Host names truncated to 64 chars

[morille]

I have an OpenSMTPD 6.6.4p1 instance running on an Ubuntu 20.04.6 LTS machine, installed using package manager.
When receiving emails from hosts with a name longer than 64 characters, the name used for the fc-rDNS check is truncated to 64 chars, and the connection is discarded.
It seems that is because HOST_NAME_MAX defaults to 64 on most linux distros.

getconf HOST_NAME_MAX
64

Is there an "easy" way to change this ?

[omar-polo]

Hello, thanks for the report.

Is this a glibc-thing? on alpine limits.h defines HOST_NAME_MAX to 255, but I can see that on debian is 64. (the VERSION section of gethostname(2) on linux seem have some commentary.)

I guess we'll have to use _POSIX_HOST_NAME_MAX which is 255 on all the systems I managed to get my hands on (OpenBSD, FreeBSD, alpine and debian)

[morille]

In limits.h there is an interesting note saying that the minimum acceptable value for HOST_NAME_MAX is _POSIX_HOST_NAME_MAX.
So when both are defined, smtpd should use the bigger one.
Is this something you could do ? (I did not put my hands on C for almost 20 years...)

[s4uliu5]

I'm using OpenSMTPD 7.5.0p0, on Funtoo Linux with glibc-2.33, with rdns and fcrdns filters enabled.

And my system is affected by this issue.

Before applying morille@b9a09d6, I had a lot of fcrdns errors similar to this

Jul  1 08:56:15 smtp01 smtpd[7838]: 33c297056fc0b7f5 smtp connected address=40.107.149.96 host=mail-germanywestcentralazon11022096.outbound.protection.outlook.
Jul  1 08:56:15 smtp01 smtpd[7838]: 33c297056fc0b7f5 smtp failed-command command="" result="550 fcrDNS error"

as host names longer than 64 chars were truncated that caused fcrdns filter to fail, e.g. from mail-germanywestcentralazon11022096.outbound.protection.outlook.com to mail-germanywestcentralazon11022096.outbound.protection.outlook.