Category: Technology

badssl.com

Today I discovered badssl.com, a very useful website to test various exceptional TLS/SSL cases. E.g., I tested what will happen if client supports TLS 1.3 only while server supports TLS 1.2 only:

# openssl s_client -connect tls-v1-2.badssl.com:1012 -tls1_3
CONNECTED(00000005)
01000000:error:0A000410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:ssl/record/rec_layer_s3.c:1584:SSL alert number 40
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 7 bytes and written 253 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
---

The gotcha of logging gdb output

By default, gdb‘s output file is appended, not overwrote. E.g: debug the same program for 2 times:

$ gdb foo
......
(gdb) set logging on
Copying output to gdb.txt.
Copying debug output to gdb.txt.
(gdb) r
......
$ ll gdb.txt
-rw-rw-r-- 1 nanxiao nanxiao 1067 Jul  9 18:06 gdb.txt
$ gdb foo
......
(gdb) set logging on
Copying output to gdb.txt.
Copying debug output to gdb.txt.
(gdb) r
......
$ ll gdb.txt
-rw-rw-r-- 1 nanxiao nanxiao 2134 Jul  9 18:08 gdb.txt

After second debug, the gdb.txt‘s size is doubled. To overwrite the output file, execute set logging overwrite on before set logging on:

$ gdb foo
......
(gdb) set logging overwrite on
(gdb) set logging on
Copying output to gdb.txt.
Copying debug output to gdb.txt.
(gdb) r
......
$ ll gdb.txt
-rw-rw-r-- 1 nanxiao nanxiao 1067 Jul  9 18:10 gdb.txt

A trick of setting breakpoint in pdb

When using pdb to debug a python program:

python -m pdb foo.py

I want to set a breakpoint, but meet following error:

(Pdb) b bar.py:46
*** 'bar.py' not found from sys.path

A small trick is setting breakpoint in main first and run the program:

(Pdb) b main
Breakpoint 1 at ......
(Pdb) r
......

After breakpoint set for main is hit, set breakpoint again at bar.py:46. This time it should work:

(Pdb) b bar.py:46
Breakpoint 2 at ......