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niklasb / libc-database

Build a database of libc offsets to simplify exploitation
https://libc.rip/
MIT License
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Retry rpm download with --no-dns-cache #52

Closed blukat29 closed 3 years ago

blukat29 commented 3 years ago

Retry in get_rpm() without dns cache.

Unlike other distros, CentOS download site uses something like round-robin DNS. The mirror.centos.org domain maps to one of many IPs. Sometimes an IP can be too slow or unreachable for some reason. In my case, 147.75.93.107 is unreachable from my location (South Korea).

wget automatically retries on such network failure, but with DNS cached. The solution to this problem is either

  1. Restart wget process, as in get_from_filelistgz(). wget's DNS cache is only in memory.
  2. Use --no-dns-cache option that tells wget to use different IP every retry.

This patch applies (2) to get_rpm().

A quick test show that the retry makes the download more robust.

#10 16.06 Getting centos-glibc
#10 16.06   -> Location: http://mirror.centos.org/centos//./7.9.2009/updates/x86_64/Packages/glibc-2.17-323.el7_9.i686.rpm
#10 16.07   -> ID: glibc-2.17-323.el7_9.i686
#10 16.07   -> Downloading package
#10 16.07 --2021-08-15 02:33:04--  http://mirror.centos.org/centos//7.9.2009/updates/x86_64/Packages/glibc-2.17-323.el7_9.i686.rpm
#10 16.07 Resolving mirror.centos.org (mirror.centos.org)... 147.75.93.107, 2604:1380:3000:2800::1
#10 16.58 Connecting to mirror.centos.org (mirror.centos.org)|147.75.93.107|:80... failed: Connection timed out.
#10 46.58 Connecting to mirror.centos.org (mirror.centos.org)|2604:1380:3000:2800::1|:80... failed: Cannot assign requested address.
#10 46.58 Retrying.
#10 46.58
#10 47.58 --2021-08-15 02:33:35--  (try: 2)  http://mirror.centos.org/centos//7.9.2009/updates/x86_64/Packages/glibc-2.17-323.el7_9.i686.rpm
#10 47.58 Resolving mirror.centos.org (mirror.centos.org)... 160.116.15.18, 2406:da1a:fcb:2f01:b6e2:c6:795:b503
#10 48.00 Connecting to mirror.centos.org (mirror.centos.org)|160.116.15.18|:80... connected.
#10 48.30 HTTP request sent, awaiting response... 200 OK
#10 48.61 Length: 4465468 (4.3M) [application/x-rpm]
#10 48.61 Saving to: ‘/tmp/tmp.hTApue5xnM/pkg.rpm’
#10 48.61
#10 48.61      0K .......... .......... .......... .......... ..........  1% 1.64M 3s
#10 48.64     50K .......... .......... .......... .......... ..........  2% 10.8M 1s

This feature may be applied to other download functions like get_debian(), but this problem seems unlikely in other download sites.

peace-maker commented 3 years ago

This seems to be a very specific issue, but the fix looks reasonable.