[Kea-users] How is tsid generated

[Munroe Sollog]

I was refering to 'xid'.  So it sounds like it is expected for the same
client to use the same xid across different negotiations, which would
explain what I am seeing.

On Tue, Dec 12, 2017 at 8:23 AM, Tomek Mrugalski <tomasz at isc.org> wrote:

> W dniu 11.12.2017 o 22:38, Munroe Sollog pisze:
> > Can someone help me understand how the tsid field is generated?  What is
> > used to generate that hash?  I’m tracking DHCP performance based on the
> > tsid and I’m seeing a very small percentage of long transaction time
> > that may be explained by colliding tsids.
> Are you asking about transaction-id, a 32 (in DHCPv4) or 24 (in DHCPv6)
> bit field in the DHCP message or tsig, a signature used to protect DNS
> updates? You mentioned a hash, which suggests the latter. Anyway, here
> are brief answers to both.
>
> xid, or transaction-id, is not a hash. It is supposed to be set by a
> client to a random value, but some clients set it a special value. Kea
> doesn't pay much attention to it, except it being echoed back in its
> responses. This value is used by clients to match responses to their
> outstanding transmissions. For details, see RFC2131, Section 2, page 10.
>
> tsig, or transaction signature is used to sign DNS updates. Kea supports
> a number of algorithms (hmac-md5, hmac-sha1 and others, see Section
> 11.3.2 for details:
> http://kea.isc.org/docs/kea-guide.html#d2-tsig-key-list-config). This
> mechanism is defined in RFC2845. I haven't looked at the details, but I
> presume it protects the whole content of the DNS message, so everything
> in the DNS update message, a timestamp and a secret key are used to
> generate that digest.
>
> Hope that helps.
> Tomek
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-- 
Munroe Sollog
Senior Network Engineer
munroe at lehigh.edu
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