ClientHello with small first TCP fragment

Some servers will reject a ClientHello if when sent the first TCP fragment is less than 9 bytes. This commit adds a test for that problem by sending ClientHello messages such such short TCP fragments. It tries initial fragments lengths of 5, 6, 7, and 8 bytes, and tries each of these lengths twice, and reports a problem only if the connection fails only every attempt. The reason for the repeated attempts is that some servers will occasionally fail these tests, and the goal is to only report a problem if testssl#1113 would have reported a problem.

Author: David Cooper

testssl.sh

@@ -9934,12 +9934,30 @@ code2network() {
 # sockets inspired by http://blog.chris007.de/?p=238
 # ARG1: hexbytes separated by commas, with a leading comma
 # ARG2: seconds to sleep
+# ARG3: the size of the first message fragment to send
 socksend_clienthello() {
      local data=""
-
+     local -i i first_fragment_size=0
+     local first_fragment
+     
+     [[ -n "$3" ]] && first_fragment_size="$3"
      code2network "$1"
      data="$NW_STR"
      [[ "$DEBUG" -ge 4 ]] && echo && echo "\"$data\""
+     if [[ $first_fragment_size -ne 0 ]]; then
+          for (( i=1; i < ${#data}; i++ )); do
+               [[ "${data:i:1}" == '\' ]] && first_fragment_size=$first_fragment_size-1
+               [[ $first_fragment_size -eq 0 ]] && break
+          done
+          first_fragment="${data:0:i}"
+          data="${data:i}"
+          if [[ -z "$PRINTF" ]] ;then
+               # We could also use "dd ibs=1M obs=1M" here but is seems to be at max 3% slower
+               printf -- "$first_fragment" | cat >&5 2>/dev/null &
+          else
+               $PRINTF -- "$first_fragment" 2>/dev/null >&5 2>/dev/null &
+          fi
+     fi
      if [[ -z "$PRINTF" ]] ;then
           # We could also use "dd ibs=1M obs=1M" here but is seems to be at max 3% slower
           printf -- "$data" | cat >&5 2>/dev/null &
@@ -12426,7 +12444,7 @@ generate_key_share_extension() {
 # ARG4: (optional) additional request extensions
 # ARG5: (optional): "true" if ClientHello should advertise compression methods other than "NULL"
 # ARG6: (optional): "false" if prepare_tls_clienthello() should not open a new socket
-#
+# ARG7: (optional): the size of the first message fragment to send.
 prepare_tls_clienthello() {
      local tls_low_byte="$1" tls_legacy_version="$1"
      local process_full="$3"
@@ -12447,6 +12465,7 @@ prepare_tls_clienthello() {
      local extensions_key_share="" extn_type supported_groups_c2n="" extn_psk_mode=""
      local extra_extensions extra_extensions_list="" extension_supported_versions=""
      local offer_compression=false compression_methods
+     local first_fragment_size="$7"
 
      # TLSv1.3 ClientHello messages MUST specify only the NULL compression method.
      [[ "$5" == true ]] && [[ "0x$tls_low_byte" -le "0x03" ]] && offer_compression=true
@@ -12825,7 +12844,7 @@ prepare_tls_clienthello() {
      fi
 
      debugme echo -n "sending client hello... "
-     socksend_clienthello "$TLS_CLIENT_HELLO$all_extensions" $USLEEP_SND
+     socksend_clienthello "$TLS_CLIENT_HELLO$all_extensions" $USLEEP_SND $first_fragment_size
 
      if [[ "$tls_low_byte" -gt 0x03 ]]; then
           TLS_CLIENT_HELLO="$(tolower "$NW_STR")"
@@ -13043,6 +13062,7 @@ resend_if_hello_retry_request() {
 # arg4: (optional) additional request extensions
 # arg5: (optional) "true" if ClientHello should advertise compression methods other than "NULL"
 # arg6: (optional) "false" if the connection should not be closed before the function returns.
+# arg7: (optional) the size of the first message fragment to send.
 # return: 0: successful connect   | 1: protocol or cipher not available | 2: as (0) but downgraded
 #         6: couldn't open socket | 7: couldn't open temp file
 tls_sockets() {
@@ -13075,7 +13095,7 @@ tls_sockets() {
      cipher_list_2send="$NW_STR"
 
      debugme echo -en "\nsending client hello... "
-     prepare_tls_clienthello "$tls_low_byte" "$cipher_list_2send" "$process_full" "$4" "$offer_compression"
+     prepare_tls_clienthello "$tls_low_byte" "$cipher_list_2send" "$process_full" "$4" "$offer_compression" "" "$7"
      ret=$?                             # 6 means opening socket didn't succeed, e.g. timeout
 
      # if sending didn't succeed we don't bother
@@ -15677,6 +15697,21 @@ run_grease() {
           fi
      fi
 
+     if "$normal_hello_ok"; then
+          for i in 5 6 7 8 5 6 7 8; do
+               debugme echo -e "\nSending ClientHello with first message fragment of length $i bytes."
+               tls_sockets "$proto" "$cipher_list" "" "" "" "" $i
+               success=$?
+               [[ $success -eq 0 ]] || [[ $success -eq 2 ]] && break
+               debugme tm_svrty_low "\nConnection failed on ClientHello with first message fragment of length $i bytes."
+          done
+          if [[ $success -ne 0 ]] && [[ $success -ne 2 ]]; then
+               prln_svrty_low " Server fails if ClientHello sent with first message fragment less than 9 bytes."
+               fileout "$jsonID" "LOW" "Server fails if ClientHello sent with first message fragment less than 9 bytes."
+               bug_found=true
+          fi
+     fi
+
      # Check that server ignores unrecognized cipher suite values
      # see https://datatracker.ietf.org/doc/draft-ietf-tls-grease
      if "$normal_hello_ok"; then