Match-and-replace: header rewrite rules

Implements build-order step 6, scoped to headers only for this pass -
see PLAN.md for why bodies are a separate problem (request-body capture
currently depends on streaming straight through, which a body-rewriting
rule would have to interrupt; deciding what "exact" means for a
rule-modified request needs its own pass, not a rushed add-on to this
one).

internal/rules: Rule type and ApplyHeaders, which serializes a Header
map to a raw "Name: value\r\n" block, runs enabled rules' match/replace
over that text, and reparses it - operating on text rather than
per-value substitution is what lets a rule add or remove a header, not
just rewrite one, matching how Burp's header match/replace works.
Invalid rule output (bad regex, unparseable result) leaves the header
map untouched rather than corrupting the request.

internal/store: rules table + CRUD. internal/proxy: forward() fetches
enabled rules for each scope and applies them to outReq.Header /
resp.Header, positioned so the existing capture/history pipeline is
untouched - request_raw keeps showing what the client actually sent and
response_raw what the origin actually sent, while the wire itself
reflects the rules. Deliberate split: match-and-replace transforms
traffic, it doesn't rewrite the audit trail. internal/ipc gains
rules_list/rules_save/rules_delete/rules_toggle. cmd/mitmux gains a
rules view ('m' from history) with add/edit/delete/toggle and a small
form (name, match, replace, scope, regex).

Verified live against real external traffic, not just local echoes:
a request-scope rule rewriting User-Agent, confirmed via httpbin.org's
own header echo that the origin received the rewritten value while curl
sent the real one; a response-scope rule rewriting the Server header,
confirmed the client actually received the rewritten value; disabling a
rule confirmed via a follow-up request that it stops applying; and
throughout, history continued showing the pre-rule original on both
sides, confirming the capture/transform split holds.

1 files changed, 325 insertions, 2 deletions

diff --git a/cmd/mitmux/main.go b/cmd/mitmux/main.go
index f47461c..68d42ed 100644
--- a/cmd/mitmux/main.go
+++ b/cmd/mitmux/main.go
@@ -20,6 +20,7 @@ import (
 
 	"mitmux/internal/ca"
 	"mitmux/internal/ipc"
+	"mitmux/internal/rules"
 	"mitmux/internal/store"
 )
 
@@ -71,6 +72,7 @@ const (
 	viewList viewMode = iota
 	viewDetail
 	viewRepeater
+	viewRules
 )
 
 type detailTab int
@@ -87,6 +89,16 @@ const (
 	focusResponse
 )
 
+type ruleField int
+
+const (
+	fieldName ruleField = iota
+	fieldMatch
+	fieldReplace
+	fieldScope
+	fieldRegex
+)
+
 type model struct {
 	client *ipc.Client
 	subCh  <-chan store.Summary
@@ -112,6 +124,19 @@ type model struct {
 	repeaterResult *ipc.EntryDetail
 	sending        bool
 
+	rulesTable table.Model
+	ruleRows   []rules.Rule
+
+	ruleForm      bool // true while the add/edit form is active (vs the rule list)
+	ruleEditingID int64
+	ruleEnabled   bool
+	ruleName      textinput.Model
+	ruleMatch     textinput.Model
+	ruleReplace   textinput.Model
+	ruleScope     string // "request" or "response"
+	ruleRegex     bool
+	ruleField     ruleField
+
 	statusMsg string
 	width     int
 	height    int
@@ -145,6 +170,24 @@ func newModel(client *ipc.Client, subCh <-chan store.Summary) *model {
 	si.Prompt = "/"
 	si.Placeholder = "search - plain text, or host:example.com / AND / OR / NOT"
 
