weird hang and tcp retransmissions in dropbear ssh client 2014.63

Gui Iribarren gui at
Mon Sep 8 21:30:04 WST 2014

Hello devs,
i stumbled upon a rare but very exasperating bug in openwrt's dropbear,

steps to reproduce:
connecting from a debian7 with openssh-client_1:6.6p1-4~bpo70
to an openwrt bb-rc3 running dropbear 2014.63 as server
with agent forwarding enabled on client,
then try to "daisy-chain" into another host


so, ssh into openwrtA, and from there do "ssh openwrtB"

expected behaviour (this works with dropbear_2012.55):
successfully login into openwrtB without being asked for a password,
since openwrtA's dropbear forwards the credentials from debian7 to openwrtB

actual behaviour: (with 2014.63, and also 2013.59 IIRC)
openwrtA dropbear client hangs for a minute or more, finally dying with
a timeout error.


i'm so used to using pubkeys, that it never ocurred to me that it'd make
a difference to disable agent forwarding. Today i discovered that if I
disable agent forwarding on my openssh-client, then connect to openwrtA,
and from there ssh into openwrtB, it will ask for a password and let me
login just fine. Hope that serves as a pointer in the right direction :)

attached you'll find two pcaps, taken with tcpdump on openwrtA.

in the first one, to simplify the setup, i simply did "ssh localhost"
(so openwrtA = openwrtB) which works as expected in 2012.55, and
produces this bizarre pcap in 2014.63.

as a contrast, i also send the result of doing "ssh debian7"
( from openwrtA. openssh-server reacts just fine, and i
can login back to my computer from dropbear-client running on openwrtA

in both cases, agent forwarding was enabled on my debian7 openssh-client

hope i didn't confuse the hell out of you,
sorry to ruin your delightful weekend with such an intricate report,
and thanks in advance for any help :)


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