patch to select pkalg

[Richard E. Silverman]

Suppose an SSH server has both RSA and DSA host keys for protocol 2, but I
only have the DSA key, and I want to use that.  I'm stuck; the OpenSSH client
is hard-wired to offer both algorithms in the key exchange, and will select
ssh-rsa if it's available (see myproposal.h, KEX_DEFAULT_PK_ALG).

Below is a patch adding the client configuration option "PKAlgorithms" for
this purpose.  It doesn't validate the supplied list; I'm not sure if that's
really necessary or desirable.

This situation raises a couple of questions.  The first is about the protocol,
which forces the client to commit to a choice of host key algorithm before it
sees the keys, and hence before it can determine whether that choice has any
chance of succeeding -- and if it fails, the whole connection fails.  I wonder
if there's a better way of doing this?

The second is about implementation -- given the protocol as it is now, would
it be a good idea to automatically retry connections that fail because of an
unverifiable host key, using the next available host key algorithm?

--
  Richard Silverman
  slade at shore.net

================================================================================

*** ../../openssh-2.5.1p2/readconf.c	Wed Feb 14 22:02:00 2001
--- readconf.c	Wed Mar  7 00:59:44 2001
***************
*** 109,115 ****
  	oCompressionLevel, oKeepAlives, oNumberOfPasswordPrompts,
  	oUsePrivilegedPort, oLogLevel, oCiphers, oProtocol, oMacs,
  	oGlobalKnownHostsFile2, oUserKnownHostsFile2, oPubkeyAuthentication,
! 	oKbdInteractiveAuthentication, oKbdInteractiveDevices, oHostKeyAlias
  } OpCodes;
  
  /* Textual representations of the tokens. */
--- 109,116 ----
  	oCompressionLevel, oKeepAlives, oNumberOfPasswordPrompts,
  	oUsePrivilegedPort, oLogLevel, oCiphers, oProtocol, oMacs,
  	oGlobalKnownHostsFile2, oUserKnownHostsFile2, oPubkeyAuthentication,
! 	oKbdInteractiveAuthentication, oKbdInteractiveDevices, oHostKeyAlias,
! 	oPKAlgorithms
  } OpCodes;
  
  /* Textual representations of the tokens. */
***************
*** 171,176 ****
--- 172,178 ----
  	{ "keepalive", oKeepAlives },
  	{ "numberofpasswordprompts", oNumberOfPasswordPrompts },
  	{ "loglevel", oLogLevel },
+ 	{ "pkalgorithms", oPKAlgorithms },
  	{ NULL, 0 }
  };
  
***************
*** 516,521 ****
--- 518,532 ----
  			options->macs = xstrdup(arg);
  		break;
  
+ 	case oPKAlgorithms:
+ 		arg = strdelim(&s);
+ 		if (!arg || *arg == '\0')
+ 			fatal("%.200s line %d: Missing argument.", filename, linenum);
+ 		/* XXX validate pkalg list? */
+ 		if (*activep && options->pkalgorithms == NULL)
+ 			options->pkalgorithms = xstrdup(arg);
+ 		break;
+ 
  	case oProtocol:
  		intptr = &options->protocol;
  		arg = strdelim(&s);
***************
*** 708,713 ****
--- 719,725 ----
  	options->cipher = -1;
  	options->ciphers = NULL;
  	options->macs = NULL;
+ 	options->pkalgorithms = NULL;
  	options->protocol = SSH_PROTO_UNKNOWN;
  	options->num_identity_files = 0;
  	options->hostname = NULL;
***************
*** 797,802 ****
--- 809,815 ----
  		options->cipher = SSH_CIPHER_NOT_SET;
  	/* options->ciphers, default set in myproposals.h */
  	/* options->macs, default set in myproposals.h */
+ 	/* options->pkalgorithms, default set in myproposals.h */
  	if (options->protocol == SSH_PROTO_UNKNOWN)
  		options->protocol = SSH_PROTO_1|SSH_PROTO_2|SSH_PROTO_1_PREFERRED;
  	if (options->num_identity_files == 0) {

*** ../../openssh-2.5.1p2/readconf.h	Wed Feb 14 22:02:00 2001
--- readconf.h	Tue Mar  6 23:58:08 2001
***************
*** 69,74 ****
--- 69,75 ----
  	int     cipher;		/* Cipher to use. */
  	char   *ciphers;	/* SSH2 ciphers in order of preference. */
  	char   *macs;		/* SSH2 macs in order of preference. */
+ 	char   *pkalgorithms;	/* SSH2 server key types in order of preference. */
  	int	protocol;	/* Protocol in order of preference. */
  	char   *hostname;	/* Real host to connect. */
  	char   *host_key_alias;	/* hostname alias for .ssh/known_hosts */

*** ../../openssh-2.5.1p2/sshconnect2.c	Thu Feb 15 20:34:57 2001
--- sshconnect2.c	Wed Mar  7 00:06:05 2001
***************
*** 94,99 ****
--- 94,102 ----
  		myproposal[PROPOSAL_MAC_ALGS_CTOS] =
  		myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
  	}
+ 	if (options.pkalgorithms != NULL) {
+ 	        myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = options.pkalgorithms;
+ 	}
  
  	/* buffers with raw kexinit messages */
  	server_kexinit = xmalloc(sizeof(*server_kexinit));

================================================================================