[mrtg] MRTG uses an IPv6 address even if w/o EnableIPv6 or with explicit EnableIPv6: no?

笹山 / SASAYAMA sasayama at otsl.jp
Fri Sep 30 07:35:43 CEST 2011

I am not new to MRTG but recently I saw a strange behavior in my
setting. It seems that MRTG 2.16.4 uses IPv6 even if mrtg.cfg contains
no EnableIPv6 or explicit EnableIPv6: no. In my setting, target hosts
are specified by host names which can be resolved to IPv6 addresses.

I investigated why it happens and found SNMP_Session.pm uses
Socket::inet_aton to resolve host names to addresses in IPv4-only
settings. Unfortunately, Socket::inet_aton may return an IPv6 address
when used to resolve host names and MRTG appears to use it without any
checks that it really is an IPv4 address. (Socket::inet_aton appears to
use C inet_aton and gethostbyname, and gethostbyname by nature may
return an IPv6 address.)

I do not know if it should be fixed in Perl-side, but IMO, it is unsafe
to use Socket::inet_aton to resolve host names in IPv4-only settings. I
managed to avoid this behavior by using IPv4-only host names, but it is
not very intuitive and should be handled properly in MRTG.
仕事用電話 080-7006-9731 <tel:08070069731> (出られないときは私用電話に転
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