Per RFC4253 4.2 clients must be able to process other lines of data
before the version string, server behavior is not defined neither
with MUST/SHOULD nor with MAY.
If server process up to 50 lines too - it may cause too long hanging
session with invalid/evil client that consume host resources and
potentially may lead to DDoS on poor embedded boxes.
Let's require first line from client to be version string and fail
early if it's not - matches both RFC and real OpenSSH behavior.
This is Dropbear, a smallish SSH server and client.
https://matt.ucc.asn.au/dropbear/dropbear.html
INSTALL has compilation instructions.
MULTI has instructions on making a multi-purpose binary (ie a single binary
which performs multiple tasks, to save disk space)
SMALL has some tips on creating small binaries.
Please contact me if you have any questions/bugs found/features/ideas/comments etc :)
There is also a mailing list http://lists.ucc.gu.uwa.edu.au/mailman/listinfo/dropbear
Matt Johnston
matt@ucc.asn.au
In the absence of detailed documentation, some notes follow:
============================================================================
Server public key auth:
You can use ~/.ssh/authorized_keys in the same way as with OpenSSH, just put
the key entries in that file. They should be of the form:
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAwVa6M6cGVmUcLl2cFzkxEoJd06Ub4bVDsYrWvXhvUV+ZAM9uGuewZBDoAqNKJxoIn0Hyd0Nk/yU99UVv6NWV/5YSHtnf35LKds56j7cuzoQpFIdjNwdxAN0PCET/MG8qyskG/2IE2DPNIaJ3Wy+Ws4IZEgdJgPlTYUBWWtCWOGc= someone@hostname
You must make sure that ~/.ssh, and the key file, are only writable by the
user. Beware of editors that split the key into multiple lines.
Dropbear supports some options for authorized_keys entries, see the manpage.
============================================================================
Client public key auth:
Dropbear can do public key auth as a client, but you will have to convert
OpenSSH style keys to Dropbear format, or use dropbearkey to create them.
If you have an OpenSSH-style private key ~/.ssh/id_rsa, you need to do:
dropbearconvert openssh dropbear ~/.ssh/id_rsa ~/.ssh/id_rsa.db
dbclient -i ~/.ssh/id_rsa.db <hostname>
Dropbear does not support encrypted hostkeys though can connect to ssh-agent.
============================================================================
If you want to get the public-key portion of a Dropbear private key, look at
dropbearkey's '-y' option.
============================================================================
To run the server, you need to generate server keys, this is one-off:
./dropbearkey -t rsa -f dropbear_rsa_host_key
./dropbearkey -t dss -f dropbear_dss_host_key
./dropbearkey -t ecdsa -f dropbear_ecdsa_host_key
./dropbearkey -t ed25519 -f dropbear_ed25519_host_key
or alternatively convert OpenSSH keys to Dropbear:
./dropbearconvert openssh dropbear /etc/ssh/ssh_host_dsa_key dropbear_dss_host_key
You can also get Dropbear to create keys when the first connection is made -
this is preferable to generating keys when the system boots. Make sure
/etc/dropbear/ exists and then pass '-R' to the dropbear server.
============================================================================
If the server is run as non-root, you most likely won't be able to allocate a
pty, and you cannot login as any user other than that running the daemon
(obviously). Shadow passwords will also be unusable as non-root.
============================================================================
The Dropbear distribution includes a standalone version of OpenSSH's scp
program. You can compile it with "make scp", you may want to change the path
of the ssh binary, specified by _PATH_SSH_PROGRAM in options.h . By default
the progress meter isn't compiled in to save space, you can enable it by
adding 'SCPPROGRESS=1' to the make commandline.
2301b6ac0b