Fix dynamic upstreams: make use of ReplaceKnown at provision (#235)

README.md

@@ -590,7 +590,7 @@ Environment variables having `{$VAR}` syntax are supported in Caddyfile only. Th
 
 Runtime placeholders having `{...}` syntax, including environment variables referenced as `{env.VAR}`, are supported in both Caddyfile and pure JSON, with some caveats described below.
 - Options of *int*, *float*, *big.int*, *duration*, and other numeric types don't support runtime placeholders at all.
-- Options of *string* type containing IPs or CIDRs (e.g. `dial` in `upstream` of `proxy` handler), regular expressions (e.g. `cookie_hash_regexp` of `rdp` matcher), or special values (e.g. `commands` and `credentials` of `socks5` handler)  support runtime placeholders, but they are evaluated __once at provision__ due to the existing optimizations.
+- Options of *string* type containing IPs or CIDRs (e.g. `remote_ip` matcher), regular expressions (e.g. `cookie_hash_regexp` of `rdp` matcher), or special values (e.g. `commands` and `credentials` of `socks5` handler)  support runtime placeholders, but they are evaluated __once at provision__ due to the existing optimizations. A special case is `dial` in `upstream` of `proxy` handler: it is evaluated 2 times: at handler provision for all known placeholders (e.g. `{env.*}`) and at dial for all placeholders (e.g. `{l4.*}`).
 - Other options of *string* type (e.g. `alpn` of `tls` matcher) generally support runtime placeholders, and they are evaluated __each time at match or handle__. However, there are some exceptions, e.g. `tls_*` options inside `upstream` of `proxy` handler, and all options inside `connection_policy` of `tls` handler, that don't support runtime placeholders at all.
 
 Please note that runtime placeholders support depends on handler/matcher implementations. Given some matchers and handlers are outside of this repository, it's up to their developers to support or restrict usage of runtime placeholders.

modules/l4proxy/upstream.go

@@ -59,7 +59,9 @@ func (u *Upstream) provision(ctx caddy.Context, h *Handler) error {
 	repl := caddy.NewReplacer()
 	for _, dialAddr := range u.Dial {
 		// replace runtime placeholders
-		replDialAddr := repl.ReplaceAll(dialAddr, "")
+		// Note: ReplaceKnown is used here instead of ReplaceAll to let unknown placeholders be replaced later
+		// in Handler.dialPeers. E.g. {l4.tls.server_name}:443 will allow for dynamic TLS SNI based upstreams.
+		replDialAddr := repl.ReplaceKnown(dialAddr, "")
 
 		// parse and validate address
 		addr, err := caddy.ParseNetworkAddress(replDialAddr)