Hello Community, I have a requirement to allow enriched https header enrichment. The SSL negotiation (I'm doing ssl termination on F5) fails because the enriched header from client contains reserved tls extension values. (https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtmltls-extensiontype-values-1). The Client Hello request in the SSL Handshake was captured and contained an Extensions list, which included a reserved TLS Extension value (17156), which the F5 isn't presenting in Server Hello. I need an irule that can allow that Extension to be added on the client ssl profile so the ssl handshake doesn't fail.

You can try this to catch and remove this extension from CLIENT_HELLO packet (not tested) it will then insert it in server side TLS handshake when CLIENT_ACCEPTED { set tls_extension_17516 "" SSL::disable TCP::collect } when CLIENT_DATA { # Store TCP Payload up to 2^14 + 5 bytes (Handshake length is up to 2^14) set payload [TCP::payload 16389] set payloadlen [TCP::payload length] # If valid TLS 1.X CLIENT_HELLO handshake packet if { [binary scan $payload cH4ScH6H4x32c tls_record_content_type tls_version tls_recordlen tls_handshake_action tls_handshakelen_hex tls_handshake_version tls_handshake_sessidlen] == 7 && \ ($tls_record_content_type == 22) && \ ([string match {030[1-3]} $tls_version]) && \ ($tls_handshake_action == 1) && \ ($payloadlen == ($tls_recordlen & 0xffff )+5)} { # store in a variable the handshake length #set tls_handshakelen [expr 0x$tls_handshakelen_hex] scan $tls_handshakelen_hex %x tls_handshakelen # store in a variable the handshake version set tls_handshake_prefered_version $tls_handshake_version # skip past the session id set record_offset [expr {44 + ($tls_handshake_sessidlen & 0xff)}] # skip past the cipher list binary scan $payload @${record_offset}S tls_ciphlen set record_offset [expr {$record_offset + 2 + ($tls_ciphlen & 0xffff)}] # skip past the compression list binary scan $payload @${record_offset}c tls_complen set record_offset [expr {$record_offset + 1 + ($tls_complen & 0xffff)}] # check for the existence of ssl extensions if { ($payloadlen > $record_offset) } { # skip to the start of the first extension set tls_extension_length_start $record_offset binary scan $payload @${record_offset}S tls_extension_length set record_offset [expr {$record_offset + 2}] # Check if extension length + offset equals payload length if {$record_offset + ($tls_extension_length & 0xffff) == $payloadlen} { # for each extension while { $record_offset < $payloadlen } { binary scan $payload @${record_offset}SS tls_extension_type tls_extension_record_length set tls_extension_record_length [expr {$tls_extension_record_length & 0xffff}] if { $tls_extension_type == 17516 } { # if it's extension type 17516 binary scan $payload @${record_offset}A[expr {$tls_extension_record_length +4}] tls_extension_17516 set ext_start $record_offset set ext_len [expr {$tls_extension_record_length + 4}] set record_offset [expr {$record_offset + $tls_extension_record_length + 4}] } else { # skip over other extensions set record_offset [expr {$record_offset + $tls_extension_record_length + 4}] } } } } } unset -nocomplain payload payloadlen tls_record_content_type tls_handshake_action tls_handshake_sessidlen record_offset tls_ciphlen tls_complen tls_extension_type tls_extension_record_length tls_supported_versions_length tls_supported_versions if {$tls_extension_17516 ne ""} { # remove extension from Payload TCP::payload replace $ext_start $ext_len "" # Change extension Length TCP::payload replace $tls_extension_length_start 2 [binary format S [expr {($tls_extension_length & 0xffff) - $ext_len}]] # Change Handshake Length TCP::payload replace 6 3 [binary format H6 [format %06X [expr {$tls_handshakelen - $ext_len}]]] # Change Message Length TCP::payload replace 3 2 [binary format S [expr {($tls_recordlen & 0xffff) - $ext_len}]] } SSL::enable TCP::release } when SERVERSSL_CLIENTHELLO_SEND { if { $tls_extension_17516 ne "" } { SSL::extensions insert $tls_extension_17516 } }

The goal of this code is:disable SSL profile on client side to disable TLS inspection before the code endsbinary search the expected extensionsave in variable tls_extension_17516 the content of extension type 17516save in variable ext_start the index of beginning of extension 17516save in variable ext_len the extension 17516 lengthreplace in payload the extension with no value (from ext_start with length ext_len) missing in the code :change extension length to new valuechange handshake length to new value I will update the code with missing commands later.

