CTFZone 2019 Quals Writeup

2019-12-16T15:55:08+08:00

# PPC
### Fridge
In the $$n\times n$$ matrix, each $$(i,j)$$ operation will add a matrix to the original one, and modulo each entry with some $$P$$. (For the first several levels, $$P=2$$, and $$P=8$$ or $$16$$ later)
We can compute a basis to solve this problem.
```python
import socket
import random
import sys
from copy import deepcopy

_debug=False

class Remote:
	def __init__(self,ip,port):
		self.s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
		self.s.connect((ip,port))
		self.buf=''
	def send(self,s):
		self.s.send(s.encode())
	def recvbyte(self):
		if len(self.buf)==0:
			self.buf=self.s.recv(1024).decode().replace('\r','')
		res=self.buf[0]
		self.buf=self.buf[1:]
		if _debug:
			sys.stdout.write(res)
			sys.stdout.flush()
		return res
	def unrecv(self,s):
		self.buf=s+self.buf
	def recvuntil(self,s):
		res=''
		while res[-len(s):]!=s:
			res+=self.recvbyte()
		return res

r=Remote('ppc-fridge.ctfz.one',31337)

def getlvl():
	s=[]
	s+=[list(map(lambda x:int(x,16),r.recvuntil('\n').split(' ')))]
	for i in range(len(s[0])-1):
		s+=[list(map(lambda x:int(x,16),r.recvuntil('\n').split(' ')))]
	return s

def test(x,y):
	r.send(str(x)+','+str(y)+'\n')
	t=r.recvbyte()
	if t=='t':
		r.recvuntil('\n')
		return True
	r.unrecv(t)
	return False

def hash(x):
	res=0
	for i in x:
		for j in i:
			res=res*2+j
	return res

def umax(s):
	res=0
	for i in s:
		res=max(res,max(i))
	return res

def test8(cur,n,m):
	print('test8=============================')
	P=umax(cur)+1
	def add(x,y):
		res=deepcopy(x)
		for i in range(n):
			for j in range(m):
				res[i][j]=(res[i][j]+y[i][j])%P
		return res
	def sub(x,y):
		res=deepcopy(x)
		for i in range(n):
			for j in range(m):
				res[i][j]=(res[i][j]+P-y[i][j])%P
		return res
	op=[[0 for i in range(m)]for j in range(n)]
	for i in range(n):
		for j in range(m):
			if test(i,j):return
			nxt=getlvl()
			op[i][j]=sub(nxt,cur)
			print(i,j,op[i][j])
			cur=nxt
	for i in range(10):
		x=random.randint(0,n-1)
		y=random.randint(0,m-1)
		print('random test:',x,y)
		if test(x,y):return
		nxt=getlvl()
		assert nxt==add(op[x][y],cur)
		cur=nxt
	sc=[[0 for i in range(m)]for j in range(n)]
	sv=[[0 for i in range(m)]for j in range(n)]
	for i in range(n):
		for j in range(m):
			oc=op[i][j]
			ov=[[0 for i in range(m)]for j in range(n)]
			ov[i][j]=1
			flag=False
			for x in range(n):
				if flag:continue
				for y in range(m):
					if oc[x][y]:
						if sc[x][y]==0:
							print(x,y,oc)
							sc[x][y]=oc
							sv[x][y]=ov
							flag=True
							break
						tc=sc[x][y]
						tv=sv[x][y]
						assert tc[x][y]
						while oc[x][y]:
							o=tc[x][y]//oc[x][y]
							for uu in range(o):
								tc=sub(tc,oc)
								tv=sub(tv,ov)
							oc,tc=tc,oc
							ov,tv=tv,ov
						sc[x][y]=tc
						sv[x][y]=tv
	oc=cur
	ov=[[0 for i in range(m)]for j in range(n)]
	print('try to find sol')
	for x in range(n):
		for y in range(m):
			if oc[x][y]:
				tc=sc[x][y]
				tv=sv[x][y]
				while oc[x][y]:
					oc=add(oc,tc)
					ov=add(ov,tv)
	for x in range(n):
		for y in range(m):
			assert oc[x][y]==0
	print('find sol:',ov)
	for i in range(n):
		for j in range(m):
			for k in range(ov[i][j]):
				print('work:',i,j)
				if test(i,j):return
				cur=getlvl()

def work():
	global _debug
	print('try to get lvl')
	_debug=True
	s=getlvl()
	_debug=False
	n=len(s)
	m=len(s[0])
	print(s)
	test8(s,n,m)

while True:
	work()
```

