From cc19c7801fd5c0382951c5d1d7773632c38918c0 Mon Sep 17 00:00:00 2001
From: Raphael Fakhri <153192858+RaphaelFakhri@users.noreply.github.com>
Date: Sat, 24 Oct 2026 15:59:13 +0000
Subject: [PATCH] cmd/tailcat: accept a bare IPv6 address in socks --listen

The socks --listen flag documents that a bare address means an
OS-assigned port, but normalizeListenAddrPort appended ":0" to an
unbracketed IPv6 address, so --listen=::1 became "::1:0" and the
proxy exited with "too many colons in address". Join a bare IP
address with port 0 via net.JoinHostPort, which brackets IPv6.
---
 cmd/tailcat/socks_test.go | 10 ++++++++++
 cmd/tailcat/tailcat.go    |  6 +++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/cmd/tailcat/socks_test.go b/cmd/tailcat/socks_test.go
index bb288ff48..a3d5a2b3f 100644
--- a/cmd/tailcat/socks_test.go
+++ b/cmd/tailcat/socks_test.go
@@ -203,6 +203,16 @@ func TestNormalizeListenAddrPort(t *testing.T) {
 			input: "[2001:db8::1]",
 			want:  "[2001:db8::1]:0",
 		},
+		{
+			name:  "omit port with unbracketed IPv6 address",
+			input: "::1",
+			want:  "[::1]:0",
+		},
+		{
+			name:  "omit port with unbracketed IPv6 unspecified address",
+			input: "::",
+			want:  "[::]:0",
+		},
 		{
 			name:  "others",
 			input: "foo",
diff --git a/cmd/tailcat/tailcat.go b/cmd/tailcat/tailcat.go
index 31d0ab4ec..7bb847262 100644
--- a/cmd/tailcat/tailcat.go
+++ b/cmd/tailcat/tailcat.go
@@ -999,8 +999,12 @@ func normalizeListenAddrPort(s string) string {
 		return net.JoinHostPort(host, port)
 	} else if port, err := strconv.ParseUint(s, 10, 16); err == nil {
 		return net.JoinHostPort("127.0.0.1", strconv.Itoa(int(port)))
+	} else if _, err := netip.ParseAddr(s); err == nil {
+		// A bare IP address. JoinHostPort brackets an IPv6 address
+		// such as "::1", which appending ":0" would leave ambiguous.
+		return net.JoinHostPort(s, "0")
 	}
-	// Assume it's a hostname or IP without a port.
+	// Assume it's a hostname without a port.
 	return s + ":0"
 }