Experimental IETF-standard HTTP/3 support (known issue exists) (#2727)

* Begin WIP integration of HTTP/3 support

* http3: Set actual Handler, make fakeClosePacketConn type for UDP sockets

Also use latest quic-go for ALPN fix

* Manually keep track of and close HTTP/3 listeners

* Update quic-go after working through some http3 bugs

* Fix go mod

* Make http3 optional for now

6 files changed, 153 insertions, 6 deletions

diff --git a/cmd/main.go b/cmd/main.go
index 3b44a85..5b97992 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -33,7 +33,7 @@ import (
 func Main() {
 	caddy.TrapSignals()
 
-	if len(os.Args) <= 1 {
+	if len(os.Args) < 2 {
 		fmt.Println(usageString())
 		return
 	}
diff --git a/go.mod b/go.mod
index 2f0222c..0a91bba 100644
--- a/go.mod
+++ b/go.mod
@@ -17,6 +17,7 @@ require (
 	github.com/imdario/mergo v0.3.7 // indirect
 	github.com/klauspost/compress v1.7.1-0.20190613161414-0b31f265a57b
 	github.com/klauspost/cpuid v1.2.1
+	github.com/lucas-clemente/quic-go v0.7.1-0.20190908032346-fc962d18373a
 	github.com/mholt/certmagic v0.6.2
 	github.com/mitchellh/go-ps v0.0.0-20170309133038-4fdf99ab2936
 	github.com/muhammadmuzzammil1998/jsonc v0.0.0-20190902132743-e4903c4dea48
@@ -26,7 +27,6 @@ require (
 	github.com/starlight-go/starlight v0.0.0-20181207205707-b06f321544f3
 	go.starlark.net v0.0.0-20190604130855-6ddc71c0ba77
 	golang.org/x/net v0.0.0-20190603091049-60506f45cf65
-	golang.org/x/sys v0.0.0-20190228124157-a34e9553db1e // indirect
 	golang.org/x/time v0.0.0-20190308202827-9d24e82272b4
 	gopkg.in/yaml.v2 v2.2.2 // indirect
 )
diff --git a/go.sum b/go.sum
index 4c691a6..3a84810 100644
--- a/go.sum
+++ b/go.sum
@@ -6,23 +6,36 @@ github.com/Masterminds/semver v1.4.2 h1:WBLTQ37jOCzSLtXNdoo8bNM8876KhNqOKvrlGITg
 github.com/Masterminds/semver v1.4.2/go.mod h1:MB6lktGJrhw8PrUyiEoblNEGEQ+RzHPF078ddwwvV3Y=
 github.com/Masterminds/sprig v2.20.0+incompatible h1:dJTKKuUkYW3RMFdQFXPU/s6hg10RgctmTjRcbZ98Ap8=
 github.com/Masterminds/sprig v2.20.0+incompatible/go.mod h1:y6hNFY5UBTIWBxnzTeuNhlNS5hqE0NB0E6fgfo2Br3o=
+github.com/alangpierce/go-forceexport v0.0.0-20160317203124-8f1d6941cd75 h1:3ILjVyslFbc4jl1w5TWuvvslFD/nDfR2H8tVaMVLrEY=
+github.com/alangpierce/go-forceexport v0.0.0-20160317203124-8f1d6941cd75/go.mod h1:uAXEEpARkRhCZfEvy/y0Jcc888f9tHCc1W7/UeEtreE=
 github.com/andybalholm/brotli v0.0.0-20190704151324-71eb68cc467c h1:pBKtfXLqKZ+GPHGjlBheGaXK2lddydUG3XhWGrYjxOA=
 github.com/andybalholm/brotli v0.0.0-20190704151324-71eb68cc467c/go.mod h1:+lx6/Aqd1kLJ1GQfkvOnaZ1WGmLpMpbprPuIOOZX30U=
 github.com/cenkalti/backoff v2.1.1+incompatible h1:tKJnvO2kl0zmb/jA5UKAt4VoEVw1qxKWjE/Bpp46npY=
 github.com/cenkalti/backoff v2.1.1+incompatible/go.mod h1:90ReRw6GdpyfrHakVjL/QHaoyV4aDUVVkXQJJJ3NXXM=
+github.com/cheekybits/genny v1.0.0 h1:uGGa4nei+j20rOSeDeP5Of12XVm7TGUd4dJA9RDitfE=
+github.com/cheekybits/genny v1.0.0/go.mod h1:+tQajlRqAUrPI7DOSpB0XAqZYtQakVtB7wXkRAgjxjQ=
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dustin/go-humanize v1.0.0 h1:VSnTsYCnlFHaM2/igO1h6X3HA71jcobQuxemgkq4zYo=
 github.com/dustin/go-humanize v1.0.0/go.mod h1:HtrtbFcZ19U5GC7JDqmcUSB87Iq5E25KnS6fMYU6eOk=
+github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV9I=
+github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/go-acme/lego v2.5.0+incompatible/go.mod h1:yzMNe9CasVUhkquNvti5nAtPmG94USbYxYrZfTkIn0M=