### Labyrinth
A maze. However, there are some hidden mines in the map. So just mark them and avoid them.
```python
import socket
import random
import sys
from copy import deepcopy
import json
import os
from pwn import *

_debug=True

r=remote('ppc-labyrinth.ctfz.one',4340)

N=41
M=201

def recvmap():
	res=[]
	for i in range(min(N,curx+601)):
		res.append(r.recvuntil('\n').decode().strip())
		if res[-1][:9]=='Exception':
			print(res[-1])
			markmine()
			r.interactive()
	return res

known_map={}

curx=0

def rmap():
	global cury
	fe=False
	t=recvmap()
	n=len(t)
	for i in range(n):
		if len(t[i])!=201:
			if t[i][0]!='#':print(t)
			assert t[i][0]=='#' and t[i][-1]=='#'
			while len(t[i])<201:
				t[i]+=' '
		for j in range(M):
			if t[i][j]=='@':
				cury=j
				ux=i
				t[i]=t[i][:j]+' '+t[i][j+1:]
			elif t[i][j]=='E':
				t[i]=t[i][:j]+' '+t[i][j+1:]
				fe=True
			elif t[i][j]!='#' and t[i][j]!=' ':
				print('find strange point:',i,j)
				print(t[i])
	for i in range(n):
		px=i-ux+curx
		if px in known_map:
			for j in range(201):
				assert known_map[px][j]=='*' or known_map[px][j]==t[i][j]
		else:
			known_map[px]=t[i]

vis={}

def getm(x,y,tx,ty):
	if tx==x+1:return 'down'
	if tx==x-1:return 'up'
	if ty==y+1:return 'right'
	return 'left'

def upmove(ss):
	global curx,guessy
	guessy=cury
	for s in ss:
		if s=='up':curx-=1
		if s=='down':curx+=1
		if s=='left':guessy=guessy-1
		if s=='right':guessy=guessy+1
	r.send(','.join(ss)+'\n')
	rmap()

guessy=0
def markmine():
	t=known_map[curx]
	t=t[:guessy]+'*'+t[guessy+1:]
	known_map[curx]=t
	save()

def find_path(sx,sy,tx,ty):
	if sx==sy and tx==ty:return []
	q=[(tx,ty)]
	fa={(tx,ty):0}
	i=0
	res=[]
	while i<len(q):
		x,y=q[i];i+=1
		def chk(nx,ny):
			if ny<201 and nx in known_map and known_map[nx][ny]!='#' and known_map[nx][ny]!='*' and (nx,ny) not in fa:
				fa[(nx,ny)]=(x,y)
				q.append((nx,ny))
				if nx==sx and ny==sy:
					while nx!=tx or ny!=ty:
						gx,gy=fa[(nx,ny)]
						res.append(getm(nx,ny,gx,gy))
						nx=gx;ny=gy
		chk(x+1,y)
		chk(x-1,y)
		chk(x,y+1)
		chk(x,y-1)
		if len(res):return res

def get_reach(sx,sy):
	q=[(sx,sy)]
	fa={(sx,sy):0}
	i=0
	while i<len(q):
		x,y=q[i];i+=1
		def chk(nx,ny):
			#print(nx,ny)
			if ny<201 and nx in known_map and known_map[nx][ny]!='#' and known_map[nx][ny]!='*' and (nx,ny) not in fa:
				fa[(nx,ny)]=(x,y)
				q.append((nx,ny))
		chk(x+1,y)
		chk(x-1,y)
		chk(x,y+1)
		chk(x,y-1)
	return fa

def print_whole_map():
	t=list(known_map)
	t.sort()
	for i in t:
		u=''
		for j in range(201):
			u+='@' if curx==i and cury==j else known_map[i][j]
		print(u)

def print_reachable():
	t=list(known_map)
	t.sort()
	rcs=get_reach(curx,cury)
	for i in t:
		u=''
		for j in range(201):
			if curx==i and cury==j:
				u+='@'
			elif (i,j) in rcs:
				u+='-'
			else:
				u+=known_map[i][j]
		print(u)

def save():
	open('map.txt','w').write(json.dumps(known_map))
def load():
	global known_map
	t=json.loads(open('map.txt').read())
	for i in t:
		known_map[int(i)]=t[i]

def random_walk():
	while True:
		nx=random.randint(-300,0)
		ny=random.randint(150,200)
		if known_map[nx][ny]==' ':
			break
	pt=find_path(curx,cury,nx,ny)
	for i in range(0,len(pt),20):
		upmove(pt[i:i+20])
		print('R',i,len(pt),curx,cury,pt[i])

if os.path.exists('map.txt'):load()
rmap()
print_whole_map();#exit()
print()
print_reachable()
save();#exit()

while True:
	t=list(known_map)
	t.sort()
	pt=False
	for i in t:
		for j in range(201):
			if known_map[i][j]==' ' and (i,j) not in vis:
				pt=find_path(curx,cury,i,j)
				if pt:break
				print(i,j,'failed')
		if pt:break
	print(pt)
	i=-40
	if curx>=-500:
		for i in range(0,len(pt),40):
			upmove(pt[i:i+40])
			print(i,len(pt),curx,cury,pt[i])
			if curx<-500:break
	st=i+40
	for i in range(st,len(pt),1):
		upmove(pt[i:i+1])
		print(i,len(pt),curx,cury,pt[i])
	print_whole_map()
	print(curx,cury)
	save()
```

