diff --git a/src/os/hostname.zig b/src/os/hostname.zig
index a75ca1cbb..9f25e4c35 100644
--- a/src/os/hostname.zig
+++ b/src/os/hostname.zig
@@ -148,6 +148,91 @@ pub const LocalHostnameValidationError = error{
     Unexpected,
 };
 
+/// Validates a hostname according to [RFC 1123](https://www.rfc-editor.org/rfc/rfc1123)
+///
+/// std.net.isValidHostname is (currently) too generous. It considers strings like
+/// ".example.com", "exa..mple.com", and "-example.com" to be valid hostnames, which
+/// is incorrect.
+pub fn isValid(hostname: []const u8) bool {
+    if (hostname.len == 0) return false;
+    if (hostname[0] == '.') return false;
+
+    // Ignore trailing dot (FQDN). It doesn't count toward our length.
+    const end = if (hostname[hostname.len - 1] == '.') end: {
+        if (hostname.len == 1) return false;
+        break :end hostname.len - 1;
+    } else hostname.len;
+
+    if (end > 253) return false;
+
+    // Hostnames are divided into dot-separated "labels", which:
+    //
+    // - Start with a letter or digit
+    // - Can contain letters, digits, or hyphens
+    // - Must end with a letter or digit
+    // - Have a minimum of 1 character and a maximum of 63
+    var label_start: usize = 0;
+    var label_len: usize = 0;
+    for (hostname[0..end], 0..) |c, i| {
+        switch (c) {
+            '.' => {
+                if (label_len == 0 or label_len > 63) return false;
+                if (!std.ascii.isAlphanumeric(hostname[label_start])) return false;
+                if (!std.ascii.isAlphanumeric(hostname[i - 1])) return false;
+
+                label_start = i + 1;
+                label_len = 0;
+            },
+            '-' => {
+                label_len += 1;
+            },
+            else => {
+                if (!std.ascii.isAlphanumeric(c)) return false;
+                label_len += 1;
+            },
+        }
+    }
+
+    // Validate the final label
+    if (label_len == 0 or label_len > 63) return false;
+    if (!std.ascii.isAlphanumeric(hostname[label_start])) return false;
+    if (!std.ascii.isAlphanumeric(hostname[end - 1])) return false;
+
+    return true;
+}
+
+test isValid {
+    const testing = std.testing;
+
+    // Valid hostnames
+    try testing.expect(isValid("example"));
+    try testing.expect(isValid("example.com"));
+    try testing.expect(isValid("www.example.com"));
+    try testing.expect(isValid("sub.domain.example.com"));
+    try testing.expect(isValid("example.com."));
+    try testing.expect(isValid("host-name.example.com."));
+    try testing.expect(isValid("123.example.com."));
+    try testing.expect(isValid("a-b.com"));
+    try testing.expect(isValid("a.b.c.d.e.f.g"));
+    try testing.expect(isValid("127.0.0.1")); // Also a valid hostname
+    try testing.expect(isValid("a" ** 63 ++ ".com")); // Label exactly 63 chars (valid)
+    try testing.expect(isValid("a." ** 126 ++ "a")); // Total length 253 (valid)
+
+    // Invalid hostnames
+    try testing.expect(!isValid(""));
+    try testing.expect(!isValid(".example.com"));
+    try testing.expect(!isValid("example.com.."));
+    try testing.expect(!isValid("host..domain"));
+    try testing.expect(!isValid("-hostname"));
+    try testing.expect(!isValid("hostname-"));
+    try testing.expect(!isValid("a.-.b"));
+    try testing.expect(!isValid("host_name.com"));
+    try testing.expect(!isValid("."));
+    try testing.expect(!isValid(".."));
+    try testing.expect(!isValid("a" ** 64 ++ ".com")); // Label length 64 (too long)
+    try testing.expect(!isValid("a." ** 126 ++ "ab")); // Total length 254 (too long)
+}
+
 /// Checks if a hostname is local to the current machine. This matches
 /// both "localhost" and the current hostname of the machine (as returned
 /// by `gethostname`).