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.

5 files changed, 344 insertions, 2 deletions

diff --git a/internal/ipc/ipc.go b/internal/ipc/ipc.go
index f890715..19c9c23 100644
--- a/internal/ipc/ipc.go
+++ b/internal/ipc/ipc.go
@@ -11,12 +11,13 @@ import (
 	"net"
 	"sync"
 
+	"mitmux/internal/rules"
 	"mitmux/internal/store"
 )
 
 // Request is sent by a client to the daemon.
 type Request struct {
-	Type     string `json:"type"` // "list", "get", "subscribe", or "repeat"
+	Type     string `json:"type"` // "list", "get", "subscribe", "repeat", "rules_list", "rules_save", "rules_delete", or "rules_toggle"
 	Limit    int    `json:"limit,omitempty"`
 	BeforeID int64  `json:"before_id,omitempty"`
 	ID       int64  `json:"id,omitempty"`
@@ -30,14 +31,22 @@ type Request struct {
 	Scheme string `json:"scheme,omitempty"`
 	Host   string `json:"host,omitempty"`
 	Raw    []byte `json:"raw,omitempty"`
+
+	// For "rules_save": add (Rule.ID == 0) or update (Rule.ID != 0) a
+	// match-and-replace rule. For "rules_delete"/"rules_toggle": RuleID
+	// (and RuleEnabled for toggle) identify the target.
+	Rule        *rules.Rule `json:"rule,omitempty"`
+	RuleID      int64       `json:"rule_id,omitempty"`
+	RuleEnabled bool        `json:"rule_enabled,omitempty"`
 }
 
 // Response is sent by the daemon to a client.
 type Response struct {
-	Type    string          `json:"type"`              // "list", "get", "new", "repeat", or "error"
+	Type    string          `json:"type"`              // "list", "get", "new", "repeat", "rules", or "error"
 	Entries []store.Summary `json:"entries,omitempty"` // for "list"
 	Detail  *EntryDetail    `json:"detail,omitempty"`  // for "get" and "repeat"
 	New     *store.Summary  `json:"new,omitempty"`     // for "new" (subscribe push)
+	Rules   []rules.Rule    `json:"rules,omitempty"`   // for "rules"
 	Error   string          `json:"error,omitempty"`
 }
 
@@ -144,6 +153,58 @@ func (c *Client) Repeat(scheme, host string, raw []byte) (*EntryDetail, error) {
 	return resp.Detail, nil
 }
 
+// ListRules returns every match-and-replace rule.
+func (c *Client) ListRules() ([]rules.Rule, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.rulesRoundTrip(Request{Type: "rules_list"})
+}
+
+// SaveRule adds r (if r.ID == 0) or updates the existing rule with that
+// ID, and returns its ID.
+func (c *Client) SaveRule(r rules.Rule) (int64, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	saved, err := c.rulesRoundTrip(Request{Type: "rules_save", Rule: &r})
+	if err != nil {
+		return 0, err
+	}
+	if len(saved) == 0 {
+		return 0, errors.New("rules_save: daemon returned no rule")
+	}
+	return saved[0].ID, nil
+}
+
+// DeleteRule removes a rule.
+func (c *Client) DeleteRule(id int64) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	_, err := c.rulesRoundTrip(Request{Type: "rules_delete", RuleID: id})
+	return err
+}
+
+// SetRuleEnabled toggles a rule without touching its other fields.
+func (c *Client) SetRuleEnabled(id int64, enabled bool) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	_, err := c.rulesRoundTrip(Request{Type: "rules_toggle", RuleID: id, RuleEnabled: enabled})
+	return err
+}
+
+func (c *Client) rulesRoundTrip(req Request) ([]rules.Rule, error) {
+	if err := c.enc.Encode(req); err != nil {
+		return nil, err
+	}
+	var resp Response
+	if err := c.dec.Decode(&resp); err != nil {
+		return nil, err
+	}
+	if resp.Type == "error" {
+		return nil, errors.New(resp.Error)
+	}
+	return resp.Rules, nil
+}
+
 // Subscribe opens a dedicated connection that streams newly captured
 // history entries as they happen. The returned channel is closed when
 // the connection ends; call the returned close func to stop early.
diff --git a/internal/ipc/server.go b/internal/ipc/server.go
index 69044af..1fe8d8f 100644
--- a/internal/ipc/server.go
+++ b/internal/ipc/server.go
@@ -8,6 +8,7 @@ import (
 	"net"
 	"sync"
 