+	rulesCols := []table.Column{
+		{Title: "On", Width: 3},
+		{Title: "Name", Width: 16},
+		{Title: "Scope", Width: 9},
+		{Title: "Match", Width: 24},
+		{Title: "Replace", Width: 24},
+		{Title: "Regex", Width: 5},
+	}
+	rt := table.New(table.WithColumns(rulesCols), table.WithFocused(true))
+	rt.SetStyles(st)
+
+	nameIn := textinput.New()
+	nameIn.Placeholder = "rule name"
+	matchIn := textinput.New()
+	matchIn.Placeholder = "match text or regex"
+	replaceIn := textinput.New()
+	replaceIn.Placeholder = "replacement"
+
 	return &model{
 		client:      client,
 		subCh:       subCh,
@@ -152,6 +195,11 @@ func newModel(client *ipc.Client, subCh <-chan store.Summary) *model {
 		table:       t,
 		reqArea:     ta,
 		searchInput: si,
+		rulesTable:  rt,
+		ruleName:    nameIn,
+		ruleMatch:   matchIn,
+		ruleReplace: replaceIn,
+		ruleScope:   "request",
 	}
 }
 
@@ -227,6 +275,106 @@ func (m *model) enterRepeater(d *ipc.EntryDetail) {
 	m.statusMsg = ""
 }
 
+type rulesLoadedMsg struct {
+	rules []rules.Rule
+	err   error
+}
+
+type ruleWriteDoneMsg struct {
+	action string // "saved", "deleted", "toggled" - for the status line
+	err    error
+}
+
+func (m *model) loadRules() tea.Msg {
+	rs, err := m.client.ListRules()
+	return rulesLoadedMsg{rules: rs, err: err}
+}
+
+func (m *model) saveRule(r rules.Rule) tea.Cmd {
+	return func() tea.Msg {
+		_, err := m.client.SaveRule(r)
+		return ruleWriteDoneMsg{action: "saved", err: err}
+	}
+}
+
+func (m *model) deleteSelectedRule() tea.Cmd {
+	row := m.rulesTable.Cursor()
+	if row < 0 || row >= len(m.ruleRows) {
+		return nil
+	}
+	id := m.ruleRows[row].ID
+	return func() tea.Msg {
+		err := m.client.DeleteRule(id)
+		return ruleWriteDoneMsg{action: "deleted", err: err}
+	}
+}
+
+func (m *model) toggleSelectedRule() tea.Cmd {
+	row := m.rulesTable.Cursor()
+	if row < 0 || row >= len(m.ruleRows) {
+		return nil
+	}
+	r := m.ruleRows[row]
+	return func() tea.Msg {
+		err := m.client.SetRuleEnabled(r.ID, !r.Enabled)
+		return ruleWriteDoneMsg{action: "toggled", err: err}
+	}
+}
+
+// enterRuleForm opens the add/edit form. r is nil to add a new rule.
+func (m *model) enterRuleForm(r *rules.Rule) {
+	m.ruleForm = true
+	m.ruleField = fieldName
+	if r == nil {
+		m.ruleEditingID = 0
+		m.ruleEnabled = true
+		m.ruleName.SetValue("")
+		m.ruleMatch.SetValue("")
+		m.ruleReplace.SetValue("")
+		m.ruleScope = "request"
+		m.ruleRegex = false
+	} else {
+		m.ruleEditingID = r.ID
+		m.ruleEnabled = r.Enabled
+		m.ruleName.SetValue(r.Name)
+		m.ruleMatch.SetValue(r.Match)
+		m.ruleReplace.SetValue(r.Replace)
+		m.ruleScope = r.Scope
+		m.ruleRegex = r.IsRegex
+	}
+	m.ruleName.Focus()
+	m.ruleMatch.Blur()
+	m.ruleReplace.Blur()
+}
+
+func (m *model) ruleFromForm() rules.Rule {
+	return rules.Rule{
+		ID:      m.ruleEditingID,
+		Enabled: m.ruleEnabled,
+		Name:    m.ruleName.Value(),
+		Scope:   m.ruleScope,
+		Part:    "header",
+		Match:   m.ruleMatch.Value(),
+		Replace: m.ruleReplace.Value(),
+		IsRegex: m.ruleRegex,
+	}
+}
+
+// focusRuleField moves input focus to m.ruleField, blurring the others.
+func (m *model) focusRuleField() {
+	m.ruleName.Blur()
+	m.ruleMatch.Blur()
+	m.ruleReplace.Blur()
+	switch m.ruleField {
+	case fieldName:
+		m.ruleName.Focus()
+	case fieldMatch:
+		m.ruleMatch.Focus()
+	case fieldReplace:
+		m.ruleReplace.Focus()
+	}
+}
+
 func (m *model) Init() tea.Cmd {
 	return tea.Batch(m.loadList, m.waitForEntry)
 }
@@ -245,6 +393,13 @@ func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.reqArea.SetWidth(msg.Width)
 		m.reqArea.SetHeight(reqHeight)
 		m.respView = viewport.New(msg.Width, msg.Height-6-reqHeight)
+
+		m.rulesTable.SetWidth(msg.Width)
+		m.rulesTable.SetHeight(msg.Height - 5)
+		formWidth := msg.Width - 12
+		m.ruleName.Width = formWidth
+		m.ruleMatch.Width = formWidth
+		m.ruleReplace.Width = formWidth
 		return m, nil
 
