SNMP::__construct() fails to resolve hostname with warning "Unknown failure while resolving hostname"

[dshovchko]

Description

The following code:

<?php

$session = new SNMP(SNMP::VERSION_2C, '127.0.0.1', 'public');
var_dump($session);

Resulted in this output:

PHP Warning:  SNMP::__construct(): Unknown failure while resolving '127.0.0.1' in /tmp/snmp_create_session.php on line 3
object(SNMP)#1 (8) {
  ["info"]=>
  array(3) {
    ["hostname"]=>
    string(0) ""
    ["timeout"]=>
    int(-1)
    ["retries"]=>
    int(-1)
  }
  ["max_oids"]=>
  NULL
  ["valueretrieval"]=>
  int(0)
  ["quick_print"]=>
  bool(false)
  ["enum_print"]=>
  bool(false)
  ["oid_output_format"]=>
  int(0)
  ["oid_increasing_check"]=>
  bool(false)
  ["exceptions_enabled"]=>
  int(0)
}

But I expected this output instead:

object(SNMP)#1 (8) {
  ["info"]=>
  array(3) {
    ["hostname"]=>
    string(9) "127.0.0.1"
    ["timeout"]=>
    int(-1)
    ["retries"]=>
    int(-1)
  }
  ["max_oids"]=>
  NULL
  ["valueretrieval"]=>
  int(0)
  ["quick_print"]=>
  bool(false)
  ["enum_print"]=>
  bool(false)
  ["oid_output_format"]=>
  int(0)
  ["oid_increasing_check"]=>
  bool(true)
  ["exceptions_enabled"]=>
  int(0)
}

The problem reproduces on 8.5.1, 8.4.16, and 8.4.15.

PHP Version

PHP 8.4.15 (cli) (built: Dec 11 2025 18:42:43) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.4.15, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.15, Copyright (c), by Zend Technologies

PHP 8.5.1 (cli) (built: Dec 21 2025 12:04:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.5.1, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.1, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04.3 LTS

[ndossche]

Doesn't reproduce on my system.
Does this reproduce consistently for you? Do you have any special networking configuration that may interfere with this?

[devnexen]

I do not have my usual ubuntu machine with me, however your script has the expected output on my mac with php 8.5.1.
So for whatever reason, netsnmp_session_init() fails for you, I may have a look next week unless someone does it faster.

[devnexen]

ah well sorry Niels :)

[dshovchko]

I have Cact (https://www.cacti.net/)i running on PHP 8.3.27 on my VPS. I decided to try upgrading to 8.4.15 and got an error in the web interface saying that it was impossible to retrieve data via SNMP. But the console agent written in C++ worked and collected data. I took an interest in this case and found that the problem was in PHP with the SNMP constructor. One of the features of the VPS is that it does not have IPv6, only IPv4. What other details do you need?

[azotov]

Got this problem on FreeBSD 14.3 after upgrading net-snmp from 5.9.4 to 5.9.5.2 and recompiling php-snmp. Upgrading php from 8.4.16 to 8.5.1 did not resolve the problem.

I have found at least one bug in snmp.c in snmp_session_init. In my building environment (after upgrading net-snmp to 5.9.5.2) HAVE_GETADDRINFO is defined but HAVE_IPV6 is not defined. As a result session->peername variable is never "filled properly" after resetting it to '\0'.

Deleting #if defined(HAVE_GETADDRINFO) && defined(HAVE_IPV6) and corresponging #endif partially solved the problem - SNMP::__construct() stopped returning the error "Unknown failure..". But SNMP::walk() for this session returned the error "Could not open snmp connection: Unknown host (127.0.0.1) (Can't assign requested address)". Trying snmprealwalk() with same parameters did not return any errors.

I recommend checking entire php-snmp code for compatibility with new net-snmp 5.9.5.2 version.

[devnexen]

Thanks for the hints @azotov I ll have a look soon-ish