From: Duncan Roe <duncan_roe@optusnet.com.au>
To: Netfilter Mailing List <netfilter@vger.kernel.org>
Subject: Re: manipulating the ttl
Date: Thu, 13 Feb 2020 21:00:18 +1100 [thread overview]
Message-ID: <20200213100018.GA6347@dimstar.local.net> (raw)
In-Reply-To: <82e00d85-680c-2932-42e2-3b1087bb1013@street-artists.org>
On Sat, Jan 25, 2020 at 05:17:05PM -0800, Daniel Lakeland wrote:
> On 1/25/20 5:02 PM, Daniel Lakeland wrote:
> > As far as I can tell in nftables there's no way to manipulate the TTL
> > field in packets, along the lines of iptables -A foo -j TTL --ttl-set 2
> > or the like
> >
> > This becomes a problem for handling certain multicast scenarios. Is this
> > on the horizon?
> >
> >
> >
> hmmm in further inspection I see that you can *set* the ttl, something like:
>
> nft add rule inet mytable mychain ip ttl set 2
>
> but I don't see how I could do something like decrement the ttl by 4 or
> basically do anything where you'd calculate the TTL as a function of its
> current value.
>
> In general calculating simple arithmetic in order to manipulate fields isn't
> necessarily obvious in nftables. Any pointers?
>
>
(apologies for late reply)
You are able to make arbitrary changes via a netfilter_queue (nfq) program: send
packets that you wish to manipulate to a QUEUE. Unlike with xtables, in nft this
is not a final verdict: other chains in the same table will see the packet after
manipulation as long as they run at a lower priority than the chain that did the
queuing. (I.e. as long as the nfq program accepts the packet).
Cheers ... Duncan.
prev parent reply other threads:[~2020-02-13 10:00 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-26 1:02 manipulating the ttl Daniel Lakeland
2020-01-26 1:17 ` Daniel Lakeland
2020-02-13 10:00 ` Duncan Roe [this message]
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