Body match-and-replace rules

Extends match-and-replace rules to request/response bodies, not just
headers. A body rule materializes the body into memory (bounded by
the same maxCaptureBytes cap as history capture) instead of streaming
it straight through - the opposite of the normal path, so it's only
paid when a body rule is actually configured. A body over the cap
passes through byte-exact and unmodified rather than being partially
rewritten.

Response Content-Length is recomputed explicitly when a rule changes
body length: unlike http.Request.Write, http.ResponseWriter doesn't
derive it from resp.ContentLength on its own, so a stale header would
otherwise corrupt response framing for the client.

The history audit trail still shows the original, pre-rule bytes on
both legs; only the wire traffic reflects the rewrite. Verified live
against a real daemon: origin receives the rewritten request body,
client receives the rewritten response body with correct
Content-Length, and history keeps the unmodified bytes.

Adds a Part selector (header/body) to the Rules add/edit form and
table in the TUI.

1 files changed, 26 insertions, 8 deletions

diff --git a/cmd/mitmux/main.go b/cmd/mitmux/main.go
index f248ef0..d11d18b 100644
--- a/cmd/mitmux/main.go
+++ b/cmd/mitmux/main.go
@@ -134,6 +134,7 @@ const (
 	fieldMatch
 	fieldReplace
 	fieldScope
+	fieldPart
 	fieldRegex
 )
 
@@ -177,6 +178,7 @@ type model struct {
 	ruleMatch     textinput.Model
 	ruleReplace   textinput.Model
 	ruleScope     string // "request" or "response"
+	rulePart      string // "header" or "body"
 	ruleRegex     bool
 	ruleField     ruleField
 
@@ -298,6 +300,7 @@ func newModel(client *ipc.Client, subCh <-chan store.Summary, socketPath string)
 		{Title: "On", Width: 3},
 		{Title: "Name", Width: 16},
 		{Title: "Scope", Width: 9},
+		{Title: "Part", Width: 7},
 		{Title: "Match", Width: 24},
 		{Title: "Replace", Width: 24},
 		{Title: "Regex", Width: 5},
@@ -374,6 +377,7 @@ func newModel(client *ipc.Client, subCh <-chan store.Summary, socketPath string)
 		ruleMatch:        matchIn,
 		ruleReplace:      replaceIn,
 		ruleScope:        "request",
+		rulePart:         "header",
 		intruderTemplate: itmpl,
 		intruderPayloads: ipayloads,
 		intruderResults:  iresults,
@@ -808,6 +812,7 @@ func (m *model) enterRuleForm(r *rules.Rule) {
 		m.ruleMatch.SetValue("")
 		m.ruleReplace.SetValue("")
 		m.ruleScope = "request"
+		m.rulePart = "header"
 		m.ruleRegex = false
 	} else {
 		m.ruleEditingID = r.ID
@@ -816,6 +821,10 @@ func (m *model) enterRuleForm(r *rules.Rule) {
 		m.ruleMatch.SetValue(r.Match)
 		m.ruleReplace.SetValue(r.Replace)
 		m.ruleScope = r.Scope
+		m.rulePart = r.Part
+		if m.rulePart == "" {
+			m.rulePart = "header"
+		}
 		m.ruleRegex = r.IsRegex
 	}
 	m.ruleName.Focus()
@@ -829,7 +838,7 @@ func (m *model) ruleFromForm() rules.Rule {
 		Enabled: m.ruleEnabled,
 		Name:    m.ruleName.Value(),
 		Scope:   m.ruleScope,
-		Part:    "header",
+		Part:    m.rulePart,
 		Match:   m.ruleMatch.Value(),
 		Replace: m.ruleReplace.Value(),
 		IsRegex: m.ruleRegex,
@@ -1546,24 +1555,31 @@ func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 				case "ctrl+s":
 					return m, m.saveRule(m.ruleFromForm())
 				case "tab":
-					m.ruleField = (m.ruleField + 1) % 5
+					m.ruleField = (m.ruleField + 1) % 6
 					m.focusRuleField()
 					return m, nil
 				case "shift+tab":
-					m.ruleField = (m.ruleField + 4) % 5
+					m.ruleField = (m.ruleField + 5) % 6
 					m.focusRuleField()
 					return m, nil
 				}
-				if m.ruleField == fieldScope || m.ruleField == fieldRegex {
+				if m.ruleField == fieldScope || m.ruleField == fieldPart || m.ruleField == fieldRegex {
 					switch msg.String() {
 					case "left", "right", "enter", " ":
-						if m.ruleField == fieldScope {
+						switch m.ruleField {
+						case fieldScope:
 							if m.ruleScope == "request" {
 								m.ruleScope = "response"
 							} else {
 								m.ruleScope = "request"
 							}
-						} else {
+						case fieldPart:
+							if m.rulePart == "header" {
+								m.rulePart = "body"
+							} else {
+								m.rulePart = "header"
+							}
+						default:
 							m.ruleRegex = !m.ruleRegex
 						}
 						return m, nil
@@ -2204,7 +2220,7 @@ func viModeLabel(v *viTextarea) 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(titleStyle.Render(fmt.Sprintf(" match/replace rules (%d) ", len(m.ruleRows))))
 	b.WriteString("\n")
 	b.WriteString(m.rulesTable.View())
 	b.WriteString("\n")
@@ -2241,6 +2257,8 @@ func (m *model) ruleFormView() string {
 
 	scopeText := fmt.Sprintf("Scope: %s  (◀▶ to change)", m.ruleScope)
 	b.WriteString(label(fieldScope, scopeText) + "\n\n")
+	partText := fmt.Sprintf("Part: %s  (◀▶ to change)", m.rulePart)
+	b.WriteString(label(fieldPart, partText) + "\n\n")
 	regexText := "Regex: off  (◀▶ to change)"
 	if m.ruleRegex {
 		regexText = "Regex: on  (◀▶ to change)"
@@ -2300,7 +2318,7 @@ func rulesRowsFor(rs []rules.Rule) []table.Row {
 		if r.IsRegex {
 			regex = "yes"
 		}
-		rows[i] = table.Row{on, r.Name, r.Scope, r.Match, r.Replace, regex}
+		rows[i] = table.Row{on, r.Name, r.Scope, r.Part, r.Match, r.Replace, regex}
 	}
 	return rows
 }