src/usr.bin/ssh/sshd.8 - diff

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Diff for /src/usr.bin/ssh/sshd.8 between version 1.284 and 1.285

version 1.284, 2016/02/17 07:38:19

version 1.285, 2016/08/15 12:32:04

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.Nm sshd

.Bk -words

.Op Fl 46DdeiqTt

.Op Fl b Ar bits

.Op Fl C Ar connection_spec

.Op Fl c Ar host_certificate_file

.Op Fl E Ar log_file

.Op Fl f Ar config_file

.Op Fl g Ar login_grace_time

.Op Fl h Ar host_key_file

.Op Fl k Ar key_gen_time

.Op Fl o Ar option

.Op Fl p Ar port

.Op Fl u Ar len

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Forces

.Nm

to use IPv6 addresses only.

.It Fl b Ar bits

Specifies the number of bits in the ephemeral protocol version 1

server key (default 1024).

.It Fl C Ar connection_spec

Specify the connection parameters to use for the

.Fl T

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is not run as root (as the normal

host key files are normally not readable by anyone but root).

The default is

.Pa /etc/ssh/ssh_host_key

for protocol version 1, and

.Pa /etc/ssh/ssh_host_dsa_key ,

.Pa /etc/ssh/ssh_host_ecdsa_key .

.Pa /etc/ssh/ssh_host_ecdsa_key ,

.Pa /etc/ssh/ssh_host_ed25519_key

and

.Pa /etc/ssh/ssh_host_rsa_key

.Pa /etc/ssh/ssh_host_rsa_key .

for protocol version 2.

It is possible to have multiple host key files for

the different protocol versions and host key algorithms.

the different host key algorithms.

.It Fl i

Specifies that

.Nm

is being run from

.Xr inetd 8 .

If SSH protocol 1 is enabled,

.Nm

should not normally be run

from inetd because it needs to generate the server key before it can

respond to the client, and this may take some time.

Clients may have to wait too long if the key was regenerated every time.

.It Fl k Ar key_gen_time

Specifies how often the ephemeral protocol version 1 server key is

regenerated (default 3600 seconds, or one hour).

The motivation for regenerating the key fairly

often is that the key is not stored anywhere, and after about an hour

it becomes impossible to recover the key for decrypting intercepted

communications even if the machine is cracked into or physically

seized.

A value of zero indicates that the key will never be regenerated.

.It Fl o Ar option

Can be used to give options in the format used in the configuration file.

This is useful for specifying options for which there is no separate

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from making DNS requests unless the authentication

mechanism or configuration requires it.

Authentication mechanisms that may require DNS include

.Cm RhostsRSAAuthentication ,

.Cm HostbasedAuthentication

.Cm HostbasedAuthentication ,

and using a

.Cm from="pattern-list"

option in a key file.

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.Cm DenyUsers .

.El

.Sh AUTHENTICATION

The OpenSSH SSH daemon supports SSH protocols 1 and 2.

The OpenSSH SSH daemon supports SSH protocol 2 only.

The default is to use protocol 2 only,

though this can be changed via the

.Cm Protocol

option in

.Xr sshd_config 5 .

Protocol 1 should not be used

and is only offered to support legacy devices.

.Pp

Each host has a host-specific key,

used to identify the host.

Partial forward security for protocol 1 is provided through

an additional server key,

normally 1024 bits,

generated when the server starts.

This key is normally regenerated every hour if it has been used, and

is never stored on disk.

Whenever a client connects, the daemon responds with its public

host and server keys.

host key.

The client compares the

RSA host key against its own database to verify that it has not changed.

host key against its own database to verify that it has not changed.

The client then generates a 256-bit random number.

Forward security is provided through a Diffie-Hellman key agreement.

It encrypts this

random number using both the host key and the server key, and sends

the encrypted number to the server.

Both sides then use this

random number as a session key which is used to encrypt all further

communications in the session.