 github.com/go-acme/lego v2.6.0+incompatible h1:KxcEWOF5hKtgou4xIqPaXSRF9DoO4OJ90ndwdK6YH/k=
 github.com/go-acme/lego v2.6.0+incompatible/go.mod h1:yzMNe9CasVUhkquNvti5nAtPmG94USbYxYrZfTkIn0M=
 github.com/golang/gddo v0.0.0-20190419222130-af0f2af80721 h1:KRMr9A3qfbVM7iV/WcLY/rL5LICqwMHLhwRXKu99fXw=
 github.com/golang/gddo v0.0.0-20190419222130-af0f2af80721/go.mod h1:xEhNfoBDX1hzLm2Nf80qUvZ2sVwoMZ8d6IE2SrsQfh4=
+github.com/golang/mock v1.2.0 h1:28o5sBqPkBsMGnC6b4MvE2TzSr5/AT4c/1fLqVGIwlk=
+github.com/golang/mock v1.2.0/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfbm0A=
+github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/golang/protobuf v1.3.0 h1:kbxbvI4Un1LUWKxufD+BiE6AEExYYgkQLQmLFqA1LFk=
+github.com/golang/protobuf v1.3.0/go.mod h1:Qd/q+1AKNOZr9uGQzbzCmRO6sUih6GTPZv6a1/R87v0=
 github.com/google/go-cmp v0.3.1 h1:Xye71clBPdm5HgqGwUkwhbynsUJZhDbS20FvLhQ2izg=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/uuid v1.1.1 h1:Gkbcsh/GbpXz7lPftLA3P6TYMwjCLYm83jiFQZF/3gY=
 github.com/google/uuid v1.1.1/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/hpcloud/tail v1.0.0 h1:nfCOvKYfkgYP8hkirhJocXT2+zOD8yUNjXaWfTlyFKI=
+github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpOxQnU=
 github.com/huandu/xstrings v1.2.0 h1:yPeWdRnmynF7p+lLYz0H2tthW9lqhMJrQV/U7yy4wX0=
 github.com/huandu/xstrings v1.2.0/go.mod h1:DvyZB1rfVYsBIigL8HwpZgxHwXozlTgGqn63UyNX5k4=
 github.com/ilibs/json5 v1.0.1 h1:3e14wUQM8PyK6Hf1bM+zAQFxfG+N5oZj35x5vCNeQ58=
@@ -34,6 +47,14 @@ github.com/klauspost/compress v1.7.1-0.20190613161414-0b31f265a57b/go.mod h1:RyI
 github.com/klauspost/cpuid v1.2.0/go.mod h1:Pj4uuM528wm8OyEC2QMXAi2YiTZ96dNQPGgoMS4s3ek=
 github.com/klauspost/cpuid v1.2.1 h1:vJi+O/nMdFt0vqm8NZBI6wzALWdA2X+egi0ogNyrC/w=
 github.com/klauspost/cpuid v1.2.1/go.mod h1:Pj4uuM528wm8OyEC2QMXAi2YiTZ96dNQPGgoMS4s3ek=
+github.com/lucas-clemente/quic-go v0.7.1-0.20190908032346-fc962d18373a h1:4NDhBYEqjdNTQhlkLK8u4FGq1mMDQv4V4MLDSS5oCS0=
+github.com/lucas-clemente/quic-go v0.7.1-0.20190908032346-fc962d18373a/go.mod h1:iP3S21xvg0qlsEAz+goZ/qfptsfhshjzkbhEK6Ka3Fg=
+github.com/marten-seemann/chacha20 v0.2.0 h1:f40vqzzx+3GdOmzQoItkLX5WLvHgPgyYqFFIO5Gh4hQ=
+github.com/marten-seemann/chacha20 v0.2.0/go.mod h1:HSdjFau7GzYRj+ahFNwsO3ouVJr1HFkWoEwNDb4TMtE=
+github.com/marten-seemann/qpack v0.1.0 h1:/0M7lkda/6mus9B8u34Asqm8ZhHAAt9Ho0vniNuVSVg=
+github.com/marten-seemann/qpack v0.1.0/go.mod h1:LFt1NU/Ptjip0C2CPkhimBz5CGE3WGDAUWqna+CNTrI=
+github.com/marten-seemann/qtls v0.4.0 h1:HM9ftULNeuhGiCliIfPKvp5VDJw6pvi/Ghq6PYf7B0E=
+github.com/marten-seemann/qtls v0.4.0/go.mod h1:pxVXcHHw1pNIt8Qo0pwSYQEoZ8yYOOPXTCZLQQunvRc=
 github.com/mholt/certmagic v0.6.2 h1:yy9cKm3rtxdh12SW4E51lzG3Eo6N59LEOfBQ0CTnMms=
 github.com/mholt/certmagic v0.6.2/go.mod h1:g4cOPxcjV0oFq3qwpjSA30LReKD8AoIfwAY9VvG35NY=
 github.com/miekg/dns v1.1.3 h1:1g0r1IvskvgL8rR+AcHzUA+oFmGcQlaIm4IqakufeMM=
@@ -42,6 +63,11 @@ github.com/mitchellh/go-ps v0.0.0-20170309133038-4fdf99ab2936 h1:kw1v0NlnN+GZcU8
 github.com/mitchellh/go-ps v0.0.0-20170309133038-4fdf99ab2936/go.mod h1:r1VsdOzOPt1ZSrGZWFoNhsAedKnEd6r9Np1+5blZCWk=
 github.com/muhammadmuzzammil1998/jsonc v0.0.0-20190902132743-e4903c4dea48 h1:BM/fjd7MfvZuyoHXLv3YlWNIuNb47PLp6EyFBL1KIMg=
