ISP port blocking practice

Fri Oct 23 03:38:01 UTC 2009

On Oct 22, 2009, at 3:39 PM, Justin Shore wrote:

> Zhiyun Qian wrote:
>> Hi all,
>> What is the common practice for enforcing port blocking policy (or  
>> what is the common practice for you and your ISP)? More  
>> specifically, when ISPs try to block certain outgoing port (port 25  
>> for instance), they could do two rules:
>> 1). For any outgoing traffic, if the destination port is 25, then  
>> drop the packets.
>> 2). For any incoming traffic, if the source port is 25, then drop  
>> the packets.
>
> I block on both generally.  I block inbound and outbound for  
> residential customers in dynamic pools.  I block inbound only for  
> residential with statically-assigned IPs.  That way a customer can  
> request (and pay for) a static IP and be able to get around out  
> outbound SMTP block.  Few companies use the MSP port (tcp/587).  I'm  
> not sure why more don't make the effort but most don't.  To make up  
> for that we allow static residential customers to evade that filter  
> with a static IP.  We still block inbound though.  We also allow  
> them to use our SMTP servers and SmartHosts if they want with no  
> requirements on source domains (like some providers require,  
> essentially requiring the customer to advertise for you).  The  
> inbound block isn't really all that useful as you elude to.  However  
> I use it more often than not to look for people scanning out ranges  
> for open relays.  I use that data for feed my RTBH trigger router  
> and drop the spammer's traffic on the floor (or the poor,  
> unfortunate owner of the compromised PC that's been 0wned.
>
Blocking ports that the end user has not asked for is bad.

Doing it and refusing to unblock is worse.

Some ISPs have the even worse practice of blocking 587 and a few even
go to the horrible length to block 465.

A few hotel gateways I have encountered are dumb enough to think they  
can block TCP/53
which is always fun.

> We block several other things too.  Netbios traffic gets dropped  
> both ways.  MS-SQL traffic gets dropped both ways (a few users have  
> complained about this but very few stick to their guns when you  
> point out that their traffic is traversing the web completely  
> unencrypted).  I block default and common proxy ports such as 3128,  
> 7212, and 6588 in both directions.  Squid is too easy to  
> misconfigure (done it myself). GhostSurf and WinGate have both been  
> abusable as open proxies in various releases.  I also block 8000,  
> 8080 and 8081 towards the customers. These are some of our most  
> commonly scanned ports (usually all 3 at once plus some or all of  
> the 80xx ones).  I've encountered many compromised residential CPEs  
> that the users either enabled themselves or were enabled by  
> default.  I don't block those 3 ports on outbound flows though; too  
> many false positives.
>
> And finally we also block several different types of ICMPs.  First  
> off we block ICMP fragments.  Then we permit echo, echo-reply,  
> packet-too-big, and time-exceeded.  The rest get explicitly dropped.  
> IPv6 will change this list dramatically.  I haven't had time to  
> research ICMPv6 thoroughly enough to say any more than that.
>
> Basically I just pick out some of the really bad ports and block  
> them. This gives me a wealth of data with which to null-route  
> compromised PCs scanning my networks.
>
Lovely for you, but, not particularly helpful to your customers who  
may actually want to use some of those services.

Owen