From 94fd8780de3374b7f55d72172338689276be63a9 Mon Sep 17 00:00:00 2001
From: graham <graham@mcintire.me>
Date: Wed, 25 Mar 2026 12:25:13 -0500
Subject: [PATCH] fix/counter-wrap-detection (#164)

Reviewed-on: https://git.mcintire.me/graham/towerops-web/pulls/164
---
 lib/towerops/workers/device_poller_worker.ex | 12 +++++++++++-
 lib/towerops_web/channels/agent_channel.ex   |  7 ++++++-
 lib/towerops_web/live/graph_live/show.ex     | 12 +++++++++---
 3 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/lib/towerops/workers/device_poller_worker.ex b/lib/towerops/workers/device_poller_worker.ex
index facca833..14f4be9c 100644
--- a/lib/towerops/workers/device_poller_worker.ex
+++ b/lib/towerops/workers/device_poller_worker.ex
@@ -1326,7 +1326,10 @@ defmodule Towerops.Workers.DevicePollerWorker do
   end
 
   defp fetch_octet_counters(client_opts, hc_oids, std_oids) do
-    hc_results = Enum.map(hc_oids, &try_fetch_hc_counter(client_opts, &1))
+    # Counter64 (HC) can't be encoded in SNMPv1 responses, so upgrade to v2c
+    # for HC queries. Most devices support v2c with the same community string.
+    hc_opts = upgrade_to_v2c_for_hc(client_opts)
+    hc_results = Enum.map(hc_oids, &try_fetch_hc_counter(hc_opts, &1))
 
     hc_available = Enum.all?(hc_results, fn {_key, value} -> value != :not_available end)
 
@@ -1338,6 +1341,13 @@ defmodule Towerops.Workers.DevicePollerWorker do
     end
   end
 
+  defp upgrade_to_v2c_for_hc(client_opts) do
+    case Keyword.get(client_opts, :version) do
+      v when v in ["1", :v1] -> Keyword.put(client_opts, :version, "2c")
+      _ -> client_opts
+    end
+  end
+
   defp try_fetch_hc_counter(client_opts, {key, oid}) do
     case Client.get(client_opts, oid) do
       {:ok, value} ->
diff --git a/lib/towerops_web/channels/agent_channel.ex b/lib/towerops_web/channels/agent_channel.ex
index d9bf7ecf..f90a48fb 100644
--- a/lib/towerops_web/channels/agent_channel.ex
+++ b/lib/towerops_web/channels/agent_channel.ex
@@ -1083,9 +1083,14 @@ defmodule ToweropsWeb.AgentChannel do
   defp build_v2c_snmp_device(device, snmp_config) do
     community = snmp_config.community || ""
 
+    # Always use v2c for agent polling — Counter64 (HC counters) can't be
+    # encoded in SNMPv1 responses, causing 64-bit counter queries to fail.
+    # v2c uses the same community string and is supported by virtually all
+    # SNMP-capable devices. Without HC counters, 32-bit counters on fast
+    # links wrap undetectably (full cycle in ~69s at 500 Mbps).
     %SnmpDevice{
       ip: device.ip_address,
-      version: device.snmp_version,
+      version: "2c",
       port: device.snmp_port || 161,
       community: community
     }
diff --git a/lib/towerops_web/live/graph_live/show.ex b/lib/towerops_web/live/graph_live/show.ex
index df6979f0..c0d4b3a7 100644
--- a/lib/towerops_web/live/graph_live/show.ex
+++ b/lib/towerops_web/live/graph_live/show.ex
@@ -1291,12 +1291,18 @@ defmodule ToweropsWeb.GraphLive.Show do
 
   # Poll octets for a single interface (try 64-bit, fall back to 32-bit)
   defp poll_single_interface_octets(interface, client_opts) do
-    # Try 64-bit counters first
+    # Counter64 (HC) can't be encoded in SNMPv1 — upgrade to v2c for HC queries
+    hc_opts =
+      case Keyword.get(client_opts, :version) do
+        v when v in ["1", :v1] -> Keyword.put(client_opts, :version, "2c")
+        _ -> client_opts
+      end
+
     in_octets_oid = "1.3.6.1.2.1.31.1.1.1.6.#{interface.if_index}"
     out_octets_oid = "1.3.6.1.2.1.31.1.1.1.10.#{interface.if_index}"
 
-    with {:ok, in_octets} <- Snmp.Client.get(client_opts, in_octets_oid),
-         {:ok, out_octets} <- Snmp.Client.get(client_opts, out_octets_oid),
+    with {:ok, in_octets} <- Snmp.Client.get(hc_opts, in_octets_oid),
+         {:ok, out_octets} <- Snmp.Client.get(hc_opts, out_octets_oid),
          true <- is_number(in_octets) and is_number(out_octets) do
       build_interface_stat(interface, in_octets, out_octets)
     else