknown`, the direct peer is
-[Deployment](#deployment).
+used. `X-Real-IP` is read only when `X-Forwarded-For` is missing or empty. The
+container image does not use this default; see [Deployment](#deployment).
 
 A variable set to a value the server cannot use, such as `PORT=abc`,
 `DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from

TODO.md

@@ -26,13 +26,15 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
   ([#144](https://git.eeqj.de/sneak/netwatch/issues/144)). It took the left-most
   entry, which the client writes itself behind a proxy that appends to the
   header, so a client could choose the address the rate limit counts and the
-  request log records. With only trusted addresses in the header, or an entry
+  request log records. An entry written as an address with a port is read as
-  that is not an IP address met on the way, the direct peer is used. `X-Real-IP`
+  that address. With only trusted addresses in the header, or an entry that is
-  is read only without `X-Forwarded-For`, and every `X-Forwarded-For` header
+  not an address at all (such as `unknown`) met on the way, the direct peer is
-  line is read, not only the first. The image's nginx, and the
+  used. `X-Real-IP` is read only without `X-Forwarded-For`, and every
-  `TRUSTED_PROXIES=127.0.0.1/32` it starts the backend with, are unchanged. Unit
+  `X-Forwarded-For` header line is read, not only the first. The image's nginx,
-  tests resolve the address from chains ending in an untrusted address, in
+  and the `TRUSTED_PROXIES=127.0.0.1/32` it starts the backend with, are
-  trusted ones, of only trusted ones, and split over two header lines
+  unchanged. Unit tests resolve the address from chains ending in an untrusted
+  address, in trusted ones, of only trusted ones, with ports, with an entry that
+  is not an address, and split over two header lines
 - 2026-10-18: after an interval change, the summary's Reachable slot reads
   `--/--` in white, as at page load, until the second round since the change
   ends ([#140](https://git.eeqj.de/sneak/netwatch/issues/140)). It used to read

internal/middleware/middleware.go

@@ -139,10 +139,12 @@ func ipFromHostPort(hostPort string) string {
 // request from, so the entries on the right are written by the
 // trusted proxies and those further left by the client. The
 // client's address is therefore the right-most entry that is not
-// a trusted proxy. When every entry is a trusted proxy, or the
+// a trusted proxy. An entry written as an address with a port is
-// walk from the right meets an entry that is not an address, the
+// read as that address, both for the trust check and as the
-// peer is used. X-Real-IP is read only when there is no
+// result. When every entry is a trusted proxy, or the walk from
-// X-Forwarded-For entry at all.
+// the right meets an entry that is not an address at all (such as
+// "unknown"), the peer is used. X-Real-IP is read only when there
+// is no X-Forwarded-For entry at all.
 func clientIP(
 	remoteAddr string,
 	header http.Header,
@@ -180,7 +182,10 @@ func clientIP(
 // forwardedFor returns the entries of X-Forwarded-For in order,
 // leaving out blank ones. A proxy may add its entry as a header
 // line of its own rather than append it to the line the client
-// sent, so every line is read, not only the first.
+// sent, so every line is read, not only the first. Some proxies
+// write an entry as an address with a port, such as
+// 198.51.100.7:5000 or [2001:db8::1]:443; the port is removed so
+// that the entry is read as its address.
 func forwardedFor(header http.Header) []string {
 	value := strings.Join(header.Values("X-Forwarded-For"), ",")
 
@@ -189,7 +194,7 @@ func forwardedFor(header http.Header) []string {
 	for part := range strings.SplitSeq(value, ",") {
 		entry := strings.TrimSpace(part)
 		if entry != "" {
-			entries = append(entries, entry)
+			entries = append(entries, ipFromHostPort(entry))
 		}
 	}
 

internal/middleware/middleware_test.go

@@ -106,9 +106,27 @@ func clientIPCases() []clientIPCase {
 			want:       loopbackIP,
 		},
 		{
-			name:       "entry that is not an address uses peer",
+			name:       "entry with a port is read as its address",
 			remoteAddr: loopbackPeer,
 			xff:        "203.0.113.9, 198.51.100.7:5000",
+			want:       "198.51.100.7",
+		},
+		{
+			name:       "ipv6 entry with a port is read as its address",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, [2001:db8::1]:443",
+			want:       "2001:db8::1",
+		},
+		{
+			name:       "trusted proxy entry with a port is skipped",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, 10.0.0.5:5000",
+			want:       "203.0.113.9",
+		},
+		{
+			name:       "entry that is not an address uses peer",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, unknown",
 			want:       loopbackIP,
 		},
 		{