The rest of the session is encrypted

using a conventional cipher, currently Blowfish or 3DES, with 3DES

being used by default.

The client selects the encryption algorithm

to use from those offered by the server.

.Pp

For protocol 2,

forward security is provided through a Diffie-Hellman key agreement.

This key agreement results in a shared session key.

The rest of the session is encrypted using a symmetric cipher, currently

128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.

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.Ql #

are ignored as

comments).

Protocol 1 public keys consist of the following space-separated fields:

Public keys consist of the following space-separated fields:

options, bits, exponent, modulus, comment.

Protocol 2 public key consist of:

options, keytype, base64-encoded key, comment.

The options field is optional;

The options field is optional.

its presence is determined by whether the line starts

The keytype is

with a number or not (the options field never starts with a number).

The bits, exponent, modulus, and comment fields give the RSA key for

protocol version 1; the

comment field is not used for anything (but may be convenient for the

user to identify the key).

For protocol version 2 the keytype is

.Dq ecdsa-sha2-nistp256 ,

.Dq ecdsa-sha2-nistp384 ,

.Dq ecdsa-sha2-nistp521 ,

.Dq ssh-ed25519 ,

.Dq ssh-dss

.Dq ssh-rsa .

.Dq ssh-rsa ;

the comment field is not used for anything (but may be convenient for the

user to identify the key).

.Pp

Note that lines in this file are usually several hundred bytes long

Note that lines in this file can be several hundred bytes long

(because of the size of the public key encoding) up to a limit of

8 kilobytes, which permits DSA keys up to 8 kilobits and RSA

keys up to 16 kilobits.

You don't want to type them in; instead, copy the

.Pa identity.pub ,

.Pa id_dsa.pub ,

.Pa id_ecdsa.pub ,

.Pa id_ed25519.pub ,

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file and edit it.

.Pp

.Nm

enforces a minimum RSA key modulus size for protocol 1

enforces a minimum RSA key modulus size of 768 bits.

and protocol 2 keys of 768 bits.

.Pp

The options (if present) consist of comma-separated option

specifications.

Line 663

Line 603

its key is added to the per-user file.

.Pp

Each line in these files contains the following fields: markers (optional),

hostnames, bits, exponent, modulus, comment.

hostnames, keytype, base64-encoded key, comment.

The fields are separated by spaces.

.Pp

The marker is optional, but if it is present then it must be one of

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Only one hashed hostname may appear on a single line and none of the above

negation or wildcard operators may be applied.

.Pp

Bits, exponent, and modulus are taken directly from the RSA host key; they

The keytype and base64-encoded key are taken directly from the host key; they

can be obtained, for example, from

.Pa /etc/ssh/ssh_host_key.pub .

.Pa /etc/ssh/ssh_host_rsa_key.pub .

The optional comment field continues to the end of the line, and is not used.

.Pp

Lines starting with

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long, and you definitely don't want to type in the host keys by hand.

Rather, generate them by a script,

.Xr ssh-keyscan 1

or by taking

or by taking, for example,

.Pa /etc/ssh/ssh_host_key.pub

.Pa /etc/ssh/ssh_host_rsa_key.pub

and adding the host names at the front.

.Xr ssh-keygen 1

also offers some basic automated editing for

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but allows host-based authentication without permitting login with

rlogin/rsh.

.Pp

.It Pa /etc/ssh/ssh_host_key

.It Pa /etc/ssh/ssh_host_dsa_key

.It Pa /etc/ssh/ssh_host_ecdsa_key

.It Pa /etc/ssh/ssh_host_ed25519_key

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.Nm

does not start if these files are group/world-accessible.

.Pp

.It Pa /etc/ssh/ssh_host_key.pub

.It Pa /etc/ssh/ssh_host_dsa_key.pub

.It Pa /etc/ssh/ssh_host_ecdsa_key.pub

.It Pa /etc/ssh/ssh_host_ed25519_key.pub