 	case listLoadedMsg:
@@ -300,6 +455,24 @@ func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.respView.GotoTop()
 		return m, nil
 
+	case rulesLoadedMsg:
+		if msg.err != nil {
+			m.statusMsg = "rules error: " + msg.err.Error()
+			return m, nil
+		}
+		m.ruleRows = msg.rules
+		m.rulesTable.SetRows(rulesRowsFor(m.ruleRows))
+		return m, nil
+
+	case ruleWriteDoneMsg:
+		if msg.err != nil {
+			m.statusMsg = msg.action + " error: " + msg.err.Error()
+			return m, nil
+		}
+		m.ruleForm = false
+		m.statusMsg = "rule " + msg.action
+		return m, m.loadRules
+
 	case tea.KeyMsg:
 		switch m.mode {
 		case viewList:
@@ -341,6 +514,10 @@ func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 				m.searchInput.SetValue(m.query)
 				m.searchInput.CursorEnd()
 				return m, m.searchInput.Focus()
+			case "m":
+				m.mode = viewRules
+				m.statusMsg = ""
+				return m, m.loadRules
 			case "esc":
 				if m.query != "" {
 					m.query = ""
@@ -413,6 +590,78 @@ func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 				m.respView, cmd = m.respView.Update(msg)
 			}
 			return m, cmd
+
+		case viewRules:
+			if m.ruleForm {
+				switch msg.String() {
+				case "esc":
+					m.ruleForm = false
+					return m, nil
+				case "ctrl+c":
+					return m, tea.Quit
+				case "ctrl+s":
+					return m, m.saveRule(m.ruleFromForm())
+				case "tab":
+					m.ruleField = (m.ruleField + 1) % 5
+					m.focusRuleField()
+					return m, nil
+				case "shift+tab":
+					m.ruleField = (m.ruleField + 4) % 5
+					m.focusRuleField()
+					return m, nil
+				}
+				if m.ruleField == fieldScope || m.ruleField == fieldRegex {
+					switch msg.String() {
+					case "left", "right", "enter", " ":
+						if m.ruleField == fieldScope {
+							if m.ruleScope == "request" {
+								m.ruleScope = "response"
+							} else {
+								m.ruleScope = "request"
+							}
+						} else {
+							m.ruleRegex = !m.ruleRegex
+						}
+						return m, nil
+					}
+				}
+				var cmd tea.Cmd
+				switch m.ruleField {
+				case fieldName:
+					m.ruleName, cmd = m.ruleName.Update(msg)
+				case fieldMatch:
+					m.ruleMatch, cmd = m.ruleMatch.Update(msg)
+				case fieldReplace:
+					m.ruleReplace, cmd = m.ruleReplace.Update(msg)
+				}
+				return m, cmd
+			}
+
+			switch msg.String() {
+			case "q", "esc":
+				m.mode = viewList
+				return m, nil
+			case "ctrl+c":
+				return m, tea.Quit
+			case "a":
+				m.enterRuleForm(nil)
+				return m, nil
+			case "enter", "e":
+				if row := m.rulesTable.Cursor(); row >= 0 && row < len(m.ruleRows) {
+					sel := m.ruleRows[row]
+					m.enterRuleForm(&sel)
+				}
+				return m, nil
+			case "d":
+				m.statusMsg = ""
+				return m, m.deleteSelectedRule()
+			case " ":
+				m.statusMsg = ""
+				return m, m.toggleSelectedRule()
+			}
+			var cmd tea.Cmd
+			m.rulesTable, cmd = m.rulesTable.Update(msg)
+			return m, cmd
 		}
 	}
 	return m, nil
@@ -427,6 +676,11 @@ func (m *model) View() string {
 		return m.detailView()
 	case viewRepeater:
 		return m.repeaterView()
+	case viewRules:
+		if m.ruleForm {
+			return m.ruleFormView()
+		}
+		return m.rulesView()
 	default:
 		return m.listView()
 	}
@@ -458,9 +712,9 @@ func (m *model) listView() string {
 		b.WriteString(statusStyle.Render(m.statusMsg))
 		b.WriteString("\n")
 	}
-	help := "↑/↓ navigate · enter view · r repeater · / search · q quit"
+	help := "↑/↓ navigate · enter view · r repeater · / search · m rules · q quit"
 	if m.query != "" {
-		help = "↑/↓ navigate · enter view · r repeater · / search · esc clear filter · q quit"
+		help = "↑/↓ navigate · enter view · r repeater · / search · m rules · esc clear filter · q quit"
 	}
 	b.WriteString(helpStyle.Render(help))
 	return b.String()
@@ -523,6 +777,75 @@ func (m *model) repeaterView() string {
 	return b.String()
 }
 