### StarWars
When we successfully attacked some ship, our shield will be the max of our old one and shield of that ship, our attack will be the sum modulo 256.
The action of other players are always the same. So the current state is only related with current shield and attack, and the remaining ships.
So we can use dp to solve this. Let $$dp[i][j][k][mask]$$ be: round $$i$$, current shield $$j$$, current attack $$k$$, $$mask$$ stores remaining ships. The dp relation is obvious.
```cpp
#include<bits/stdc++.h>

typedef std::pair<int,int> pii;
template<typename T> inline T max(T a,T b){return a>b?a:b;}

#define xx first
#define yy second
#define mp(a,b) std::make_pair(a,b)
#define pb push_back
#define fo0(i,n) for(int i=0,i##end=n;i<i##end;i++)

const int system_ships_t[16][2]={
 {20,10},
 {60,10},
 {30,20},
 {80,20},
 {100,20},
 {200,25},
 {600,25},
 {400,50},
 {800,20},
 {1000,100},
 {1600,125},
 {2000,100},
 {3000,150},
 {6000,100},
 {20000,255},
 {10000,1},
};
const int ships_t[17][2]={
 {50,10},
 {30,20},
 {10,60},
 {100,6},
 {20,30},
 {50,15},
 {70,10},
 {120,5},
 {800,1},
 {350,2},
 {10,100},
 {40,15},
 {35,20},
 {70,15},
 {60,20},
 {88,8},
 {99,9},
};
const int attack_order[17][24]={
 {0,0,0,0,0,0,2,3,2,5,2,5,4,7,7,5,0,1,8,2,8,7,0,14},
 {1,2,2,1,1,3,4,3,0,4,7,15,4,11,6,0,0,2,8,6,10,7,5,8},
 {2,1,1,1,0,3,5,2,2,1,3,5,0,3,3,4,15,13,0,4,8,10,13,1},
 {2,0,0,2,0,3,5,1,5,4,2,4,8,0,15,9,4,8,3,1,7,9,0,6},
 {1,1,0,0,1,3,5,1,1,2,5,2,8,8,6,4,7,5,9,2,3,2,8,4},
 {2,1,2,0,1,5,4,2,1,0,1,1,8,5,7,2,7,5,8,4,0,15,8,3},
 {2,1,2,0,1,3,5,5,5,3,7,5,1,5,8,4,5,15,1,9,10,0,10,0},
 {1,2,0,2,1,5,2,2,3,4,3,2,8,1,3,5,7,8,1,2,2,0,4,1},
 {0,1,1,1,1,0,0,5,2,0,0,6,2,3,3,5,2,4,7,15,2,2,1,6},
 {0,2,0,0,1,4,0,3,0,2,1,8,3,6,2,7,15,4,6,2,15,6,5,0},
 {2,0,1,2,2,5,0,4,0,3,0,2,1,4,2,3,15,0,0,5,9,9,1,4},
 {2,2,0,1,0,0,3,4,15,9,0,13,4,3,4,0,5,2,7,10,9,10,1,11},
 {1,0,2,0,1,1,5,5,3,2,3,0,6,0,7,2,4,8,1,5,15,4,3,9},
 {2,1,1,1,1,4,5,4,3,2,7,8,6,4,4,0,3,6,7,2,6,2,9,3},
 {2,0,0,1,1,5,1,3,3,2,8,5,1,3,7,1,7,2,9,8,0,2,4,1},
 {0,2,2,0,2,0,5,2,3,3,0,7,0,5,4,0,6,6,0,3,6,3,8,9},
 {0,0,1,2,0,4,1,0,15,4,11,4,12,8,3,4,6,10,12,13,1,11,4,9},