+	"mitmux/internal/rules"
 	"mitmux/internal/store"
 )
 
@@ -127,6 +128,46 @@ func (s *Server) handleConn(conn net.Conn) {
 			}
 			enc.Encode(Response{Type: "repeat", Detail: detailFromEntry(e)})
 
+		case "rules_list":
+			rs, err := s.db.ListRules()
+			if err != nil {
+				enc.Encode(Response{Type: "error", Error: err.Error()})
+				continue
+			}
+			enc.Encode(Response{Type: "rules", Rules: rs})
+
+		case "rules_save":
+			if req.Rule == nil {
+				enc.Encode(Response{Type: "error", Error: "rules_save: missing rule"})
+				continue
+			}
+			r := *req.Rule
+			var err error
+			if r.ID == 0 {
+				r.ID, err = s.db.AddRule(r)
+			} else {
+				err = s.db.UpdateRule(r)
+			}
+			if err != nil {
+				enc.Encode(Response{Type: "error", Error: err.Error()})
+				continue
+			}
+			enc.Encode(Response{Type: "rules", Rules: []rules.Rule{r}})
+
+		case "rules_delete":
+			if err := s.db.DeleteRule(req.RuleID); err != nil {
+				enc.Encode(Response{Type: "error", Error: err.Error()})
+				continue
+			}
+			enc.Encode(Response{Type: "rules"})
+
+		case "rules_toggle":
+			if err := s.db.SetRuleEnabled(req.RuleID, req.RuleEnabled); err != nil {
+				enc.Encode(Response{Type: "error", Error: err.Error()})
+				continue
+			}
+			enc.Encode(Response{Type: "rules"})
+
 		case "subscribe":
 			sub := s.hub.subscribe()
 			defer s.hub.unsubscribe(sub)
diff --git a/internal/proxy/proxy.go b/internal/proxy/proxy.go
index 014c601..3b2d50d 100644
--- a/internal/proxy/proxy.go
+++ b/internal/proxy/proxy.go
@@ -35,6 +35,7 @@ import (
 	"golang.org/x/net/http2"
 
 	"mitmux/internal/ca"
+	"mitmux/internal/rules"
 	"mitmux/internal/store"
 )
 
@@ -244,6 +245,17 @@ func (s *Server) forward(dial dialer, scheme, hostname string, w http.ResponseWr
 	outReq.URL.Host = hostname
 	outReq.RequestURI = ""
 	stripHopByHop(outReq.Header)
+	// Header rules are applied to outReq only, after cloning and header
+	// stripping - history's request_raw keeps showing what the client
+	// actually sent (clientTee/reqBodyCap already capture from r, not
+	// outReq), while what actually reaches the upstream server reflects
+	// the rules. That split is deliberate: match-and-replace is a wire
+	// transform, not a rewrite of the audit trail.
+	if reqRules, err := s.enabledRules("request"); err != nil {
+		log.Printf("load request rules: %v", err)
+	} else if len(reqRules) > 0 {
+		outReq.Header = rules.ApplyHeaders(outReq.Header, reqRules)
+	}
 
 	// Only needed when the client leg isn't tee-captured (HTTP/2): tee
 	// the body as it streams through so the reconstructed capture isn't
@@ -297,6 +309,17 @@ func (s *Server) forward(dial dialer, scheme, hostname string, w http.ResponseWr
 		resp.Body = io.NopCloser(respBodyCap)
 	}
 
+	// Same split as the request side: response_raw keeps reflecting what
+	// the origin server actually sent (captured below, from upstreamTee
+	// or respBodyCap, both already wired to resp.Body independent of
+	// resp.Header), while the client actually receives the rule-modified
+	// headers.
+	if respRules, err := s.enabledRules("response"); err != nil {
+		log.Printf("load response rules: %v", err)
+	} else if len(respRules) > 0 {
+		resp.Header = rules.ApplyHeaders(resp.Header, respRules)
+	}
+
 	stripHopByHop(resp.Header)
 	for k, vv := range resp.Header {
 		for _, v := range vv {
@@ -317,6 +340,17 @@ func (s *Server) forward(dial dialer, scheme, hostname string, w http.ResponseWr
 	s.record(started, duration, scheme, hostname, r, reqRaw, reqExact, respRaw, respExact, resp.StatusCode, "")
 }
 
