diff --git a/src/shrpx_client_handler.cc b/src/shrpx_client_handler.cc
index 64e83c3a..1e05e350 100644
--- a/src/shrpx_client_handler.cc
+++ b/src/shrpx_client_handler.cc
@@ -386,8 +386,9 @@ ClientHandler::ClientHandler(Worker *worker, int fd, SSL *ssl,
       faddr_(faddr),
       worker_(worker),
       left_connhd_len_(NGHTTP2_CLIENT_MAGIC_LEN),
-      affinity_hash_(-1),
-      should_close_after_write_(false) {
+      affinity_hash_(0),
+      should_close_after_write_(false),
+      affinity_hash_computed_(false) {
 
   ++worker_->get_worker_stat()->num_connections;
 
@@ -674,23 +675,19 @@ void ClientHandler::remove_downstream_connection(DownstreamConnection *dconn) {
 }
 
 namespace {
-uint32_t hash32(const StringRef &s) {
-  /* 32 bit FNV-1a: http://isthe.com/chongo/tech/comp/fnv/ */
-  uint32_t h = 2166136261u;
-  size_t i;
+// Computes 32bits hash for session affinity for IP address |ip|.
+uint32_t compute_affinity_from_ip(const StringRef &ip) {
+  int rv;
+  std::array<uint8_t, 32> buf;
 
-  for (i = 0; i < s.size(); ++i) {
-    h ^= s[i];
-    h += (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24);
+  rv = util::sha256(buf.data(), ip);
+  if (rv != 0) {
+    // Not sure when sha256 failed.  Just fall back to another
+    // function.
+    return util::hash32(ip);
   }
 
-  return h;
-}
-} // namespace
-
-namespace {
-int32_t calculate_affinity_from_ip(const StringRef &ip) {
-  return hash32(ip) & 0x7fffffff;
+  return (buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
 }
 } // namespace
 
@@ -918,11 +915,22 @@ ClientHandler::get_downstream_connection(Downstream *downstream) {
   auto &shared_addr = group->shared_addr;
 
   if (shared_addr->affinity == AFFINITY_IP) {
-    if (affinity_hash_ == -1) {
-      affinity_hash_ = calculate_affinity_from_ip(StringRef{ipaddr_});
+    if (!affinity_hash_computed_) {
+      affinity_hash_ = compute_affinity_from_ip(StringRef{ipaddr_});
+      affinity_hash_computed_ = true;
     }
 
-    auto idx = affinity_hash_ % shared_addr->addrs.size();
+    const auto &affinity_hash = shared_addr->affinity_hash;
+
+    auto it = std::lower_bound(
+        std::begin(affinity_hash), std::end(affinity_hash), affinity_hash_,
+        [](const AffinityHash &lhs, uint32_t rhs) { return lhs.hash < rhs; });
+
+    if (it == std::end(affinity_hash)) {
+      it = std::begin(affinity_hash);
+    }
+
+    auto idx = (*it).idx;
 
     auto &addr = shared_addr->addrs[idx];
     if (addr.proto == PROTO_HTTP2) {
diff --git a/src/shrpx_client_handler.h b/src/shrpx_client_handler.h
index 73117720..bdfdafd2 100644
--- a/src/shrpx_client_handler.h
+++ b/src/shrpx_client_handler.h
@@ -174,8 +174,11 @@ private:
   Worker *worker_;
   // The number of bytes of HTTP/2 client connection header to read
   size_t left_connhd_len_;
-  int32_t affinity_hash_;
+  // hash for session affinity using client IP
+  uint32_t affinity_hash_;
   bool should_close_after_write_;
+  // true if affinity_hash_ is computed
+  bool affinity_hash_computed_;
   ReadBuf rb_;
 };
 
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index e059b276..3d27e6f2 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -2830,12 +2830,49 @@ StringRef strproto(shrpx_proto proto) {
   assert(0);
 }
 
+namespace {
+// Consistent hashing method described in
+// https://github.com/RJ/ketama.  Generate 160 32-bit hashes per |s|,
+// which is usually backend address.  The each hash is associated to
+// index of backend address.  When all hashes for every backend
+// address are calculated, sort it in ascending order of hash.  To
+// choose the index, compute 32-bit hash based on client IP address,
+// and do lower bound search in the array. The returned index is the
+// backend to use.
+int compute_affinity_hash(std::vector<AffinityHash> &res, size_t idx,
+                          const StringRef &s) {
+  int rv;
+  std::array<uint8_t, 32> buf;
+
+  for (auto i = 0; i < 20; ++i) {
+    auto t = s.str();
+    t += i;
+
+    rv = util::sha256(buf.data(), StringRef{t});
+    if (rv != 0) {
+      return -1;
+    }
+
+    for (int i = 0; i < 8; ++i) {
+      auto h = (buf[4 * i] << 24) | (buf[4 * i + 1] << 16) |
+               (buf[4 * i + 2] << 8) | buf[4 * i + 3];
+
+      res.emplace_back(idx, h);
+    }
+  }
+
+  return 0;
+}
+} // namespace
+
 // Configures the following member in |config|:
 // conn.downstream_router, conn.downstream.addr_groups,
 // conn.downstream.addr_group_catch_all.
 int configure_downstream_group(Config *config, bool http2_proxy,
                                bool numeric_addr_only,
                                const TLSConfig &tlsconf) {
+  int rv;
+
   auto &downstreamconf = *config->conn.downstream;
   auto &addr_groups = downstreamconf.addr_groups;
   auto &routerconf = downstreamconf.router;
@@ -2959,6 +2996,25 @@ int configure_downstream_group(Config *config, bool http2_proxy,
                   << util::to_numeric_addr(&addr.addr);
       }
     }
+
+    if (g.affinity == AFFINITY_IP) {
+      size_t idx = 0;
+      for (auto &addr : g.addrs) {
+        auto p = reinterpret_cast<uint8_t *>(&addr.addr.su);
+        rv = compute_affinity_hash(g.affinity_hash, idx,
+                                   StringRef{p, addr.addr.len});
+        if (rv != 0) {
+          return -1;
+        }
+
+        ++idx;
+      }
+
+      std::sort(std::begin(g.affinity_hash), std::end(g.affinity_hash),
+                [](const AffinityHash &lhs, const AffinityHash &rhs) {
+                  return lhs.hash < rhs.hash;
+                });
+    }
   }
 