};

pii ssp[16],sp[17],spo[16][25];

bool work(pii a,pii b,pii&res)
{
	int ac=a.yy?b.xx/a.yy:1e9,bc=a.xx/b.yy;
	if(ac<=bc)
	{
		res=mp(max(a.xx,b.xx),(a.yy+b.yy)&0xff);
		return 1;
	}
	return 0;
}

int hc,hid[23333],hv[33];

std::bitset<1<<16|5> f[25][33][256];

std::vector<int> find_seq(int ti,int tj,int tk,int tl)
{
	if(ti==0)return std::vector<int>();
	int i=ti-1;
	std::vector<std::pair<pii,pii>>ef;
	fo0(j,hc)fo0(k,256)
	{
		for(int l=f[i][j][k]._Find_first();l<f[i][j][k].size();l=f[i][j][k]._Find_next(l))
		{
			fo0(x,17)
			{
				pii tmp;int mask;
				if(x)
				{
					int t=x-1;
					if(l>>t&1)continue;
					if(!work(mp(hv[j],k),spo[t][i],tmp))continue;
					mask=l|1<<t;
				}
				else tmp=mp(hv[j],k),mask=l;
				if(!work(tmp,ssp[attack_order[0][i]],tmp))continue;
				if(hid[tmp.xx]==tj&&tmp.yy==tk&&mask==tl)ef.pb(mp(mp(j,k),mp(l,x)));
			}
		}
	}
	std::vector<int>res=find_seq(ti-1,ef[0].xx.xx,ef[0].xx.yy,ef[0].yy.xx);
	res.pb(ef[0].yy.yy);
	return res;
}

int main()
{
	fo0(i,16)ssp[i]=mp(system_ships_t[i][0],system_ships_t[i][1]);
	fo0(i,17)sp[i]=mp(ships_t[i][0],ships_t[i][1]);
	memset(hid,0xff,sizeof hid);
	fo0(i,16)if(!~hid[ssp[i].xx])hv[hid[ssp[i].xx]=hc++]=ssp[i].xx;
	fo0(i,17)if(!~hid[sp[i].xx])hv[hid[sp[i].xx]=hc++]=sp[i].xx;
	fo0(i,16)
	{
		spo[i][0]=sp[i+1];
		fo0(j,24)
		{
			assert(work(spo[i][j],ssp[attack_order[i+1][j]],spo[i][j+1]));
		}
	}
	f[0][hid[sp[0].xx]][sp[0].yy]=1;
	fo0(i,24)
	{
		fo0(j,hc)
		{
			fo0(k,256)
			{
				for(int l=f[i][j][k]._Find_first();l<f[i][j][k].size();l=f[i][j][k]._Find_next(l))
				{
					fo0(x,17)
					{
						pii tmp;int mask;
						if(x)
						{
							int t=x-1;
							if(l>>t&1)continue;
							if(!work(mp(hv[j],k),spo[t][i],tmp))continue;
							mask=l|1<<t;
						}
						else tmp=mp(hv[j],k),mask=l;
						if(mask==65535)
						{
							std::vector<int>ans=find_seq(i,j,k,l);
							ans.pb(x);
							for(auto i:ans)printf("%d ",i);puts("");
							exit(0);
						}
						if(!work(tmp,ssp[attack_order[0][i]],tmp))continue;
						f[i+1][hid[tmp.xx]][tmp.yy][mask]=1;
					}
				}
			}
		}
	}
}
```

