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Comment by codeka | Hacker News Reader

> In that case, with UDP, spoofed headers are allowed. Connectionless.

rachelbythebay is asking which ISPs allow spoofed UDP packets.

The way this attack works is you send a query to an open resolver, using the target's IP address as the "source" IP address in the UDP header instead of your own.

However, ISPs can (and should) block UDP packets where the source IP address is outside the IP-blocks they own. Why don't ISPs do this?

I'm not really sure what the rest of your post has to do with any of this.

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So you are saying it is common practice to block outgoing UDP packets based on source IP? I did not know this. Does your ISP do that? Everyone is expected to block ingress with spoofed IP. But I can't find an BCP for blocking UDP egress based on source IP. Does it exist?

As for the rest the comment, this appears to be an "informational advertising" style marketing piece for Cloudflare so I think it's relevant.

You can't tell that incoming UDP is spoofed. The only filtering you can reliably do is outgoing.

Right. As the top post also points out. There's no way to distinguish incoming UDP traffic as "spoofed".

My question is does anyone filter UDP egress based on source IP? Is there guidance somewhere that tells admins to do this?

Let me put it another way: If it was a workable solution to get admins to do this - to filter outgoing UDP based on source IP, then why are people trying to get network admins to change their DNS server settings as a way to reduce the possibility of DNS-based DDOS? That seems like a far more difficult task given that there hundreds of thousands of open resolvers and most admins understand working with firewall rulesets better than DNS configuration.

Of course they do.

Any responsible host filters all outgoing packets to limit them to <only IP addresses we own>.

For example, linode does this afaik.

Thanks.

http://tools.ietf.org/html/rfc4732

Sure you can at the carrier level, it's called unicast reverse path forwarding. Any incoming packets from a network with a source IP not being advertised via BGP by that network would be spoofed.

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