   return 0;
diff --git a/src/shrpx_config.h b/src/shrpx_config.h
index 25a34dac..2ea76015 100644
--- a/src/shrpx_config.h
+++ b/src/shrpx_config.h
@@ -370,12 +370,24 @@ struct DownstreamAddrConfig {
   bool tls;
 };
 
+// Mapping hash to idx which is an index into
+// DownstreamAddrGroupConfig::addrs.
+struct AffinityHash {
+  AffinityHash(size_t idx, uint32_t hash) : idx(idx), hash(hash) {}
+
+  size_t idx;
+  uint32_t hash;
+};
+
 struct DownstreamAddrGroupConfig {
   DownstreamAddrGroupConfig(const StringRef &pattern)
       : pattern(pattern.c_str(), pattern.size()), affinity(AFFINITY_NONE) {}
 
   ImmutableString pattern;
   std::vector<DownstreamAddrConfig> addrs;
+  // Bunch of session affinity hash.  Only used if affinity ==
+  // AFFINITY_IP.
+  std::vector<AffinityHash> affinity_hash;
   // Session affinity
   shrpx_session_affinity affinity;
 };
diff --git a/src/shrpx_worker.cc b/src/shrpx_worker.cc
index f815149b..5927a8d5 100644
--- a/src/shrpx_worker.cc
+++ b/src/shrpx_worker.cc
@@ -192,6 +192,7 @@ void Worker::replace_downstream_config(
 
     shared_addr->addrs.resize(src.addrs.size());
     shared_addr->affinity = src.affinity;
+    shared_addr->affinity_hash = src.affinity_hash;
 
     size_t num_http1 = 0;
     size_t num_http2 = 0;
diff --git a/src/shrpx_worker.h b/src/shrpx_worker.h
index d187743c..3503d0f7 100644
--- a/src/shrpx_worker.h
+++ b/src/shrpx_worker.h
@@ -127,6 +127,9 @@ struct WeightedPri {
 
 struct SharedDownstreamAddr {
   std::vector<DownstreamAddr> addrs;
+  // Bunch of session affinity hash.  Only used if affinity ==
+  // AFFINITY_IP.
+  std::vector<AffinityHash> affinity_hash;
   // List of Http2Session which is not fully utilized (i.e., the
   // server advertized maximum concurrency is not reached).  We will
   // coalesce as much stream as possible in one Http2Session to fully
diff --git a/src/util.cc b/src/util.cc
index d24c5c8d..af7f187d 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -54,8 +54,11 @@
 #include <iostream>
 #include <fstream>
 
+#include <openssl/evp.h>
+
 #include <nghttp2/nghttp2.h>
 
+#include "ssl_compat.h"
 #include "timegm.h"
 
 namespace nghttp2 {
@@ -1325,6 +1328,59 @@ double int_pow(double x, size_t y) {
   return res;
 }
 
+uint32_t hash32(const StringRef &s) {
+  /* 32 bit FNV-1a: http://isthe.com/chongo/tech/comp/fnv/ */
+  uint32_t h = 2166136261u;
+  size_t i;
+
+  for (i = 0; i < s.size(); ++i) {
+    h ^= s[i];
+    h += (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24);
+  }
+
+  return h;
+}
+
+#if !OPENSSL_101_API
+namespace {
+EVP_MD_CTX *EVP_MD_CTX_new(void) { return EVP_MD_CTX_create(); }
+} // namespace
+
+namespace {
+void EVP_MD_CTX_free(EVP_MD_CTX *ctx) { EVP_MD_CTX_destroy(ctx); }
+} // namespace
+#endif
+
+int sha256(uint8_t *res, const StringRef &s) {
+  int rv;
+
+  auto ctx = EVP_MD_CTX_new();
+  if (ctx == nullptr) {
+    return -1;
+  }
+
+  auto ctx_deleter = defer(EVP_MD_CTX_free, ctx);
+
+  rv = EVP_DigestInit_ex(ctx, EVP_sha256(), nullptr);
+  if (rv != 1) {
+    return -1;
+  }
+
+  rv = EVP_DigestUpdate(ctx, s.c_str(), s.size());
+  if (rv != 1) {
+    return -1;
+  }
+
+  unsigned int mdlen = 32;
+
+  rv = EVP_DigestFinal_ex(ctx, res, &mdlen);
+  if (rv != 1) {
+    return -1;
+  }
+
+  return 0;
+}
+
 } // namespace util
 
 } // namespace nghttp2
diff --git a/src/util.h b/src/util.h
index 375dacfd..5ae8cfb5 100644
--- a/src/util.h
+++ b/src/util.h
@@ -680,6 +680,12 @@ OutputIterator copy_lit(OutputIterator it, CharT(&s)[N]) {
 // Returns x**y
 double int_pow(double x, size_t y);
 
+uint32_t hash32(const StringRef &s);
+
+// Computes SHA-256 of |s|, and stores it in |buf|.  This function
+// returns 0 if it succeeds, or -1.
+int sha256(uint8_t *buf, const StringRef &s);
+
 } // namespace util
 
 } // namespace nghttp2