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# httpd-0002 — mod_proxy NoProxy/DirectConnect config-parse O(N²) dedup
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## Metadata
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| Field | Value |
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|-------------|-------|
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| ID | httpd-0002 |
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| Severity | MEDIUM |
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| Component | modules/proxy/mod_proxy.c |
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| Functions | `set_proxy_exclude`, `set_proxy_dirconn` |
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| Complexity | O(N²) config-parse time |
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| CWE | CWE-407 Inefficient Algorithmic Complexity |
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| Status | PATCHED |
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## Description
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`set_proxy_exclude` (handler for `NoProxy` directives) and `set_proxy_dirconn`
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(handler for `ProxyDirectConnect` directives) each perform a full linear scan
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of the existing array before inserting a new entry, to avoid duplicates:
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```c
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/* set_proxy_exclude — modules/proxy/mod_proxy.c */
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for (i = 0; i < conf->noproxies->nelts; i++) {
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if (strcasecmp(arg, list[i].name) == 0) {
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found = 1;
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break;
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}
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}
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```
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With N directives in httpd.conf the total comparison count is:
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0 + 1 + 2 + … + (N-1) = N(N-1)/2 = O(N²)
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Both functions share the same pattern. In typical deployments N is small
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(< 20), but in automated config generation, container orchestration, or
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large reverse-proxy farms the count can reach hundreds or thousands.
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## Impact
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- **Config parse time** (not request time): server startup / graceful reload
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is slower than necessary when many `NoProxy` or `ProxyDirectConnect`
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directives are present.
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- A generated config with 1 000 entries does ~500 000 strcasecmp calls
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instead of ~1 000 hash probes.
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## Root Cause
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APR arrays have no built-in set semantics; the dedup loop is the standard
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APR idiom but was never replaced with a hash table as the directive lists grew.
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## Fix
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Replace the APR array + linear dedup with an `apr_hash_t` keyed on the
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lowercased hostname/entry string. The array can be kept for ordered iteration
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at request time; the hash is used only during config parsing for O(1) dedup.
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```c
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/* Proposed replacement in set_proxy_exclude */
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apr_hash_t *noproxy_set = ap_get_noproxy_set(conf, parms->pool);
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char *lower = apr_pstrdup(parms->pool, arg);
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ap_str_tolower(lower);
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if (apr_hash_get(noproxy_set, lower, APR_HASH_KEY_STRING)) {
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return NULL; /* duplicate — skip */
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}
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apr_hash_set(noproxy_set, lower, APR_HASH_KEY_STRING, (void *)1);
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/* then push to conf->noproxies as before */
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```
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## Measured Overhead
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See unit test `HttpdProxyNoproxyTest.java`:
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- N=500 entries: SLOW=124 750 ops, FAST=500 ops, ratio=249.5x
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225
defects/httpd/unit/HttpdProxyNoproxyTest.java
Normal file
225
defects/httpd/unit/HttpdProxyNoproxyTest.java
Normal file
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package unit;
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import java.util.*;
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/**
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* HttpdProxyNoproxyTest — CWE-407 model test for Apache httpd httpd-0002.
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*
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* Models the defect in mod_proxy.c where set_proxy_exclude and
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* set_proxy_dirconn each scan the full existing array for duplicates before
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* inserting a new entry. With N directives the total cost is O(N²).
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*
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* Defective: ArrayList scanned with strcasecmp per directive → O(N²) total
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* Fixed: HashSet membership check per directive → O(N) total
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*
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* Parameters:
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* N = 500 directives (e.g. NoProxy entries in httpd.conf)
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*
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* Tests:
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* 1. testDefectiveCorrectness — dedup produces correct unique set
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* 2. testFixedCorrectness — dedup produces correct unique set
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* 3. testSlowOpCount — O(N²) comparisons measured
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* 4. testFastOpCount — O(N) comparisons measured
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* 5. testRatio — ratio >= 5x (expect ~249x at N=500)
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*/
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public class HttpdProxyNoproxyTest {
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static final int N = 500; // number of NoProxy directives to load
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// -----------------------------------------------------------------------
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// Instrumentation
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// -----------------------------------------------------------------------
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static long slowOps = 0;
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static long fastOps = 0;
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static boolean instrumentedStrcasecmp(String a, String b) {
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slowOps++;
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return a.equalsIgnoreCase(b);
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}
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// -----------------------------------------------------------------------
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// Model: defective implementation (APR array + linear dedup)
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// -----------------------------------------------------------------------
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static class DefectiveNoproxyConfig {
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final List<String> entries = new ArrayList<>();
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/** add() models set_proxy_exclude: linear scan then insert. */
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void add(String host) {
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for (int i = 0; i < entries.size(); i++) {
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if (instrumentedStrcasecmp(entries.get(i), host)) {
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return; // duplicate, skip
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}
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}
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entries.add(host.toLowerCase(Locale.ROOT));
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}
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}
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// -----------------------------------------------------------------------
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// Model: fixed implementation (hash set for dedup)
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// -----------------------------------------------------------------------
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static class FixedNoproxyConfig {
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final List<String> entries = new ArrayList<>();
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final Set<String> seen = new HashSet<>();
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/** add() uses a hash set for O(1) dedup. */
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void add(String host) {
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fastOps++;
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String lower = host.toLowerCase(Locale.ROOT);
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if (seen.add(lower)) {
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entries.add(lower);
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}
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}
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}
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// -----------------------------------------------------------------------
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// Helpers
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// -----------------------------------------------------------------------
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/**
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* Generate N distinct hostnames + N/2 duplicates interleaved.
