A very hardcore but simple way to view open connections and ports open for listening in any Linux container. Use the following command even inside any bare bones container without netstat and such tools:

grep -v "rem_address" /proc/net/tcp

The output will be something like this

   0: 00000000:1F40 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 6106109 1 ffff889f5ff35800 100 0 0 10 0
   1: 00000000:0C6D 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 6112091 1 ffff889ee7e96000 100 0 0 10 0
   2: 611F820A:0C6D E61E820A:A5E6 01 00000000:00000000 00:00000000 00000000     0        0 6122922 1 ffff889d2b712800 20 0 0 10 -1
   3: 611F820A:0C6D E61E820A:A5EE 01 00000000:00000000 00:00000000 00000000     0        0 6118270 1 ffff889e736e3000 20 4 21 10 -1
   4: 611F820A:1F40 F21F820A:DE3E 01 00000000:00000000 00:00000000 00000000     0        0 6119808 1 ffff889e78be7000 20 4 3 10 -1
   5: 611F820A:0C6D E61E820A:A5FC 01 00000000:00000000 00:00000000 00000000     0        0 6128773 1 ffff889e78bf2000 20 4 33 10 -1

The two left columns are source address and port and destination address and port in hex. The first two rows in this example correlate to listening ports (the destination is all zeros) and the rest are open connections.

To get this in a bit more human readable form you can run the following command that should also work inside most containers:

grep -v "rem_address" /proc/net/tcp | awk 'function hextonum(str, ret, n, i, k, c) {if (str ~ /^0[xX][0-9a-fA-F]+$/) {str = substr(str, 3);n = length(str);ret = 0;for (i = 1; i <= n; i++) {c = substr(str, i, 1);c = tolower(c);k = index("123456789abcdef", c);ret = ret * 16 + k}} else ret = "NOT-A-NUMBER";return ret} {y=hextonum("0x"substr($2,index($2,":")-2,2));x=hextonum("0x"substr($3,index($3,":")-2,2));for (i=5; i>0; i-=2) {x = x"."hextonum("0x"substr($3,i,2));y = y"."hextonum("0x"substr($2,i,2));} print y":"hextonum("0x"substr($2,index($2,":")+1,4))" "x":"hextonum("0x"substr($3,index($3,":")+1,4));}'

The output will be similar to this:

0.0.0.0:8000 0.0.0.0:0
0.0.0.0:3181 0.0.0.0:0
10.130.31.97:3181 10.130.30.230:42470
10.130.31.97:3181 10.130.30.230:42478
10.130.31.97:8000 10.130.31.242:56894

If you want to understand how this command works - leave a comment!