github.com/miekg/dns v1.1.48: Msg.SetEdns0, OPT

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify dns.Msg.SetEdns0 in pkg:golang/github.com/miekg/dns@v1.1.48
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/miekg/dns@v1.1.48
Demonstrate these symbols/APIs:
  - dns.Msg.SetEdns0
  - dns.OPT

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:85012ad6750b86c75298e642e5adf5d39ce129ed533e65425bc56fe019285106","contract":["dns.Msg.SetEdns0 initializes OPT pseudo-RR with specified UDP buffer size and DO flag","dns.Msg.IsEdns0 retrieves pointer to OPT resource record from message extra section","dns.OPT supports configuring and extracting EDNS0 Client Subnet option with IPv4 and IPv6","dns.Msg.Pack and Unpack preserve EDNS0 OPT record and subnet options across wire serialization"],"goal":"verify dns.Msg.SetEdns0 in pkg:golang/github.com/miekg/dns@v1.1.48","kind":"HOW","packages":["pkg:golang/github.com/miekg/dns@v1.1.48"],"schemaVersion":1,"symbols":["dns.Msg.SetEdns0","dns.OPT"]},"contractCommand":["go","test","-v","./..."],"environment":{"arch":"x64","compiler":"go","compilerVersion":"1.26.6","distro":"ubuntu","ecosystem":"golang","language":"go","languageVersion":"1.26.6","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","packageManagerVersion":"1.26.6","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/miekg/dns@v1.1.48"],"schemaVersion":1,"subject":"pkg:golang/github.com/miekg/dns@v1.1.48","symbols":["dns.Msg.SetEdns0","dns.OPT"],"verifierAdapter":"golang@1"}

package ednssample

import (
	"fmt"
	"net"

	"github.com/miekg/dns"
)

// CreateMessageWithEdns0 initializes a DNS query message with EDNS0 support.
func CreateMessageWithEdns0(domain string, qtype uint16, udpSize uint16, do bool) *dns.Msg {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(domain), qtype)
	m.SetEdns0(udpSize, do)
	return m
}

// AddClientSubnetOption appends an EDNS0 Client Subnet option to the message's OPT record.
func AddClientSubnetOption(msg *dns.Msg, ip net.IP, mask uint8) error {
	opt := msg.IsEdns0()
	if opt == nil {
		return fmt.Errorf("message does not have an EDNS0 OPT record")
	}

	subnet := new(dns.EDNS0_SUBNET)
	subnet.Code = dns.EDNS0SUBNET
	subnet.SourceScope = 0

	if ip4 := ip.To4(); ip4 != nil {
		subnet.Family = 1 // IPv4
		subnet.SourceNetmask = mask
		subnet.Address = ip4
	} else if ip6 := ip.To16(); ip6 != nil {
		subnet.Family = 2 // IPv6
		subnet.SourceNetmask = mask
		subnet.Address = ip6
	} else {
		return fmt.Errorf("invalid IP address")
	}

	opt.Option = append(opt.Option, subnet)
	return nil
}

// ExtractClientSubnetOption retrieves the EDNS0 Client Subnet option from a message.
func ExtractClientSubnetOption(msg *dns.Msg) (*dns.EDNS0_SUBNET, error) {
	opt := msg.IsEdns0()
	if opt == nil {
		return nil, fmt.Errorf("no OPT record found")
	}

	for _, o := range opt.Option {
		if subnet, ok := o.(*dns.EDNS0_SUBNET); ok {
			return subnet, nil
		}
	}

	return nil, fmt.Errorf("no EDNS0 subnet option found")
}

// PackAndUnpack encodes the message into wire format and decodes it back.
func PackAndUnpack(msg *dns.Msg) (*dns.Msg, []byte, error) {
	wire, err := msg.Pack()
	if err != nil {
		return nil, nil, fmt.Errorf("failed to pack DNS message: %w", err)
	}

	unpacked := new(dns.Msg)
	if err := unpacked.Unpack(wire); err != nil {
		return nil, nil, fmt.Errorf("failed to unpack DNS message: %w", err)
	}

	return unpacked, wire, nil
}

module example.com/ednssample

go 1.26.6

require github.com/miekg/dns v1.1.48

require (
	golang.org/x/mod v0.4.2 // indirect
	golang.org/x/net v0.0.0-20210726213435-c6fcb2dbf985 // indirect
	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c // indirect
	golang.org/x/tools v0.1.6-0.20210726203631-07bc1bf47fb2 // indirect
	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
)