After running this, we find the unique sequence that leads to win.
However, the output part failed to run on my computer, so here is a python code to output the flag:
```python
import hashlib

s=[0,0,0,0,0,0,0,11,16,0,1,10,6,13,3,2,9,15,8,14,12,5,4,7]
hs=hashlib.sha256(bytes(s)).digest()
u=[514320763,1323926996,700138656,849420235,427957186,1264587960,734406206,1885512490]
ut=[]
for i in u:
	ut.append(i&255)
	ut.append(i>>8&255)
	ut.append(i>>16&255)
	ut.append(i>>24&255)
ut=bytes(ut)
ans=[]
for i in range(32):
	v=(ut[i]^hs[i])%127
	ans.append(v if v>=32 else v+32)
print(bytes(ans))
```

# Reverse

### Baby rev
A dos executable. It reads a file and divide it into parts. In each part, it’s compared each byte to some value.
The following script is to extract these compared values from the asm presented by IDA.
```python
s=[-1 for i in range(480)]
for i in open('tmp.txt').readlines():
	t=i[23:].strip()
	if t.startswith('cmp     byte ptr') and '+' in t:
		t=t[t.find('+')+1:]
		o=t[:t.find('h')]
		if ']' in o:o=o[:o.find(']')]
		o=int(o,16)
		t=t[t.find(',')+1:]
		v=t[:t.find('h')]
		v=int(v,16)
		s[o]=v

s[0]=0xdb
for i in range(480):
	if s[i]==-1:print(i)
open('test.bin','wb').write(bytes(s))
```
File tmp.txt contains asm like:
```
seg000:0214 loc_10214:                              ; CODE XREF: sub_101FA+15↑j
seg000:0214                 cmp     byte ptr [si+1], 0DBh
seg000:0218                 jz      short loc_1021D
seg000:021A                 jmp     loc_10426
seg000:021D ; ---------------------------------------------------------------------------
seg000:021D
seg000:021D loc_1021D:                              ; CODE XREF: sub_101FA+1E↑j
seg000:021D                 cmp     byte ptr [si+2], 0DBh
seg000:0221                 jz      short loc_10226
seg000:0223                 jmp     loc_10426
```
Finally run “BABY_REV test.bin” we got the flag.

### CSES
The program requires lua, and it communicates with the server to execute commands.
In the main function, an array is decoded, and then it’s the lua bytecode.
I used luadec to decompile the bytecode, but some functions were incorrect.
However, in the handle_answer function, we saw the command.
```lua
handle_answer = function(answer)
  -- function num : 0_14 , upvalues : _ENV
  enc_index = (string.find)(answer, "encrypt::")
  dec_index = (string.find)(answer, "decrypt::")
  command = rc4_cipher("ctfzone2019", base64_dec("1bC87lzEebgL"))
  bin_index = (string.find)(answer, command)
  if enc_index == 1 or dec_index == 1 or bin_index == 1 then
    print_result(answer)
  end
End
```
I patched this function, and find that command is “gpsjdeadk”.
Then I send this to the server, an large base64 encoded string is returned. It contains an ELF file, the server.
The server encrypts flag with an hard coded AES key, and use the encrypted flag as IV and key for later encryption.
The following script is to find the encrypted flag. The final decryption is too easy, and I won’t put it here.
```python
import socket
import random
import sys
from copy import deepcopy
from pwn import *

r=remote('re-cses.ctfz.one',3607)

def encrypt(s):
	r.send('encrypt::'+s+'\n')
	return r.recvuntil('\0')[:-2].decode('base_64')

def decrypt(s):
	r.send('decrypt::'+s.encode('base_64').strip()+'\n')
	res=''
	for i in range(len(s)):
		t=r.recv(1)
		if t=='\0':break
		res+=t
	return res

def xor(a,b):
	r=''
	for i in range(len(a)):
		r+=chr(ord(a[i])^ord(b[i]))
	return r

a='1'*16
x=encrypt(a)[:16]

xu=decrypt(x+x)
b=xu[:16]
c=xu[16:]
print b.encode('hex'),c.encode('hex')

rc=xor(c,x)
flag_enc=xor(rc,a)
print(flag_enc.encode('hex'))
```

### Strange_pdf
The comment starting with % in pdf file will be ignored by pdf-viewer, and there is one program in the comment just after header. Also, there is anther comment later which is “55 AA”, the end signature of MBR. So the program is a 16-bit x86 program and it print 99399 on screen, which is x.