+// enabledRules fetches the current enabled match-and-replace rules for
+// scope ("request" or "response") fresh from the store on every call -
+// simple and always current, and cheap enough (a local, in-process
+// SQLite query) not to bother caching for how this is actually used.
+func (s *Server) enabledRules(scope string) ([]rules.Rule, error) {
+	if s.store == nil {
+		return nil, nil
+	}
+	return s.store.EnabledRules(scope)
+}
+
 // record stores one history entry and notifies OnEntry.
 func (s *Server) record(started time.Time, duration time.Duration, scheme, host string, r *http.Request,
 	reqRaw []byte, reqExact bool, respRaw []byte, respExact bool, status int, errMsg string) {
diff --git a/internal/rules/rules.go b/internal/rules/rules.go
new file mode 100644
index 0000000..1f50ec4
--- /dev/null
+++ b/internal/rules/rules.go
@@ -0,0 +1,97 @@
+// Package rules implements match-and-replace: user-defined rules that
+// rewrite request/response headers as they pass through the proxy.
+// Deliberately headers-only for now - see ApplyHeaders for why bodies
+// are a separate, harder problem (noted as a follow-up in PLAN.md).
+package rules
+
+import (
+	"bufio"
+	"net/http"
+	"net/textproto"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// Rule is one match-and-replace rule.
+type Rule struct {
+	ID      int64
+	Enabled bool
+	Name    string
+	Scope   string // "request" or "response"
+	Part    string // "header" (only part supported so far)
+	Match   string
+	Replace string
+	IsRegex bool
+	// Position orders rule application (ascending) when several rules
+	// could touch the same text.
+	Position int
+}
+
+// ApplyHeaders rewrites h in place by serializing it to a raw
+// "Name: value\r\n" block, running every enabled rule with Part=="header"
+// over that text (in Position order), and reparsing the result. Working
+// on the raw text rather than per-value substitution is what lets a rule
+// add or remove a header entirely, not just rewrite an existing value -
+// matching how Burp's header match/replace works. If a rule's output
+// doesn't parse back as valid headers, ApplyHeaders returns h unchanged
+// rather than risk sending something corrupted.
+func ApplyHeaders(h http.Header, rs []Rule) http.Header {
+	keys := make([]string, 0, len(h))
+	for k := range h {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+
+	var block strings.Builder
+	for _, k := range keys {
+		for _, v := range h[k] {
+			block.WriteString(k)
+			block.WriteString(": ")
+			block.WriteString(v)
+			block.WriteString("\r\n")
+		}
+	}
+	text := block.String()
+
+	changed := false
+	for _, r := range sortedByPosition(rs) {
+		if !r.Enabled || r.Part != "header" {
+			continue
+		}
+		if next, ok := apply(text, r); ok {
+			text, changed = next, true
+		}
+	}
+	if !changed {
+		return h
+	}
+
+	tp := textproto.NewReader(bufio.NewReader(strings.NewReader(text + "\r\n")))
+	mh, err := tp.ReadMIMEHeader()
+	if err != nil {
+		return h
+	}
+	return http.Header(mh)
+}
+
+func sortedByPosition(rs []Rule) []Rule {
+	out := make([]Rule, len(rs))
+	copy(out, rs)
+	sort.SliceStable(out, func(i, j int) bool { return out[i].Position < out[j].Position })
+	return out
+}
+
+func apply(text string, r Rule) (string, bool) {
+	if r.IsRegex {
+		re, err := regexp.Compile(r.Match)
+		if err != nil {
+			return text, false
+		}
+		return re.ReplaceAllString(text, r.Replace), true
+	}
+	if r.Match == "" {
+		return text, false
+	}
+	return strings.ReplaceAll(text, r.Match, r.Replace), true
+}
diff --git a/internal/store/store.go b/internal/store/store.go
index a3c6b03..211162e 100644
--- a/internal/store/store.go
+++ b/internal/store/store.go
@@ -12,6 +12,8 @@ import (
 	"time"
 
 	_ "modernc.org/sqlite"
+
+	"mitmux/internal/rules"
 )
 
 const schema = `
@@ -36,6 +38,18 @@ CREATE VIRTUAL TABLE IF NOT EXISTS history_fts USING fts5(
 	method, host, path, request_text, response_text,
 	tokenize = 'unicode61 remove_diacritics 2'
 );
+
+CREATE TABLE IF NOT EXISTS rules (
+	id       INTEGER PRIMARY KEY AUTOINCREMENT,
+	enabled  INTEGER NOT NULL DEFAULT 1,
+	name     TEXT NOT NULL DEFAULT '',
+	scope    TEXT NOT NULL,
+	part     TEXT NOT NULL,
+	match    TEXT NOT NULL,
+	replace  TEXT NOT NULL,
+	is_regex INTEGER NOT NULL DEFAULT 0,
+	position INTEGER NOT NULL DEFAULT 0
+);
 `
 