 github.com/muhammadmuzzammil1998/jsonc v0.0.0-20190902132743-e4903c4dea48/go.mod h1:saF2fIVw4banK0H4+/EuqfFLpRnoy5S+ECwTOCcRcSU=
+github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
+github.com/onsi/ginkgo v1.7.0 h1:WSHQ+IS43OoUrWtD1/bbclrwK8TTH5hzp+umCiuxHgs=
+github.com/onsi/ginkgo v1.7.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
+github.com/onsi/gomega v1.4.3 h1:RE1xgDvH7imwFD45h+u2SgIfERHlS2yNG4DObb5BSKU=
+github.com/onsi/gomega v1.4.3/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/rs/cors v1.6.0 h1:G9tHG9lebljV9mfp9SNPDL36nCDxmo3zTlAf1YgvzmI=
@@ -60,23 +86,37 @@ go.starlark.net v0.0.0-20190604130855-6ddc71c0ba77/go.mod h1:c1/X6cHgvdXj6pUlmWK
 golang.org/x/crypto v0.0.0-20190123085648-057139ce5d2b/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2 h1:VklqNMn3ovrHsnt90PveolxSbWFaJdECFbxSq0Mqo2M=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20190829043050-9756ffdc2472 h1:Gv7RPwsi3eZ2Fgewe3CBsuOebPwO27PoXzRpJPsvSSM=
+golang.org/x/crypto v0.0.0-20190829043050-9756ffdc2472/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/net v0.0.0-20180906233101-161cd47e91fd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20190125091013-d26f9f9a57f3/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/net v0.0.0-20190228165749-92fc7df08ae7/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190603091049-60506f45cf65 h1:+rhAzEzT3f4JtomfC371qB+0Ola2caSKcY69NUBZrRQ=
 golang.org/x/net v0.0.0-20190603091049-60506f45cf65/go.mod h1:HSz+uSET+XFnRR8LxR5pz3Of3rY3CfYBVs4xY44aLks=
+golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4 h1:YUO/7uOKsKeq9UokNS62b8FYywz3ker1l1vDZRCRefw=
 golang.org/x/sync v0.0.0-20181221193216-37e7f081c4d4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190124100055-b90733256f2e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a h1:1BGLXjeY4akVXGgbC9HugT3Jv3hCI0z56oJR5vAMgBU=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20190228124157-a34e9553db1e h1:ZytStCyV048ZqDsWHiYDdoI2Vd4msMcrDECFxS+tL9c=
-golang.org/x/sys v0.0.0-20190228124157-a34e9553db1e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20190904154756-749cb33beabd h1:DBH9mDw0zluJT/R+nGuV3jWFWLFaHyYZWD4tOT+cjn0=
+golang.org/x/sys v0.0.0-20190904154756-749cb33beabd/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/time v0.0.0-20190308202827-9d24e82272b4 h1:SvFZT6jyqRaOeXpc5h/JSfZenJ2O330aBsf7JfSUXmQ=
 golang.org/x/time v0.0.0-20190308202827-9d24e82272b4/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
+google.golang.org/genproto v0.0.0-20180831171423-11092d34479b/go.mod h1:JiN7NxoALGmiZfu7CAH4rXhgtRTLTxftemlI0sWmxmc=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/fsnotify.v1 v1.4.7 h1:xOHLXZwVvI9hhs+cLKq5+I5onOuwQLhQwiu63xxlHs4=
+gopkg.in/fsnotify.v1 v1.4.7/go.mod h1:Tz8NjZHkW78fSQdbUxIjBTcgA1z1m8ZHf0WmKUhAMys=
 gopkg.in/square/go-jose.v2 v2.2.2 h1:orlkJ3myw8CN1nVQHBFfloD+L3egixIa4FvUP6RosSA=
 gopkg.in/square/go-jose.v2 v2.2.2/go.mod h1:M9dMgbHiYLoDGQrXy7OpJDJWiKiU//h+vD76mk0e1AI=
+gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7 h1:uRGJdciOHaEIrze2W8Q3AKkepLTh2hOroT7a+7czfdQ=
+gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7/go.mod h1:dt/ZhP58zS4L8KSrWDmTeBkI65Dw0HsyUHuEVlX15mw=
+gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/listeners.go b/listeners.go
index 846059d..04ec788 100644
--- a/listeners.go
+++ b/listeners.go
@@ -16,6 +16,7 @@ package caddy
 
