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DH GEX Modulus Size Testing

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Adam Russell
2022-02-16 21:01:22 +00:00
committed by Joe Testa
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ssh-audit.1
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@ -1,4 +1,4 @@
.TH SSH-AUDIT 1 "March 2, 2021"
.TH SSH-AUDIT 1 "February 13, 2022"
.SH NAME
\fBssh-audit\fP \- SSH server & client configuration auditor
.SH SYNOPSIS
@ -51,6 +51,43 @@ Starts a server on port 2222 to audit client software configuration. Use -p/--p
.br
Enable debug output.
.TP
.B -g, \-\-gex-test=<n[,n,...] | min:pref:max[,min:pref:max,...] | n-n[:step]>
.br
Runs a Diffie-Hellman Group Exchange modulus size test against a server.
Diffie-Hellman requires the client and server to agree on a generator value and
a modulus value. In the "Group Exchange" implementation of Diffie-Hellman, the
client specifies the size of the modulus in bits by providing the server with
minimum, preferred and maximum values. The server then finds a group that best
matches the client's request, returning the corresponding generator and modulus.
For a full explanation of this process see RFC 4419 and its successors.
This test acts as a client by providing an SSH server with the size of a modulus
and then obtains the size of the modulus returned by the server.
Three types of syntax are supported:
1. <n[,n,...]>
A comma delimited list of modulus sizes.
A test is performed against each value in the list where it acts as the minimum, preferred and maximum modulus size.
2. <min:pref:max[,min:pref:max,...]>
A set of three colon delimited values denoting minimum, preferred and maximum modulus size.
A test is performed against each set.
Multiple sets can specified as a comma separated list.
3. <n-n[:step]>
A range of modulus sizes with an optional step value. Step defaults to 1 if omitted.
If the left value is greater than the right value, then the sequence operates from right to left.
A test is performed against each value in the range where it acts as the minimum, preferred and maximum modulus size.
Duplicates are excluded from the return value.
.TP
.B -j, \-\-json
.br
@ -130,7 +167,7 @@ When the -P/--policy option is used, \fBssh-audit\fP performs a policy audit. T
Policy auditing is helpful for ensuring a group of related servers are properly hardened to an exact specification.
.PP
The set of official built-in policies can be viewed with -L/--list-policies. Multiple servers can be audited with -T/--targets=<servers.txt>. Custom policies can be made from an ideal target server with -M/--make-policy=<custom_policy.txt>.
The set of official built-in policies can be viewed with -L/--list-policies. Multiple servers can be audited with -T/--targets=<servers.txt>. Custom policies can be made from an preferred target server with -M/--make-policy=<custom_policy.txt>.
.SH EXAMPLES
@ -219,6 +256,30 @@ ssh-audit -M new_policy.txt targetserver
.fi
.RE
.LP
To run a Diffie-Hellman Group Exchange modulus size test using the values 2000 bits, 3000 bits, 4000 bits and 5000 bits:
.RS
.nf
ssh-audit targetserver --gex-test=2000,3000,4000,5000
.fi
.RE
.LP
To run a Diffie-Hellman Group Exchange modulus size test where 2048 bits is the minimum, 3072 bits is the preferred and 5000 bits is the maximum:
.RS
.nf
ssh-audit targetserver --gex-test=2048:3072:5000
.fi
.RE
.LP
To run a Diffie-Hellman Group Exchange modulus size test from 0 bits to 5120 bits in increments of 1024 bits:
.RS
.nf
ssh-audit targetserver --gex-test=0-5120:1024
.fi
.RE
.SH RETURN VALUES
When a successful connection is made and all algorithms are rated as "good", \fBssh-audit\fP returns 0. Other possible return values are: