caddytls: Improve alt chain preference settings

This allows for finer-grained control when choosing alternate chains than
simply the previous/Certbot-esque behavior of "choose first chain that
contains an issuer's common name." This update allows you to sort by
length (if optimizing for efficiency on the wire) and also to select the
chain with a specific root CommonName.

3 files changed, 32 insertions, 8 deletions

diff --git a/go.mod b/go.mod
index 57df139..f116149 100644
--- a/go.mod
+++ b/go.mod
@@ -6,7 +6,7 @@ require (
 	github.com/Masterminds/sprig/v3 v3.1.0
 	github.com/alecthomas/chroma v0.8.2
 	github.com/aryann/difflib v0.0.0-20170710044230-e206f873d14a
-	github.com/caddyserver/certmagic v0.12.1-0.20201209195841-b726d1ed13c3
+	github.com/caddyserver/certmagic v0.12.1-0.20201215190346-201f83a06067
 	github.com/dustin/go-humanize v1.0.1-0.20200219035652-afde56e7acac
 	github.com/go-chi/chi v4.1.2+incompatible
 	github.com/google/cel-go v0.6.0
diff --git a/go.sum b/go.sum
index d8e38cf..27dab3b 100644
--- a/go.sum
+++ b/go.sum
@@ -85,8 +85,8 @@ github.com/bombsimon/wsl/v2 v2.0.0/go.mod h1:mf25kr/SqFEPhhcxW1+7pxzGlW+hIl/hYTK
 github.com/boombuler/barcode v1.0.0/go.mod h1:paBWMcWSl3LHKBqUq+rly7CNSldXjb2rDl3JlRe0mD8=
 github.com/bradfitz/go-smtpd v0.0.0-20170404230938-deb6d6237625/go.mod h1:HYsPBTaaSFSlLx/70C2HPIMNZpVV8+vt/A+FMnYP11g=
 github.com/buger/jsonparser v0.0.0-20181115193947-bf1c66bbce23/go.mod h1:bbYlZJ7hK1yFx9hf58LP0zeX7UjIGs20ufpu3evjr+s=
-github.com/caddyserver/certmagic v0.12.1-0.20201209195841-b726d1ed13c3 h1:NW/7kFV4p6VLwG5eQa6ECivSqxAO0MmnAsAV7esFp0o=
-github.com/caddyserver/certmagic v0.12.1-0.20201209195841-b726d1ed13c3/go.mod h1:tr26xh+9fY5dN0J6IPAlMj07qpog22PJKa7Nw7j835U=
+github.com/caddyserver/certmagic v0.12.1-0.20201215190346-201f83a06067 h1:gpjCX6/8hHRgVXxy1v2AQdoAX6XRXIA8fBUZtEpnVg0=
+github.com/caddyserver/certmagic v0.12.1-0.20201215190346-201f83a06067/go.mod h1:tr26xh+9fY5dN0J6IPAlMj07qpog22PJKa7Nw7j835U=
 github.com/census-instrumentation/opencensus-proto v0.2.0/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
 github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
 github.com/cespare/xxhash v1.1.0 h1:a6HrQnmkObjyL+Gs60czilIUGqrzKutQD6XZog3p+ko=
diff --git a/modules/caddytls/acmeissuer.go b/modules/caddytls/acmeissuer.go
index 7c79c7e..df071c4 100644
--- a/modules/caddytls/acmeissuer.go
+++ b/modules/caddytls/acmeissuer.go
@@ -74,10 +74,11 @@ type ACMEIssuer struct {
 	// is internal or for development/testing purposes.
 	TrustedRootsPEMFiles []string `json:"trusted_roots_pem_files,omitempty"`
 
-	// List of preferred certificate chains, by issuer's CommonName. If empty,
-	// or if no matching chain is found, the first chain offered by the server
-	// will be used.
-	PreferredChains []string `json:"preferred_chains,omitempty"`
+	// Preferences for selecting alternate certificate chains, if offered
+	// by the CA. By default, the first offered chain will be selected.
+	// If configured, the chains may be sorted and the first matching chain
+	// will be selected.
+	PreferredChains *ChainPreference `json:"preferred_chains,omitempty"`
 
 	rootPool *x509.CertPool
 	template certmagic.ACMEManager
@@ -163,7 +164,6 @@ func (iss *ACMEIssuer) makeIssuerTemplate() (certmagic.ACMEManager, error) {
 		CertObtainTimeout: time.Duration(iss.ACMETimeout),
 		TrustedRoots:      iss.rootPool,
 		ExternalAccount:   iss.ExternalAccount,
-		PreferredChains:   iss.PreferredChains,
 		Logger:            iss.logger,
 	}
 
@@ -182,6 +182,14 @@ func (iss *ACMEIssuer) makeIssuerTemplate() (certmagic.ACMEManager, error) {
 		template.ListenHost = iss.Challenges.BindHost
 	}
 
+	if iss.PreferredChains != nil {
+		template.PreferredChains = certmagic.ChainPreference{
+			Smallest:       iss.PreferredChains.Smallest,
+			AnyCommonName:  iss.PreferredChains.AnyCommonName,
+			RootCommonName: iss.PreferredChains.RootCommonName,
+		}
+	}
+
 	return template, nil
 }
 
@@ -407,6 +415,22 @@ func onDemandAskRequest(ask string, name string) error {
 	return nil
 }
 
+// ChainPreference describes the client's preferred certificate chain,
+// useful if the CA offers alternate chains. The first matching chain
+// will be selected.
+type ChainPreference struct {
+	// Prefer chains with the fewest number of bytes.
+	Smallest *bool `json:"smallest,omitempty"`
+
+	// Select first chain having a root with one of
+	// these common names.
+	RootCommonName []string `json:"root_common_name,omitempty"`
+
+	// Select first chain that has any issuer with one
+	// of these common names.
+	AnyCommonName []string `json:"any_common_name,omitempty"`
+}
+
 // Interface guards
 var (
 	_ certmagic.PreChecker  = (*ACMEIssuer)(nil)