+func (m *model) rulesView() string {
+	var b strings.Builder
+	b.WriteString(titleStyle.Render(fmt.Sprintf(" match/replace rules (%d) - headers only for now ", len(m.ruleRows))))
+	b.WriteString("\n")
+	b.WriteString(m.rulesTable.View())
+	b.WriteString("\n")
+	if m.statusMsg != "" {
+		b.WriteString(statusStyle.Render(m.statusMsg))
+		b.WriteString("\n")
+	}
+	b.WriteString(helpStyle.Render("a add · enter/e edit · d delete · space toggle · esc back · q quit"))
+	return b.String()
+}
+
+func (m *model) ruleFormView() string {
+	var b strings.Builder
+	title := " add rule "
+	if m.ruleEditingID != 0 {
+		title = fmt.Sprintf(" edit rule #%d ", m.ruleEditingID)
+	}
+	b.WriteString(titleStyle.Render(title))
+	b.WriteString("\n\n")
+
+	label := func(field ruleField, text string) string {
+		if m.ruleField == field {
+			return tabActive.Render(text)
+		}
+		return tabInactive.Render(text)
+	}
+
+	b.WriteString(label(fieldName, "Name") + "\n")
+	b.WriteString(m.ruleName.View() + "\n\n")
+	b.WriteString(label(fieldMatch, "Match") + "\n")
+	b.WriteString(m.ruleMatch.View() + "\n\n")
+	b.WriteString(label(fieldReplace, "Replace") + "\n")
+	b.WriteString(m.ruleReplace.View() + "\n\n")
+
+	scopeText := fmt.Sprintf("Scope: %s  (◀▶ to change)", m.ruleScope)
+	b.WriteString(label(fieldScope, scopeText) + "\n\n")
+	regexText := "Regex: off  (◀▶ to change)"
+	if m.ruleRegex {
+		regexText = "Regex: on  (◀▶ to change)"
+	}
+	b.WriteString(label(fieldRegex, regexText) + "\n\n")
+
+	if m.statusMsg != "" {
+		b.WriteString(statusStyle.Render(m.statusMsg))
+		b.WriteString("\n")
+	}
+	b.WriteString(helpStyle.Render("tab/shift+tab move · ctrl+s save · esc cancel · ctrl+c quit"))
+	return b.String()
+}
+
+func rulesRowsFor(rs []rules.Rule) []table.Row {
+	rows := make([]table.Row, len(rs))
+	for i, r := range rs {
+		on := " "
+		if r.Enabled {
+			on = "✓"
+		}
+		regex := ""
+		if r.IsRegex {
+			regex = "yes"
+		}
+		rows[i] = table.Row{on, r.Name, r.Scope, r.Match, r.Replace, regex}
+	}
+	return rows
+}
+
 func exactSuffix(exact bool) string {
 	if exact {
 		return ", exact"