dnsdist lua nxdomain spoof echoes flags and EDNS options

[Habbie]

• Program: dnsdist
• Issue type: Bug report

Short description

When spoofing NXDOMAINs from a lua rule in dnsdist, the flags and EDNS options from the query blindly get echoed in the response completely, which is bad.

Environment

• Operating system:
• Software version: git master at 385aa46
• Software source: git

Steps to reproduce

1. `dnsdist.conf:

newServer('8.8.8.8')
function luarule(dq) if (dq.qtype==dnsdist.NAPTR) then return DNSAction.Nxdomain, "" else return DNSAction.Allow, "" end end
addLuaAction(AllRule(), luarule)

2. [sudo] ./dnsdist -C dnsdist.conf
3. dig nu.nl @127.0.0.1 -t naptr +subnet=65.66.67.68/20 +ednsopt=512:6563686f21206563686f21

Expected behaviour

; <<>> DiG 9.11.2 <<>> nu.nl @127.0.0.1 -t naptr +subnet=65.66.67.68/20 +ednsopt=512:6563686f21206563686f21
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 14245
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;nu.nl.				IN	NAPTR

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Mar 14 21:21:04 CET 2018
;; MSG SIZE  rcvd: 72

Actual behaviour

; <<>> DiG 9.11.2 <<>> nu.nl @127.0.0.1 -t naptr +subnet=65.66.67.68/20 +ednsopt=512:6563686f21206563686f21
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 14245
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; CLIENT-SUBNET: 65.66.64.0/20/0
; COOKIE: e2faf6c3333ad720 (echoed)
; OPT=512: 65 63 68 6f 21 20 65 63 68 6f 21 ("echo! echo!")
;; QUESTION SECTION:
;nu.nl.				IN	NAPTR

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Mar 14 21:21:04 CET 2018
;; MSG SIZE  rcvd: 72

Note the CLIENT-SUBNET, COOKIE and OPT lines. Also note that the flags: have been copied verbatim. Sending +noad' will remove ad from the answer. Same for +norec and rd, etc.

[Habbie]

@rgacogne points out that a non-lua RcodeAction should also trigger this issue.

[RobinGeuze]

I've been trying to figure out how to best fix this. I could either remove the entire OPT record, just unset all flags and options, or just unset specific options and flags. The first two options might have unwanted side effects, and the last option might become alot of work to maintain. It might be better to allow you to specify what flags and options to set somehow, however for example the DO-bit needs to be influenced by the query.