一次（可能算是）失败的 BGP 尝试

2019-12-06T19:39:00+08:00

### 背景
教育网的 ipv6 大多是免流的。那么问题是如何找一个合适的梯子实现高速免流。教育网 ipv6 的出国线路大概有 HE、Cogentco、HKIX（有多个 ISP 都会经过 HKIX，就不详细列出了）。
HE 是大多数 ip 都会走线路，于是他 QoS 也比较严重。Cogentco 和 HKIX 则 QoS 的比较少。走 HKIX 的 VPS 还是有不少（自行搜索），不过价格一般还是不便宜。
而 Cogentco 线路有一些大服务商有，比如 vultr 的达拉斯回程和亚特兰大的去程，但是这两个的相反方向却都只是普通的HE 线路。要综合这两个线路得到一个双向 Cogentco 线路的梯子，其实可以通过一些操作，使得本地给 A 发包，如何 B 返回结果。但是感觉设置比较麻烦，就咕了。
这时我想到我恰好有一个（算是）闲置的 ASN 和一些 ipv6 资源，能不能用 BGP 实现这个双向 Cogentco 呢？
### 前提条件
一个 ASN，一些 ip 段。这些许多商家有售（比如 hostus）。比较便宜的可以去各大论坛或者群里问（比如 [https://t.me/MoeQing](https://t.me/MoeQing)）

首先在 vultr 开两台机，装好必要的软件。

接下来在两台机器上都配置广播 ip，可以参考 [https://blog.ni-co.moe/public/560.html](https://blog.ni-co.moe/public/560.html)。

### 配置去程
由于我们想让这个 ip 段强制走其中一台进入，需要把他们之间连起来，并且再设置一下路由优先级。

把他们之间连起来可以参考 [https://lantian.pub/article/modify-website/join-dn42-experimental-network.lantian/](https://lantian.pub/article/modify-website/join-dn42-experimental-network.lantian/)。这里需要注意，为了让 zerotier 能正常分配公网 ipv6，需要运行 `zerotier-cli set 你的网络id allowGlobal=1`。

接下来是设置路由优先级。这里的需求很简单，只是为了让路由全都走某一台，那么可以让另一台的 AS Path 足够长，这样就不会被优先选择了。

具体来说，在 `/etc/bird/bird6.conf` 中，`protocol bgp vultr` 一段里，把 `export all;` 改成：
```
export filter {
    bgp_path.prepend(你的 AS 号);
    bgp_path.prepend(你的 AS 号);
    ...
    bgp_path.prepend(你的 AS 号);
    bgp_path.prepend(你的 AS 号);
};
```
这里要加多少个就自己决定了，只要让别人走不过来就行。

配好之后，重启 bird6，等一会路由更新，然后 trace 一下，发现去程确实走了 Cogentco。

### 配置回程
回程按理来说是不需要配置的，但是在达拉斯的机器上运行
```
mtr 2402:f000:2:f001::240:1
```
是走 GTT 到 Cogentco；而运行
```
mtr -a 你的ip 2402:f000:2:f001::240:1
```
却是走 NTT 到 HE。

为了配置这一部分，需要想办法调整回程线路，也就是 VPS 上的出口线路。

而根据 Vultr 给出的一些参考（[https://www.vultr.com/docs/as20473-bgp-customer-guide](https://www.vultr.com/docs/as20473-bgp-customer-guide)），可以用 BGP Community 来控制宣告。

比如可以在 `export filter` 中加入
```
bgp_community.add((64609, 3257));
bgp_community.add((64699, 3257));
bgp_community.add((20473, 6000));
```
来只对外宣告到 AS3257（GTT）。

然而，这样仍然不能使上面的 mtr 都走 Cogentco。

### 问题分析
我找 Vultr 客服把我的 BGP 模式改成了全表。在所有出口路径中，我搜索了 45576（贵清的 ASN），发现只有
```
64515 65534 20473 6939 23911 23910 24348 45576
```
一条。也就是直接走 HE 的。

那么 GTT 和 Cogentco 是怎么走出来的就非常奇怪了。

这里我也分析不下去了，如果有人知道原因欢迎联系我。

后来我换了几个不同的贵清 ip 测试，发现有的是两种 mtr 都走 NTT-HE，有的是两种都走 GTT-Cogentco。

### 总结
BGP 实在是玄妙啊。。。

mcfx's blog - 2019年12月

CTFZone 2019 Quals Writeup

一次（可能算是）失败的 BGP 尝试