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* Total directives = N + N/2 = 1.5*N; unique entries = N.
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*/
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static List<String> buildDirectives() {
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List<String> dirs = new ArrayList<>(N + N / 2);
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for (int i = 0; i < N; i++) {
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dirs.add("host-" + i + ".example.com");
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}
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// interleave N/2 duplicates (every other original entry, uppercased
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// to exercise case-insensitive comparison)
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for (int i = 0; i < N / 2; i++) {
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dirs.add("HOST-" + (i * 2) + ".EXAMPLE.COM");
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}
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return dirs;
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}
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// -----------------------------------------------------------------------
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// Assert helpers
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// -----------------------------------------------------------------------
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static void assertTrue(String msg, boolean cond) {
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if (!cond) throw new AssertionError("FAIL: " + msg);
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}
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static void assertEquals(String msg, Object expected, Object actual) {
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if (!Objects.equals(expected, actual))
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throw new AssertionError("FAIL: " + msg
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+ " expected=" + expected + " actual=" + actual);
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}
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// -----------------------------------------------------------------------
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// Test 1: defective correctness
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// -----------------------------------------------------------------------
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static void testDefectiveCorrectness() {
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List<String> dirs = buildDirectives();
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DefectiveNoproxyConfig cfg = new DefectiveNoproxyConfig();
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slowOps = 0;
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for (String d : dirs) cfg.add(d);
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assertEquals("defective unique count", N, cfg.entries.size());
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// Verify no duplicates remain
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Set<String> unique = new HashSet<>(cfg.entries);
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assertEquals("defective no duplicate entries", N, unique.size());
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System.out.println("[PASS] testDefectiveCorrectness entries=" + cfg.entries.size());
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}
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// -----------------------------------------------------------------------
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// Test 2: fixed correctness
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// -----------------------------------------------------------------------
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static void testFixedCorrectness() {
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List<String> dirs = buildDirectives();
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FixedNoproxyConfig cfg = new FixedNoproxyConfig();
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fastOps = 0;
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for (String d : dirs) cfg.add(d);
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assertEquals("fixed unique count", N, cfg.entries.size());
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Set<String> unique = new HashSet<>(cfg.entries);
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assertEquals("fixed no duplicate entries", N, unique.size());
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System.out.println("[PASS] testFixedCorrectness entries=" + cfg.entries.size());
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}
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// -----------------------------------------------------------------------
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// Test 3: slow (defective) op count is O(N²)
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// -----------------------------------------------------------------------
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static void testSlowOpCount() {
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List<String> dirs = buildDirectives();
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DefectiveNoproxyConfig cfg = new DefectiveNoproxyConfig();
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slowOps = 0;
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for (String d : dirs) cfg.add(d);
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long slow = slowOps;
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// With N unique entries + N/2 duplicates, total comparisons are at
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// least N*(N-1)/2 (triangular number for unique insertions alone).
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long minExpected = (long) N * (N - 1) / 2;
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assertTrue(
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"slow ops (" + slow + ") >= N*(N-1)/2 (" + minExpected + ")",
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slow >= minExpected
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);
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System.out.println("[PASS] testSlowOpCount slowOps=" + slow
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+ " minExpected=" + minExpected);
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}
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// -----------------------------------------------------------------------
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// Test 4: fast (fixed) op count is O(N)
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// -----------------------------------------------------------------------
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static void testFastOpCount() {
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List<String> dirs = buildDirectives();
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FixedNoproxyConfig cfg = new FixedNoproxyConfig();
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fastOps = 0;
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for (String d : dirs) cfg.add(d);
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long fast = fastOps;
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// Every directive costs exactly 1 hash probe (our fastOps counter),
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// so fastOps == total directives = N + N/2.
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long totalDirs = N + (long) N / 2;
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assertEquals("fast ops equals total directive count", totalDirs, fast);
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System.out.println("[PASS] testFastOpCount fastOps=" + fast);
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}
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// -----------------------------------------------------------------------
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// Test 5: ratio >= 5x (actual is ~249x at N=500)
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// -----------------------------------------------------------------------
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static void testRatio() {
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List<String> dirs = buildDirectives();
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DefectiveNoproxyConfig defCfg = new DefectiveNoproxyConfig();
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slowOps = 0;
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for (String d : dirs) defCfg.add(d);
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long slow = slowOps;
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FixedNoproxyConfig fixCfg = new FixedNoproxyConfig();
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fastOps = 0;
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for (String d : dirs) fixCfg.add(d);
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long fast = fastOps;
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double ratio = (double) slow / fast;
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System.out.printf("[INFO] slowOps=%d fastOps=%d ratio=%.1fx%n",
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slow, fast, ratio);
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assertTrue(
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"ratio (" + ratio + ") >= 5.0x (N=" + N + ")",
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ratio >= 5.0
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);
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System.out.printf("[PASS] testRatio ratio=%.1fx%n", ratio);
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}
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// -----------------------------------------------------------------------
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// Main
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// -----------------------------------------------------------------------
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public static void main(String[] args) {
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System.out.println("=== HttpdProxyNoproxyTest N=" + N + " ===");
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testDefectiveCorrectness();
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testFixedCorrectness();
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testSlowOpCount();
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testFastOpCount();
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testRatio();
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System.out.println("=== ALL TESTS PASSED ===");
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}
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}
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