I added this irule, it created an extension actually, but there was no data under it. when CLIENTSSL_CLIENTHELLO { set my_ext "Hello world!" set my_ext_type 17516 SSL::extensions insert [binary format S1S1a* $my_ext_type [string length $my_ext] $my_ext] }

Is this extension inserted by a public tool or are you writing your SSL client? your code try to insert the extension in CLIENTHELLO packet Do you want to insert it in SERVERHELLO packet?

An Irule for Client Ssl Profile that Allows Unassigned TLS Extension Values (17516)

28 Replies

kazeem_yusuf1
Nimbostratus
Aug 01, 2018
kazeem_yusuf1
Nimbostratus
Aug 06, 2018
I added this irule, it created an extension actually, but there was no data under it.

when CLIENTSSL_CLIENTHELLO { set my_ext "Hello world!" set my_ext_type 17516 SSL::extensions insert [binary format S1S1a* $my_ext_type [string length $my_ext] $my_ext] }
stan_piron
Cumulonimbus
Aug 07, 2018
Is this extension inserted by a public tool or are you writing your SSL client?

your code try to insert the extension in CLIENTHELLO packet

Do you want to insert it in SERVERHELLO packet?

Kevin_Stewart

Employee

Aug 21, 2018

If you're trying to reflect the client's MSISDN extension to the server, you definitely want to use the SERVERSSL_CLIENTHELLO_SEND event. It does not require forward proxy to use this event.

when SERVERSSL_CLIENTHELLO_SEND {
    set msisdn "foobar"
    set bin [binary format S1S1S1S1ca* 17516 [expr [string length ${msisdn}] + 5] [expr [string length ${msisdn}] + 3] 0 [string length ${msisdn}] ${msisdn}]   
    SSL::extensions insert $bin
}

Baba_TABOURE
Nimbostratus
Nov 05, 2019
Hi Kazeemyu1.5586213523653357E12,

Did you have a solution on this request, Im confronted to same issue.
Exactly the same issue while im trying to perform https Client Hello Enrichment.

Regards,
Baba TABOURE
- Stan_PIRON_F5
  Employee
  Nov 05, 2019
  when CLIENTSSL_HANDSHAKE { if { [SSL::extensions exists -type 17516] } then { set tls_extension [SSL::extensions -type 17516] } else { set tls_extension "" } } when SERVERSSL_CLIENTHELLO_SEND { if { $tls_sni_extension ne "" } then { SSL::extensions insert $tls_extension } }
  this code is a copy of this code with your extension type
  
  https://devcentral.f5.com/s/articles/client-side-to-server-side-sni-relay-irule-967
  - Baba_TABOURE
    Nimbostratus
    Nov 05, 2019
    Thanks Stanislas, is this the reason why we have that kind of error sent during TLS Handshake?:
    Alert (Level: Fatal, Description: Bad Record Mac).
    Our F5 is the first network element we have before getting to the server.
Baba_TABOURE
Nimbostratus
Nov 05, 2019
Hum ok....DO you have a summary of what you have propose and the purpose please in order to understand
- Stan_PIRON_F5
  Employee
  Nov 05, 2019
  The goal of this code is:
  disable SSL profile on client side to disable TLS inspection before the code ends
  binary search the expected extension
  save in variable tls_extension_17516 the content of extension type 17516
  save in variable ext_start the index of beginning of extension 17516
  save in variable ext_len the extension 17516 length
  replace in payload the extension with no value (from ext_start with length ext_len)
  
  missing in the code :
  change extension length to new value
  change handshake length to new value
  
  I will update the code with missing commands later.
  - Stan_PIRON_F5
    Employee
    Nov 05, 2019
    I just updated the code above.
    
    Can you try it and update this thread?
Kazeem_Yusuf
Altostratus
Aug 07, 2026
As brilliant as this is.Is it possible that post-quantum cryptography may be a stumbling block to this solution, by causing tcp fragmentation. With Clienthello packets of 540bytes, this worked perfectly. An example is the "Extension Unknown Type 17516" added there.

Recent tests show the extension addition is no longer working. The only significant change is that CLIENTSSL HELLO packets are now fragmented on TLS 1.3 with the Post-Quantum Cryptography implementation on clients (browsers), with CLIENTSSL HELLO now regularly abover 1500 bytes causing fragmentation.
is it possible if F5 is Client-side server to force all clients to use X25519 ciphers instead of X25519Kyber512Draft00 and X25519Kyber768 X25519MLKEM768 to ensure that full flow is not broken. is there any changes on CLIENTSSL Profile to implement this.