 import (
 	"fmt"
+	"log"
 	"net"
 	"strconv"
 	"strings"
@@ -53,13 +54,41 @@ func Listen(network, addr string) (net.Listener, error) {
 	return &fakeCloseListener{usage: &lnUsage.usage, key: lnKey, Listener: ln}, nil
 }
 
+// ListenPacket returns a net.PacketConn suitable for use in a Caddy module.
+// Always be sure to close the PacketConn when you are done.
+func ListenPacket(network, addr string) (net.PacketConn, error) {
+	lnKey := network + "/" + addr
+
+	listenersMu.Lock()
+	defer listenersMu.Unlock()
+
+	// if listener already exists, increment usage counter, then return listener
+	if lnUsage, ok := listeners[lnKey]; ok {
+		atomic.AddInt32(&lnUsage.usage, 1)
+		log.Printf("[DEBUG] %s: Usage counter should not go above 2 or maybe 3, is now: %d", lnKey, atomic.LoadInt32(&lnUsage.usage)) // TODO: remove
+		return &fakeClosePacketConn{usage: &lnUsage.usage, key: lnKey, PacketConn: lnUsage.pc}, nil
+	}
+
+	// or, create new one and save it
+	pc, err := net.ListenPacket(network, addr)
+	if err != nil {
+		return nil, err
+	}
+
+	// make sure to start its usage counter at 1
+	lnUsage := &listenerUsage{usage: 1, pc: pc}
+	listeners[lnKey] = lnUsage
+
+	return &fakeClosePacketConn{usage: &lnUsage.usage, key: lnKey, PacketConn: pc}, nil
+}
+
 // fakeCloseListener's Close() method is a no-op. This allows
 // stopping servers that are using the listener without giving
 // up the socket; thus, servers become hot-swappable while the
 // listener remains running. Listeners should be re-wrapped in
 // a new fakeCloseListener each time the listener is reused.
 type fakeCloseListener struct {
-	closed int32  // accessed atomically
+	closed int32  // accessed atomically - TODO: this needs to be shared across the whole app instance, not to cross instance boundaries... hmmm... see #2658 (still relevant?)
 	usage  *int32 // accessed atomically
 	key    string
 	net.Listener
@@ -146,6 +175,34 @@ func (fcl *fakeCloseListener) fakeClosedErr() error {
 	}
 }
 