github.com/miekg/dns v1.1.48 h1:Ucfr7IIVyMBz4lRE8qmGUuZ4Wt3/ZGu9hmcMT3Uu4tQ=
github.com/miekg/dns v1.1.48/go.mod h1:e3IlAVfNqAllflbibAZEWOXOQ+Ynzk/dDozDxY7XnME=
github.com/yuin/goldmark v1.3.5/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1Zlc8k=
golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
golang.org/x/mod v0.4.2 h1:Gz96sIWK3OalVv/I/qNygP42zyoKp3xptRVCWRFEBvo=
golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4/go.mod h1:p54w0d4576C0XHj96bSt6lcn1PtDYWL6XObtHCRCNQM=
golang.org/x/net v0.0.0-20210726213435-c6fcb2dbf985 h1:4CSI6oo7cOjJKajidEljs9h+uP0rRZBPPPhcCbj5mw8=
golang.org/x/net v0.0.0-20210726213435-c6fcb2dbf985/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
golang.org/x/sync v0.0.0-20210220032951-036812b2e83c h1:5KslGYwFpkhGh+Q16bwMP3cOontH8FOep7tGV86Y7SQ=
golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
golang.org/x/sys v0.0.0-20210330210617-4fbd30eecc44/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
golang.org/x/sys v0.0.0-20210510120138-977fb7262007/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c h1:F1jZWGFhYfh0Ci55sIpILtKKK8p3i2/krTr0H1rg74I=
golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
golang.org/x/tools v0.1.6-0.20210726203631-07bc1bf47fb2 h1:BonxutuHCTL0rBDnZlKjpGIQFTjyUVTexFOdWkB6Fg0=
golang.org/x/tools v0.1.6-0.20210726203631-07bc1bf47fb2/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=
golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

{
  "schemaVersion": 1,
  "goal": "verify dns.Msg.SetEdns0 in pkg:golang/github.com/miekg/dns@v1.1.48",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/miekg/dns@v1.1.48"
  ],
  "symbols": [
    "dns.Msg.SetEdns0",
    "dns.OPT"
  ]
}

package test

import (
	"bytes"
	"net"
	"testing"

	"example.com/ednssample"
	"github.com/miekg/dns"
)

func TestEdns0InitializationAndFlags(t *testing.T) {
	domain := "example.com"
	var udpSize uint16 = 4096
	msg := ednssample.CreateMessageWithEdns0(domain, dns.TypeA, udpSize, true)

	if len(msg.Question) != 1 {
		t.Fatalf("expected 1 question, got %d", len(msg.Question))
	}
	if msg.Question[0].Name != "example.com." {
		t.Fatalf("expected question name 'example.com.', got '%s'", msg.Question[0].Name)
	}

	opt := msg.IsEdns0()
	if opt == nil {
		t.Fatalf("expected OPT record to be present")
	}

	if opt.UDPSize() != udpSize {
		t.Fatalf("expected UDP size %d, got %d", udpSize, opt.UDPSize())
	}

	if !opt.Do() {
		t.Fatalf("expected DNSSEC OK (DO) flag to be true")
	}
}

func TestEdns0SubnetOption(t *testing.T) {
	msg := ednssample.CreateMessageWithEdns0("example.com", dns.TypeAAAA, 2048, false)
	clientIP := net.ParseIP("192.0.2.0")
	var mask uint8 = 24

	err := ednssample.AddClientSubnetOption(msg, clientIP, mask)
	if err != nil {
		t.Fatalf("AddClientSubnetOption failed: %v", err)
	}

	subnet, err := ednssample.ExtractClientSubnetOption(msg)
	if err != nil {
		t.Fatalf("ExtractClientSubnetOption failed: %v", err)
	}

	if subnet.Family != 1 {
		t.Fatalf("expected IPv4 family (1), got %d", subnet.Family)
	}
	if subnet.SourceNetmask != mask {
		t.Fatalf("expected netmask %d, got %d", mask, subnet.SourceNetmask)
	}
	if subnet.SourceScope != 0 {
		t.Fatalf("expected source scope 0, got %d", subnet.SourceScope)
	}
	if !subnet.Address.Equal(clientIP.To4()) {
		t.Fatalf("expected IP %v, got %v", clientIP.To4(), subnet.Address)
	}
}

func TestEdns0WireSerialization(t *testing.T) {
	msg := ednssample.CreateMessageWithEdns0("example.com", dns.TypeA, 4096, true)
	clientIP := net.ParseIP("2001:db8::")
	var mask uint8 = 56

	err := ednssample.AddClientSubnetOption(msg, clientIP, mask)
	if err != nil {
		t.Fatalf("AddClientSubnetOption IPv6 failed: %v", err)
	}

	unpacked, wire, err := ednssample.PackAndUnpack(msg)
	if err != nil {
		t.Fatalf("PackAndUnpack failed: %v", err)
	}

	if len(wire) == 0 {
		t.Fatalf("expected non-empty wire data")
	}

	opt := unpacked.IsEdns0()
	if opt == nil {
		t.Fatalf("unpacked message missing OPT record")
	}

	if opt.UDPSize() != 4096 {
		t.Fatalf("expected unpacked UDP size 4096, got %d", opt.UDPSize())
	}

	if !opt.Do() {
		t.Fatalf("expected unpacked DO flag to be true")
	}

	subnet, err := ednssample.ExtractClientSubnetOption(unpacked)
	if err != nil {
		t.Fatalf("ExtractClientSubnetOption on unpacked failed: %v", err)
	}

	if subnet.Family != 2 {
		t.Fatalf("expected IPv6 family (2), got %d", subnet.Family)
	}
	if subnet.SourceNetmask != mask {
		t.Fatalf("expected netmask %d, got %d", mask, subnet.SourceNetmask)
	}
	if !bytes.Equal(subnet.Address, clientIP.To16()) {
		t.Fatalf("expected IP %v, got %v", clientIP.To16(), subnet.Address)
	}
}