 // Store is a handle to the history database. Safe for concurrent use.
@@ -312,6 +326,101 @@ func prepareFTSQuery(q string) string {
 	return strings.Join(fields, " ")
 }
 
+// ListRules returns every match-and-replace rule, ordered for application.
+func (s *Store) ListRules() ([]rules.Rule, error) {
+	rows, err := s.db.Query(
+		`SELECT id, enabled, name, scope, part, match, replace, is_regex, position
+		 FROM rules ORDER BY position, id`,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("list rules: %w", err)
+	}
+	defer rows.Close()
+
+	var out []rules.Rule
+	for rows.Next() {
+		var r rules.Rule
+		var enabled, isRegex int
+		if err := rows.Scan(&r.ID, &enabled, &r.Name, &r.Scope, &r.Part, &r.Match, &r.Replace, &isRegex, &r.Position); err != nil {
+			return nil, fmt.Errorf("scan rule row: %w", err)
+		}
+		r.Enabled = enabled != 0
+		r.IsRegex = isRegex != 0
+		out = append(out, r)
+	}
+	return out, rows.Err()
+}
+
+// EnabledRules returns enabled rules for scope ("request" or
+// "response"), ordered for application.
+func (s *Store) EnabledRules(scope string) ([]rules.Rule, error) {
+	rows, err := s.db.Query(
+		`SELECT id, enabled, name, scope, part, match, replace, is_regex, position
+		 FROM rules WHERE enabled = 1 AND scope = ? ORDER BY position, id`,
+		scope,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("enabled rules: %w", err)
+	}
+	defer rows.Close()
+
+	var out []rules.Rule
+	for rows.Next() {
+		var r rules.Rule
+		var enabled, isRegex int
+		if err := rows.Scan(&r.ID, &enabled, &r.Name, &r.Scope, &r.Part, &r.Match, &r.Replace, &isRegex, &r.Position); err != nil {
+			return nil, fmt.Errorf("scan rule row: %w", err)
+		}
+		r.Enabled = enabled != 0
+		r.IsRegex = isRegex != 0
+		out = append(out, r)
+	}
+	return out, rows.Err()
+}
+
+// AddRule stores r and returns its assigned ID.
+func (s *Store) AddRule(r rules.Rule) (int64, error) {
+	res, err := s.db.Exec(
+		`INSERT INTO rules (enabled, name, scope, part, match, replace, is_regex, position)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
+		boolToInt(r.Enabled), r.Name, r.Scope, r.Part, r.Match, r.Replace, boolToInt(r.IsRegex), r.Position,
+	)
+	if err != nil {
+		return 0, fmt.Errorf("add rule: %w", err)
+	}
+	return res.LastInsertId()
+}
+
+// UpdateRule replaces the stored rule with the same ID as r.
+func (s *Store) UpdateRule(r rules.Rule) error {
+	_, err := s.db.Exec(
+		`UPDATE rules SET enabled = ?, name = ?, scope = ?, part = ?, match = ?, replace = ?, is_regex = ?, position = ?
+		 WHERE id = ?`,
+		boolToInt(r.Enabled), r.Name, r.Scope, r.Part, r.Match, r.Replace, boolToInt(r.IsRegex), r.Position, r.ID,
+	)
+	if err != nil {
+		return fmt.Errorf("update rule %d: %w", r.ID, err)
+	}
+	return nil
+}
+
+// SetRuleEnabled toggles a rule without touching its other fields.
+func (s *Store) SetRuleEnabled(id int64, enabled bool) error {
+	_, err := s.db.Exec(`UPDATE rules SET enabled = ? WHERE id = ?`, boolToInt(enabled), id)
+	if err != nil {
+		return fmt.Errorf("set rule %d enabled: %w", id, err)
+	}
+	return nil
+}
+
+// DeleteRule removes a rule.
+func (s *Store) DeleteRule(id int64) error {
+	if _, err := s.db.Exec(`DELETE FROM rules WHERE id = ?`, id); err != nil {
+		return fmt.Errorf("delete rule %d: %w", id, err)
+	}
+	return nil
+}
+
 func boolToInt(b bool) int {
 	if b {
 		return 1