+type fakeClosePacketConn struct {
+	closed int32  // accessed atomically - TODO: this needs to be shared across the whole app instance, not to cross instance boundaries... hmmm... see #2658 (still relevant?)
+	usage  *int32 // accessed atomically
+	key    string
+	net.PacketConn
+}
+
+func (fcpc *fakeClosePacketConn) Close() error {
+	log.Println("[DEBUG] Fake-closing underlying packet conn") // TODO: remove this
+
+	if atomic.CompareAndSwapInt32(&fcpc.closed, 0, 1) {
+		// since we're no longer using this listener,
+		// decrement the usage counter and, if no one
+		// else is using it, close underlying listener
+		if atomic.AddInt32(fcpc.usage, -1) == 0 {
+			listenersMu.Lock()
+			delete(listeners, fcpc.key)
+			listenersMu.Unlock()
+			err := fcpc.PacketConn.Close()
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
 // ErrFakeClosed is the underlying error value returned by
 // fakeCloseListener.Accept() after Close() has been called,
 // indicating that it is pretending to be closed so that the
@@ -158,6 +215,7 @@ var errFakeClosed = fmt.Errorf("listener 'closed' 😉")
 type listenerUsage struct {
 	usage int32 // accessed atomically
 	ln    net.Listener
+	pc    net.PacketConn
 }
 
 var (
diff --git a/modules/caddyhttp/caddyhttp.go b/modules/caddyhttp/caddyhttp.go
index 9dfdf36..7f33c1d 100644
--- a/modules/caddyhttp/caddyhttp.go
+++ b/modules/caddyhttp/caddyhttp.go
@@ -31,6 +31,7 @@ import (
 
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/modules/caddytls"
+	"github.com/lucas-clemente/quic-go/http3"
 	"github.com/mholt/certmagic"
 )
 
@@ -50,7 +51,9 @@ type App struct {
 	GracePeriod caddy.Duration     `json:"grace_period,omitempty"`
 	Servers     map[string]*Server `json:"servers,omitempty"`
 
-	servers []*http.Server
+	servers     []*http.Server
+	h3servers   []*http3.Server
+	h3listeners []net.PacketConn
 
 	ctx caddy.Context
 }
@@ -187,6 +190,27 @@ func (app *App) Start() error {
 						return fmt.Errorf("%s/%s: making TLS configuration: %v", network, addr, err)
 					}
 					ln = tls.NewListener(ln, tlsCfg)
+
+					/////////
+					// TODO: HTTP/3 support is experimental for now
+					if srv.ExperimentalHTTP3 {
+						log.Printf("[INFO] Enabling experimental HTTP/3 listener on %s", addr)
+						h3ln, err := caddy.ListenPacket("udp", addr)
+						if err != nil {
+							return fmt.Errorf("getting HTTP/3 UDP listener: %v", err)
+						}
+						h3srv := &http3.Server{
+							Server: &http.Server{
+								Addr:      addr,
+								Handler:   srv,
+								TLSConfig: tlsCfg,
+							},
+						}
+						go h3srv.Serve(h3ln)
+						app.h3servers = append(app.h3servers, h3srv)
+						app.h3listeners = append(app.h3listeners, h3ln)
+					}
+					/////////
 				}
 
 				go s.Serve(ln)
@@ -212,6 +236,28 @@ func (app *App) Stop() error {
 			return err
 		}
 	}
+	// TODO: Closing the http3.Server is the right thing to do,
+	// however, doing so sometimes causes connections from clients
+	// to fail after config reloads due to a bug that is yet
+	// unsolved: https://github.com/caddyserver/caddy/pull/2727
+	// for _, s := range app.h3servers {
+	// 	// TODO: CloseGracefully, once implemented upstream
+	// 	// (see https://github.com/lucas-clemente/quic-go/issues/2103)
+	// 	err := s.Close()
+	// 	if err != nil {
+	// 		return err
+	// 	}
+	// }
+	// as of September 2019, closing the http3.Server
+	// instances doesn't close their underlying listeners
+	// so we have todo that ourselves
+	// (see https://github.com/lucas-clemente/quic-go/issues/2103)
+	for _, pc := range app.h3listeners {
+		err := pc.Close()
+		if err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
diff --git a/modules/caddyhttp/server.go b/modules/caddyhttp/server.go
index dc38b19..e16a600 100644
--- a/modules/caddyhttp/server.go
+++ b/modules/caddyhttp/server.go
@@ -43,6 +43,9 @@ type Server struct {
 	MaxRehandles      *int                        `json:"max_rehandles,omitempty"`
 	StrictSNIHost     bool                        `json:"strict_sni_host,omitempty"`
 
+	// This field is not subject to compatibility promises
+	ExperimentalHTTP3 bool `json:"experimental_http3,omitempty"`
+
 	tlsApp *caddytls.TLS
 }