[NCLUG] comcast blocking port 25?

[Stephen Warren]

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DJ Eshelman wrote:
> Is it possible for mail from a dynamic IP to have a reverse DNS entry?

Yes, quite probable in fact. For example, mine:

133.80.9.24.in-addr.arpa. 86400 \
	IN      PTR   c-24-9-80-133.hsd1.co.comcast.net

Static v.s. dynamic IP has nothing to do with whether a reverse DNS
entry can be configured. The main difference between dynamic and static
IPs is that typically *end-users* of static IPs can control their own
reverse DNS entry, and users of dynamic IPs can't.

That is somewhat of an over generalization; it's all down to the ISP.
I'm sure there are ISPs who won't let their users edit their reverse DNS
even when they have a static IP. On the flip-side, Comcast technically
operates dynamic IPs (since they're given out via DHCP), but for all
practical purposes, they are static (since Comcast's DHCP servers always
give the same value to a given customer; I suspect this is a deliberate
configuration on their part), and as such, they could, at least in
theory, allow editing reverse DNS information (subject to some timing
issues if they ever re-number their network, w.r.t. re-using IPs and DNS
caching).

> How did you go about detecting if it was from a Dynamic IP or not?

There are RBLs whose specific purpose is to give that information. I
don't know which ones they are though.

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